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Archie L. Cobbs edited this page Jul 17, 2022 · 22 revisions
S3BACKER(1)                 BSD General Commands Manual                 S3BACKER(1)

     s3backer -- FUSE-based single file backing store via Amazon S3

     s3backer [options] bucket[/subdir] /mount/point

     s3backer --nbd [options] bucket[/subdir] /dev/nbdX

     s3backer --test [options] dir /mount/point

     s3backer --erase [options] bucket[/subdir]

     s3backer --reset-mounted-flag [options] bucket[/subdir]

     s3backer is a filesystem that contains a single file backed by the Amazon Sim-
     ple Storage Service (Amazon S3).  As a filesystem, it is very simple: it pro-
     vides a single normal file having a fixed size.  Underneath, the file is
     divided up into blocks, and the content of each block is stored in a unique
     Amazon S3 object.  In other words, what s3backer provides is really more like
     an S3-backed virtual hard disk device, rather than a filesystem.

     In typical usage, a `normal' filesystem is mounted on top of the file exported
     by the s3backer filesystem using a loopback mount (or disk image mount on Mac
     OS X).

     This arrangement has several benefits compared to more complete S3 filesystem

     o   By not attempting to implement a complete filesystem, which is a complex
         undertaking and difficult to get right, s3backer can stay very lightweight
         and simple.  All of the experience and knowledge about how to properly
         implement filesystems that already exists can be reused.

     o   By utilizing existing filesystems, you get full UNIX filesystem semantics.
         Subtle bugs or missing functionality relating to hard links, extended
         attributes, POSIX locking, etc. are avoided.

     o   The gap between normal filesystem semantics and Amazon S3 ``eventual con-
         sistency'' is more easily and simply solved when one can interpret S3
         objects as simple device blocks rather than filesystem objects (see

     o   When storing your data on Amazon S3 servers, which are not under your con-
         trol, the ability to encrypt and authenticate data becomes a critical
         issue.  s3backer supports secure encryption and authentication.  Alter-
         nately, the encryption capability built into the Linux loopback device can
         be used.

     o   Since S3 data is accessed over the network, local caching is also very
         important for performance reasons.  Since s3backer presents the equivalent
         of a virtual hard disk to the kernel, most of the filesystem caching can
         be done where it should be: in the kernel, via the kernel's page cache.
         However s3backer also includes its own internal block cache for increased
         performance, using asynchronous worker threads to take advantage of the
         parallelism inherent in the network.

   Consistency Guarantees
     Previously, Amazon S3 made relatively weak guarantees relating to the timing
     and consistency of reads vs. writes (collectively known as ``eventual consis-
     tency'').  s3backer includes logic and configuration parameters to work around
     these limitations, allowing the user to guarantee consistency to whatever
     level desired, up to and including 100% detection and avoidance of incorrect
     data.  These are:

     1.  s3backer enforces a minimum delay between consecutive PUT or DELETE opera-
         tions on the same block.  This ensures that Amazon S3 doesn't apply these
         operations out of order.

     2.  s3backer maintains an internal block MD5 checksum cache, which enables
         automatic detection and rejection of `stale' blocks returned by GET opera-

     This logic is configured by the following command line options:
     --md5CacheSize, --md5CacheTime, and --minWriteDelay.

     In December of 2020, Amazon announced that S3 is now fully consistent.  There-
     fore, the MD5 cache is disabled by default in s3backer versions after 1.6.3.
     Other S3-compatible services may still require it however.

     To guarantee consistency, --md5CacheTime and --minWriteDelay should be greater
     than the maximum S3 convergence time.

   Zeroed Block Optimization
     As a simple optimization, s3backer does not store blocks containing all
     zeroes; instead, they are simply deleted.  Conversely, reads of non-existent
     blocks will contain all zeroes.  In other words, the backed file is always
     maximally sparse.

     As a result, blocks do not need to be created before being used and no special
     initialization is necessary when creating a new filesystem.

     s3backer also uses a simple bitmap to track blocks that are known to be zero
     on an ongoing basis.  When the --listBlocks flag is given (recommended),
     s3backer will list all existing blocks at startup to initialize this bitmap.
     This especially affects performance with operations that read or write a large
     number of zero blocks, such as filesystem initialization, fstrim(8), bulk
     copies, etc.

   File and Block Size Auto-Detection
     As a convenience, whenever the first block of the backed file is written,
     s3backer includes as meta-data (in the ``x-amz-meta-s3backer-filesize''
     header) the total size of the file.  Along with the size of the block itself,
     this value can be checked and/or auto-detected later when the filesystem is
     remounted, eliminating the need for the --blockSize or --size flags to be
     explicitly provided and avoiding accidental mis-interpretation of an existing

   Block Cache
     s3backer includes support for an internal block cache to increase performance.
     The block cache cache is completely separate from the MD5 cache which only
     stores MD5 checksums transiently and whose sole purpose is to mitigate ``even-
     tual consistency''.  The block cache is a traditional cache containing cached
     data blocks.  When full, clean blocks are evicted as necessary in LRU order.

     Reads of cached blocks will return immediately with no network traffic.
     Writes to the cache also return immediately and trigger an asynchronous write
     operation to the network via a separate worker thread.  Because the kernel
     typically writes blocks through FUSE filesystems one at a time, performing
     writes asynchronously allows s3backer to take advantage of the parallelism
     inherent in the network, vastly improving write performance.

     The block cache can be configured to store the cached data in a local file
     instead of in memory.  This permits larger cache sizes and allows s3backer to
     reload cached data after a restart.  Reloaded data is verified via MD5 check-
     sum with Amazon S3 before reuse.

     Having said all that, Linux users may want to consider instead using the ker-
     nel "bcache" mechanism for local caching of blocks.

     The block cache is configured by the following command line options:
     --blockCacheFile, --blockCacheMaxDirty, --blockCacheNoVerify,
     --blockCacheNumProtected, --blockCacheSize, --blockCacheSync,
     --blockCacheThreads, --blockCacheTimeout, --blockCacheWriteDelay, and

   Read Ahead
     s3backer implements a simple read-ahead algorithm in the block cache.  When a
     configurable number of blocks are read in order, block cache worker threads
     are awoken to begin reading subsequent blocks into the block cache.  Read
     ahead continues as long as the kernel continues reading blocks sequentially.
     The kernel typically requests blocks one at a time, so having multiple worker
     threads already reading the next few blocks improves read performance by tak-
     ing advantage of the parallelism inherent in the network.

     Note that the kernel implements a read ahead algorithm as well; its behavior
     should be taken into consideration.  By default, s3backer passes the -o
     max_readahead=0 option to FUSE.

     Read ahead is configured by the --readAhead and --readAheadTrigger command
     line options.

   Encryption and Authentication
     s3backer supports encryption via the --encrypt, --password, and --passwordFile
     flags.  When encryption is enabled, SHA1 HMAC authentication is also automati-
     cally enabled, and s3backer rejects any blocks that are not properly encrypted
     and signed.

     Encrypting at the s3backer layer is preferable to encrypting at an upper layer
     (e.g., at the loopback device layer), because if the data s3backer sees is
     already encrypted it can't optimize away zeroed blocks or do meaningful com-

     s3backer supports block-level compression, which minimizes transfer time and
     storage costs.

     Compression is configured via the --compress and --compress-level flags.  Com-
     pression is automatically enabled when encryption is enabled.

   Server Side Encryption
     s3backer supports server side encryption via the --sse flag.

   Read-Only Access
     An Amazon S3 account is not required in order to use s3backer.  The filesystem
     must already exist and have S3 objects with ACL's configured for public read
     access (see --accessType below); users should perform the looback mount with
     the read-only flag (see mount(8)) and provide the --readOnly flag to s3backer.
     This mode of operation facilitates the creation of public, read-only filesys-

   Simultaneous Mounts
     Although it functions over the network, the s3backer filesystem is not a dis-
     tributed filesystem and does not support simultaneous read/write mounts.
     (This is not something you would normally do with a hard-disk partition
     either.)  As a safety measure, s3backer attempts to detect this situation
     using an 'already mounted' flag in the data store, and will fail to start if
     it does.

     This detection may produce a false positive if a former s3backer process was
     not shutdown cleanly; if so, the --reset-mounted-flag flag can be used to
     reset the 'already mounted' flag.  But see also BUGS below.

   Statistics File
     s3backer populates the filesystem with a human-readable statistics file.  Use
     --statsFilename to change the name of this file (default `stats').  The sta-
     tistics can be reset to zero by attempting to remove the file.

   NBD Plugin
     On platforms with ndbkit(1), s3backer is also installed as an nbdkit(1) plug-

     When using the NBD plugin, command line flags like --blockSize=SIZE should
     instead be specified as plugin parameters like blockSize=SIZE; for boolean
     flags, specify a value of true.  The bucket name (with optional subdirectory)
     must also be specified with the bucket parameter.

     The --nbd flag causes s3backer to function as a wrapper process for managing
     an NBD session; see below.

     In normal operation s3backer will log via syslog(3).  When run with the -d or
     -f flags, s3backer will log to standard error.

     Each command line flag has two forms, for example --accessFile=FILE and -o
     accessFile=FILE.  Only the first form is shown below.  Either form many be
     used; both are equivalent.  The second form allows mount options to be speci-
     fied directly in /etc/fstab and passed seamlessly to s3backer by FUSE.

             Specify a file containing `accessID:accessKey' pairs, one per-line.
             Blank lines and lines beginning with a `#' are ignored.  If no
             --accessKey or --accessKeyEnv is specified, this file will be searched
             for the entry matching the access ID specified via --accessId; if no
             --accessId is specified, the first entry in this file will be used.
             Default value is $HOME/.s3backer_passwd.

             Specify Amazon S3 access ID.  Specify an empty string to force no
             access ID.  If no access ID is specified (and none is found in the
             access file) then s3backer will still function, but only reads of pub-
             licly available filesystems will work.

             Specify Amazon S3 access key.

             To avoid publicizing this secret via the command line, use
             --accessKeyEnv or --accessFile instead of this flag.

             Specify an environment variable containing the Amazon S3 access key.

             Specify the Amazon S3 access privilege ACL type for newly written
             blocks.  The value must be one of `private', `public-read', `public-
             read-write', or `authenticated-read'.  Default is `private'.

             Download access credentials and security token in JSON document form
             every five minutes.

             This option allows S3 credentials to be provided automatically via the
             specified IAM role to s3backer when running on an Amazon EC2 instance.

             Specify how to authenticate requests. There are two supported authen-
             tication methods: aws2 is the original AWS authentication scheme.
             aws4 is the newer, recommended authentication scheme.

             aws4 is the default setting starting in version 1.4, and is required
             for certain non-US regions, while aws2 may still be required by some
             non-Amazon S3 providers.

             Specify the base URL, which must end in a forward slash. Default is

             Use this flag to specify FIPS and dual-stack endpoints.

             Note: the region name is used in authentication, so if you include a
             region name you probably also need to specify it via --region.

             Specify a file in which to store cached data blocks.  Without this
             flag, the block cache lives entirely in process memory and the cached
             data disappears when s3backer is stopped.  The file will be created if
             it doesn't exist.

             Cache files that have been created by previous invocations of s3backer
             are reusable as long as they were created with the same configured
             block size (if not, startup will fail).  This is true even if s3backer
             was stopped abruptly, e.g., due to a system crash; however, this guar-
             antee rests on the assumption that the filesystem containing the cache
             file will not reorder writes across calls to fsync(2).

             If an existing cache is used but was created with a different size,
             s3backer will automatically expand or shrink the file at startup.
             When shrinking, blocks that don't fit in the new, smaller cache are
             discarded.  This process also compacts the cache file to the extent

             By default, only clean cache blocks are recoverable after a restart.
             This means a system crash will cause dirty blocks in the cache to be
             lost (of course, that is the case with an in-memory cache as well).

             With the newer cache file format introduced in release 1.5.0, you can
             recover these dirty blocks by specifying the
             --blockCacheRecoverDirtyBlocks option.  This will cause any dirty
             blocks in the cache file to be made writable again on startup.  If
             your cache file was created with a prior release of s3backer or you do
             not specify this option, dirty blocks in the cache file are discarded
             on startup.  The window of this data loss can be limited by

             By default, when having reloaded the cache from a cache file, s3backer
             will verify the MD5 checksum of each reloaded block with Amazon S3
             before its first use.  This verify operation does not require actually
             reading the block's data, and therefore is relatively quick.  This
             guards against the cached data having unknowingly gotten out of sync
             since the cache file was last used, a situation that is otherwise
             impossible for s3backer to detect.

             Specify a limit on the number of dirty blocks in the block cache.
             When this limit is reached, subsequent write attempts will block until
             an existing dirty block is successfully written (and therefore becomes
             no longer dirty).  This flag limits the amount of inconsistency there
             can be with respect to the underlying S3 data store.

             The default value is zero, which means no limit.

             Disable the MD5 verification of blocks loaded from a cache file speci-
             fied via --blockCacheFile.  Using this flag is dangerous; use only
             when you are sure the cached file is uncorrupted and the data it con-
             tains is up to date.

             Specify the block cache size (in number of blocks).  Each entry in the
             cache will consume approximately block size plus 20 bytes.  A value of
             zero disables the block cache.  Default value is 1000.

             Set the size of the thread pool associated with the block cache (if
             enabled).  This bounds the number of simultaneous writes that can
             occur to the network.  Default value is 20.

             Specify the maximum time a clean entry can remain in the block cache
             before it will be forcibly evicted and its associated memory freed.  A
             value of zero means there is no timeout; in this case, the number of
             entries in the block cache will never decrease, eventually reaching
             the maximum size configured by --blockCacheSize and staying there.
             Configure a non-zero value if the memory usage of the block cache is a
             concern.  Default value is zero (no timeout).

             Specify the maximum time a dirty block can remain in the block cache
             before it must be written out to the network.  Blocks may be written
             sooner when there is cache pressure.  A value of zero configures a
             ``write-through'' policy; greater values configure a ``write-back''
             policy.  Larger values increase performance when a small number of
             blocks are accessed repeatedly, at the cost of greater inconsistency
             with the underlying S3 data store.  Default value is 250 milliseconds.

             Forces synchronous writes in the block cache layer.  Instead of
             returning immediately and scheduling the actual write to operation
             happen later, write requests will not return until the write has com-
             pleted.  This flag is a stricter requirement than
             --blockCacheWriteDelay=0, which merely causes the writes to be initi-
             ated as soon as possible (but still after the write request returns).

             This flag requires --blockCacheWriteDelay to be zero.  Using this flag
             is likely to drastically reduce write performance.

             An unclean dismount may leave dirty blocks (blocks written to the
             local cache file, but not yet flushed to S3) in the cache file.

             If this option is set, s3backer will recover any such dirty blocks and
             eventually write them back to S3.  If this option is not specified,
             all dirty data in the cache file are discarded on startup.

             If the filesystem has been mounted since the cache file was last used,
             s3backer will refuse to mount.  This is verified by checking a unique
             32-bit mount token in the cache file against the 'already mounted'
             flag in the data store.

             This flag requires --blockCacheFile to be set.

             Preferentially retain the first NUM blocks in the block cache.

             Some upper filesystems store highly active data (e.g., write journal)
             at the beginning of the filesystem.  This option can be used to
             improve performance by reducing network reads for these regions of the
             file.  With this option enabled, blocks after the first NUM blocks
             will be evicted before any protected blocks.

             Immediately after being read or written by the kernel, data in the
             block cache file can end up being cached twice by the kernel: once in
             the cache for the block cache file, and again in the cache for the
             s3backer file.

             When this flag is given, posix_fadvise(2) is used whenever data is
             read or written to/from the block cache file to tell the kernel not to
             cache that data on behalf of the block cache file.

             This flag is ignored if --blockCacheFile is not specified.

             Prepend random prefixes (generated deterministically from the block
             number) to block object names.  This spreads requests more evenly
             across the namespace, and prevents heavy access to a narrow range of
             blocks from all being directed to the same backend server.

             As with --prefix or subdir, this flag must be used consistently once a
             disk image is established.

             Specify the block size.  This must be a power of two and should be a
             multiple of the kernel's native page size.  The size may have an
             optional suffix 'K' for kilobytes, 'M' for megabytes, etc.  The total
             number of blocks must fit into 32 bits.

             s3backer supports partial block operations, though this forces a read
             before each write; use of the block cache and proper alignment of the
             s3backer block size with the intended use (e.g., the block size of the
             `upper' filesystem) will help minimize the extra reads.  Note that
             even when filesystems are configured for large block sizes, the kernel
             will often still write page-sized blocks.

             s3backer will attempt to auto-detect the block size by reading block
             number zero at startup.  If this option is not specified, the auto-
             detected value will be used.  If this option is specified but dis-
             agrees with the auto-detected value, s3backer will exit with an error
             unless --force is also given.  If auto-detection fails because block
             number zero does not exist, and this option is not specified, then the
             default value of 4K (4096) is used.

             Specify SSL certificate file to be used when verifying the remote
             server's identity when operating over SSL connections.  Equivalent to
             the --cacert flag documented in curl(1).

             Compress blocks using the specified compression algorithm before send-
             ing them over the network.  This should result in less network traffic
             (in both directions) and lower storage costs.

             The ALGORITHM is optional; currently, the default value is deflate.
             Also supported is zstd.

             This flag only enables compression of newly written blocks; decompres-
             sion is always enabled and applied when appropriate.  Therefore, it is
             safe to switch this flag on or off between different invocations of
             s3backer on the same filesystem.

             This flag is implied when --encrypt is used.

             When using an encrypted upper layer filesystem, this flag adds no
             value because the data will not be compressible.

             Configure the compression level for the selected compression algo-
             rithm, which must be specified via --compress.

             The interpretation of LEVEL depends on the algorithm; for deflate, it
             must be an integer between 1 (maximum speed) and 9 (maximum compres-
             sion), inclusive.

             If this flag is omitted, the default compression level for the
             selected algorithm is used.

             Insert command line flags and arguments read from the specified file
             in place of this flag.

             Leading and trailing whitespace is trimmed from each line, and blank
             lines and lines starting with # are ignored.  Each line contains
             exactly one command line argument (including internal whitespace, if

             Nested --configFile flags are allowed.

             This option may also be specified as -F FILE.

             Disable kernel caching of the backed file.  This will force the kernel
             to always pass reads and writes directly to s3backer.  This may reduce
             performance but also eliminates one source of inconsistency.

             Enable logging of debug messages.  This includes logging HTTP error
             response payloads.

             Note that this flag is different from -d, which is a flag to FUSE;
             however, the -d FUSE flag implies this flag.

             Configure libcurl(3) to print debugging information (e.g., HTTP
             request and response headers) to standard error.

             With this flag you would normally also specify -f so s3backer runs in
             the foreground and standard error is visible.

             Use this to workaround S3 backends that fail to send back the
             Content-Encoding header that was sent to them by s3backer.  If a block
             read response contains no Content-Encoding header, this value will be

             If you get errors complaining that the content was expected to be
             encrypted, try setting this to deflate,encrypt-AES-128-CBC.

             Enable encryption and authentication of block data.  See your OpenSSL
             documentation for a list of supported ciphers; the default if no
             cipher is specified is AES-128 CBC.

             Currently, only block ciphers are supported.

             The encryption password may be supplied via one of --password or
             --passwordFile.  If neither flag is given, s3backer will ask for the
             password at startup.

             Note: the actual key used is derived by hashing the password, the
             bucket name, the prefix name (if any), and the block number.  There-
             fore, encrypted data cannot be ported to different buckets or pre-

             This flag implies --compress.

             Completely erase the file system by deleting all non-zero blocks,
             clear the 'already mounted' flag, and then exit.  User confirmation is
             required unless the --force flag is also given.  Note, no simultaneous
             mount detection is performed in this case.

             This operation bypasses the caching layers, so any leftover cache file
             must be manually deleted.

             Specify the name of the backed file that appears in the s3backer
             filesystem.  Default is `file'.

             Specify the UNIX permission bits for the backed file that appears in
             the s3backer filesystem.  Default is 0600, unless --readOnly is speci-
             fied, in which case the default is 0400.

             Proceed even if the value specified by --blockSize or --size disagrees
             with the auto-detected value, or s3backer detects that another
             s3backer instance is still mounted on top of the same S3 bucket (and
             prefix).  In any of these cases, proceeding will lead to corrupted
             data, so the --force flag should be avoided for normal use.

             The simultaneous mount detection can produce a false positive when a
             previous s3backer instance was not shut down cleanly.  In this case,
             don't use --force but rather run s3backer once with the
             --reset-mounted-flag flag.

             If --erase is given, --force causes s3backer to proceed without user

     -h --help
             Print a help message and exit.

             Restrict to HTTP version 1.1.  There have been reports of errors
             and/or reduced performance with some S3-compatible backends when
             HTTP/2 is used.  This flag can be used to prevent HTTP/2 negotiation.

             Specify the initial pause time in milliseconds before the first retry
             attempt after failed HTTP operations.  Failures include network fail-
             ures and timeouts, HTTP errors, and reads of stale data (i.e., MD5
             mismatch); s3backer will make multiple retry attempts using an expo-
             nential backoff algorithm, starting with this initial retry pause
             time.  Default value is 200ms.  See also --maxRetryPause.

             Do not verify the remote server's identity when operating over SSL
             connections.  Equivalent to the --insecure flag documented in curl(1).

             Override the length of the generated block encryption key.

             Versions of s3backer prior to 1.3.6 contained a bug where the length
             of the generated encryption key was fixed but system-dependent, caus-
             ing it to be possibly incompatible on different systems for some
             ciphers.  In version 1.3.6, this bug was corrected; however, in some
             cases this changed the generated key length, making the encryption no
             longer compatible with previously written data.  This flag can be used
             to force the older, fixed key length.  The value you want to use is
             whatever is defined for EVP_MAX_KEY_LENGTH on your system, typically

             It is an error to specify a value smaller than the cipher's natural
             key length; however, a value of zero is allowed and is equivalent to
             not specifying anything.

             After starting, initiate a background query to determine which blocks
             already exist.  This enables optimizations whereby, for each block
             that does not yet exist, reads return zeroes and zeroed writes are
             omitted, thereby eliminating any network access.

             This flag is useful when creating a new backed file, or any time it is
             expected that a large number of zeroed blocks will be read or written,
             such as when initializing a new filesystem.

             In general, use of this flag is recommended, but it does create addi-
             tional network traffic during startup in proportion to the number of
             blocks that already exist.

             To minimize startup delay, the initial block enumeration of
             --listBlocks is performed in parallel using multiple threads.  This
             flag configures the number of threads used.

             Default value is 16.


             These flags set a limit on the bandwidth utilized for individual block
             uploads and downloads (i.e., the setting applies on a per-thread
             basis).  The limits only apply to HTTP payload data and do not include
             any additional overhead from HTTP or TCP headers, etc.

             The value is measured in bits per second, and abbreviations like
             `256k', `1m', etc. may be used.  By default, there is no fixed limit.

             Use of these flags may also require setting the --timeout flag to a
             higher value.

             Specify the total amount of time in milliseconds s3backer should pause
             when retrying failed HTTP operations before giving up.  Failures
             include network failures and timeouts, HTTP errors, and reads of stale
             data (i.e., MD5 mismatch); s3backer will make multiple retry attempts
             using an exponential backoff algorithm, up to this maximum total retry
             pause time.  This value does not include the time it takes to perform
             the HTTP operations themselves (use --timeout for that).  Default
             value is 30000 (30 seconds).  See also --initialRetryPause.

             Specify a minimum time in milliseconds between the successful comple-
             tion of a write and the initiation of another write to the same block.
             This delay ensures that S3 doesn't receive the writes out of order.
             This value must be set to zero when --md5CacheSize is set to zero (MD5
             cache disabled).  Default value is 500ms.

             Specify the size of the MD5 checksum cache (in number of blocks).  If
             the cache is full when a new block is written, the write will block
             until there is room.  Therefore, it is important to configure
             --md5CacheTime and --md5CacheSize according to the frequency of writes
             to the filesystem overall and to the same block repeatedly.  Alter-
             nately, a value equal to the number of blocks in the filesystem elimi-
             nates this problem but consumes the most memory when full (each entry
             in the cache is approximately 40 bytes).  A value of zero disables the
             MD5 cache.  Default value is zero (disabled).

             Specify in milliseconds the time after a block has been successfully
             written for which the MD5 checksum of the block's contents should be
             cached, for the purpose of detecting stale data during subsequent
             reads.  A value of zero means `infinite' and provides a guarantee
             against reading stale data; however, you should only do this when
             --md5CacheSize is configured to be equal to the number of blocks; oth-
             erwise deadlock will (eventually) occur.  This value must be at least
             as big as --minWriteDelay. This value must be set to zero when
             --md5CacheSize is set to zero (MD5 cache disabled).  Default value is
             10 seconds.

             The MD5 checksum cache is not persisted across restarts.  Therefore,
             to ensure the same eventual consistency protection while s3backer is
             not running, you must delay at least --md5CacheTime milliseconds
             between stopping and restarting s3backer.

     --nbd   Create a Network Block Device (NBD) instead of a FUSE filesystem.
             This is probably a more logical and direct way to do things on systems
             that support NBD.

             s3backer will start up appropriate instances of nbdkit(1) and
             nbd-client(1) in order to attach the S3 volume to the specified

             When invoked in this mode, only --doubleDash style flags are sup-
             ported, with the exception of -f and -d; any other flags (including
             any FUSE-specific flags) are disallowed, and the --nbd flag may not
             appear in a --configFile file.

             To detach the S3 volume, just kill the s3backer process and it will
             automatically disconnect the NBD client and shut down the NBD server.


             Add string as an nbdkit(1) command line parameter, either before or
             after the plugin name (respectively).

             Disable block and file size auto-detection at startup.  If this flag
             is given, then the block size defaults to 4096 and the --size flag is

             Supply the password for encryption and authentication as a command-
             line parameter.

             Read the password for encryption and authentication from (the first
             line of) the specified file.

             Specify a prefix to prepend to the resource names within bucket that
             identify each block.  By using different prefixes, multiple indepen-
             dent s3backer disks can live in the same S3 bucket.

             Instead of using this flag, it is recommended to specify a prefix
             implicitly by appending a slash and a subdirectory name to the bucket
             name (the S3 bucket namespace is flat, so the notion of a subdirectory
             is not real).  The configured prefix then becomes the given subdirec-
             tory name, followed by slash.  The AWS web console will then display
             the virtual disk's blocks grouped into a subfolder.

             For example, specifying bucket mybucket/foo/bar is equivalent to spec-
             ifying bucket mybucket along with the flag --prefix=foo/bar/ (note the
             additional trailing slash).

             Extraneous slash characters in a subdirectory name are disallowed.

             This flag (or subdir) must be used consistently once a disk image is

             The default prefix is the empty string.

             Suppress progress output during initial startup.

             Configure the number of blocks of read ahead.  This determines how
             many blocks will be read into the block cache ahead of the last block
             read by the kernel when read ahead is active.  This option has no
             effect if the block cache is disabled.  Default value is 4 in FUSE
             mode, zero in NBD mode.

             Configure the number of blocks that must be read consecutively before
             the read ahead algorithm is triggered.  Once triggered, read ahead
             will continue as long as the kernel continues reading blocks sequen-
             tially.  This option has no effect if the block cache is disabled.
             Default value is 2 in FUSE mode, zero in NBD mode.

             Assume the filesystem is going to be mounted read-only, and return
             EROFS in response to any attempt to write.  This flag also changes the
             default mode of the backed file from 0600 to 0400 and disables the MD5
             checksum cache.

             Specify an AWS region.  This flag changes the default base URL to
             include the region name and automatically sets the --vhost flag,
             unless the --no-vhost flag is used.

             The default region is us-east-1.

             Reset the 'already mounted' flag on the underlying S3 data store.

             s3backer detects simultaneous mounts by checking a special flag.  If a
             previous invocation of s3backer was not shut down cleanly, the flag
             may not have been cleared.  Running s3backer --erase will clear it
             manually.  But see also BUGS below.

             Specify the size (in bytes) of the backed file to be exported by the
             filesystem.  The size may have an optional suffix 'K' for kilobytes,
             'M' for megabytes, 'G' for gigabytes, 'T' for terabytes, 'E' for
             exabytes, 'Z' for zettabytes (if supported), or 'Y' for yottabytes (if
             supported).  The total number of blocks must fit into 32 bits.  Suf-
             fixes represent powers of two.

             s3backer will attempt to auto-detect the size of the backed file by
             reading block number zero.  If this option is not specified, the auto-
             detected value will be used.  If this option is specified but dis-
             agrees with the auto-detected value, s3backer will exit with an error
             unless --force is also given.

             Enable server side encryption.  This adds the
             x-amz-server-side-encryption header to all PUT requests.

             The value must be either AES256 or aws:kms.  In the latter case, a
             customer-supplied key is used for encryption, and the --sse-key-id
             flag is required.

             Specify the ID of the customer-supplied key for server-side encryp-
             tion.  This flag is required if --sse is given, ignored otherwise.

     --ssl   Equivalent to --baseURL

             Specify the name of the human-readable statistics file that appears in
             the s3backer filesystem.  A value of empty string disables the appear-
             ance of this file.  Default is `stats'.

             Specify storage class.

             Valid values are: STANDARD, STANDARD_IA, ONEZONE_IA,

             The default is STANDARD.

     --test  Operate in local test mode.  Filesystem blocks are stored as regular
             files in the directory dir.  No network traffic occurs.

             Note if dir is a relative pathname (and -f is not given) it will be
             resolved relative to the root directory.

             In test mode, introduce random I/O delays.

             In test mode, discard all data written, return zero for all data
             blocks read, and perform no I/O operations.  This mode is useful for
             isolating the FUSE and s3backer performance overhead.

             In test mode, introduce random I/O errors.

             Specify a time limit in seconds for one HTTP operation attempt.  This
             limits the entire operation including connection time (if not already
             connected) and data transfer time.  The default is 30 seconds; this
             value may need to be adjusted upwards to avoid premature timeouts on
             slower links and/or when using a large number of block cache worker

             See also --maxRetryPause.

             Output version and exit.

             Force virtual hosted style requests.  For example, this will cause
             s3backer to use the URL
             instead of

             This flag is required when S3 buckets have been created with location
             constraints (for example `EU buckets').  Put another way, this flag is
             required for buckets defined outside of the US region.  This flag is
             automatically set when the --region flag is used, unless the
             --no-vhost flag is used.

             Disable virtual hosted style requests (the default).

     In addition, s3backer accepts all of the generic FUSE options as well.  Here
     is a partial list:

     -o uid=UID
             Override the user ID of the backed file, which defaults to the current
             user ID.

     -o gid=GID
             Override the group ID of the backed file, which defaults to the cur-
             rent group ID.

     -o sync_read
             Do synchronous reads.

     -o max_readahead=NUM
             Set maximum read-ahead (in bytes).

     -f      Run in the foreground (do not fork).  Causes logging to be sent to
             standard error.

     -d      Enable FUSE debug mode.  Implies -f.

     -s      Run in single-threaded mode.

     In addition, s3backer passes the following flags which are optimized for
     s3backer to FUSE (unless overridden by the user on the command line):

     -o kernel_cache
     -o fsname=<baseURL><bucket>/<prefix>
     -o subtype=s3backer
     -o use_ino
     -o entry_timeout=31536000
     -o negative_timeout=31536000
     -o max_readahead=0
     -o attr_timeout=0
     -o default_permissions
     -o allow_other
     -o nodev
     -o nosuid

             Contains Amazon S3 `accessID:accessKey' pairs.
             Contains UNIX socket files used by nbd-client and nbdkit(1) when run-
             ning in --nbd mode.  For security reasons this directory is only read-
             able by root.

     curl(1), losetup(8), mount(8), nbdkit(1), umount(8), fusermount(8).

     s3backer: FUSE-based single file backing store via Amazon S3,

     Amazon Simple Storage Service (Amazon S3),

     FUSE: Filesystem in Userspace,

     MacFUSE: A User-Space File System Implementation Mechanism for Mac OS X,

     FUSE for OS X,

     Google Search for `linux page cache',

     Linux Block Layer Cache (bcache),

     Due to a design flaw in FUSE, an unmount of the s3backer filesystem will com-
     plete successfully before s3backer has finished writing back all dirty blocks.
     Therefore, when using the block cache, attempts to remount the same bucket and
     prefix may fail with an 'already mounted' error while the former s3backer
     process finishes flushing its cache.  Before assuming a false positive and
     using --reset-mounted-flag, ensure that any previous s3backer process attached
     to the same bucket and prefix has exited.  See issue #40 for details.

     For cache space efficiency, s3backer uses 32 bit values to index individual
     blocks.  Therefore, the block size must be increased beyond the default 4K
     when very large filesystems (greater than 16 terabytes) are created.

     On Mac OS X, the kernel imposes its own timeout (600 seconds) on FUSE opera-
     tions, and automatically unmounts the filesystem when this limit is reached.
     This can happen when a combination of --maxRetryPause and/or --timeout set-
     tings allow HTTP retries to take longer than this value.  A warning is emitted
     on startup in this case.

     Filesystem size is limited by the maximum allowable size of a single file.

     The default block size of 4k is non-optimal from a compression and cost per-
     spective.  Typically, users will want a larger value to maximize compression
     and minimize transaction costs, e.g., 1m.

     When running under NBD, the stats file in not available.

     Archie L. Cobbs <>

BSD                              September 7, 2009                              BSD