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

Overview
  s3backer is a filesystem that contains a single file backed by the Amazon
  Simple Storage Service (Amazon S3).  As a filesystem, it is very simple:
  it provides 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 implementations:

  o   By not attempting to implement a complete filesystem, which is a com-
      plex undertaking and difficult to get right, s3backer can stay very
      lightweight and simple. Only three HTTP operations are used: GET,
      PUT, and DELETE.  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
      consistency'' is more easily and simply solved when one can interpret
      S3 objects as simple device blocks rather than filesystem objects
      (see below).

  o   When storing your data on Amazon S3 servers, which are not under your
      control, the ability to encrypt and authenticate data becomes a crit-
      ical issue.  s3backer supports secure encryption and authentication.
      Alternately, 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 filesys-
      tem 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
  Amazon S3 makes 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
      operations on the same block.  This ensures that Amazon S3 doesn't
      receive 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 operations.

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

Zeroed Block Optimization
  As a simple optimization, s3backer does not store blocks containing all
  zeroes; instead, they are simply deleted.  Conversely, reads of non-exis-
  tent 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.

  When the --listBlocks flag is given, s3backer will list all existing
  blocks at startup so it knows ahead of time exactly which blocks are
  empty.

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-inter-
  pretation of an existing filesystem.

Block Cache
  s3backer includes support for an internal block cache to increase perfor-
  mance.  The block cache cache is completely separate from the MD5 cache
  which only stores MD5 checksums transiently and whose sole purpose is to
  mitigate ``eventual 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 veri-
  fied before reuse.

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

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 read-
  ing 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 taking 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 com-
  mand line options.

Encryption and Authentication
  s3backer supports encryption via the --encrypt, --password, and
  --passwordFile flags.  When encryption is enabled, SHA1 HMAC authentica-
  tion is also automatically 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 compression.

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 cre-
  ation of public, read-only filesystems.

Simultaneous Mounts
  Although it functions over the network, the s3backer filesystem is not a
  distributed filesystem and does not support simultaneous read/write
  mounts.  (This is not something you would normally do with a hard-disk
  partition either.)  s3backer does not detect this situation; it is up to
  the user to ensure that it doesn't happen.

Statistics File
  s3backer populates the filesystem with a human-readable statistics file.
  See --statsFilename below.

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

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Home page: http://s3backer.googlecode.com/

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See INSTALL for installation instructions. After installing, see the s3backer(1)
man page for how to run it.

See COPYING for license.

See CHANGES for change history.

Enjoy!

$Id$