A file system on Amazon S3, with write-back caching of data and a persistent metadata cache
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s3fslite is a fork of s3fs, originally written by Randy Rizun. It is a file system that stores all data in an Amazon S3 bucket. It allows access to a bucket as though it were a local file system. It is useful for publishing static web data that can be read easily by a browser, or for backing up private or shared data.

This fork is intended to work better when using rsync to copy data to an S3 mount.

Quick start

Start by installing the dependencies. In Ubuntu Linux, the following commands should do the trick:

sudo apt-get install build-essential pkg-config libxml2-dev
sudo apt-get install libcurl4-openssl-dev libsqlite3-dev
sudo apt-get install libfuse2 libfuse-dev fuse-utils

Next, download the latest source:

git clone git://github.com/russross/s3fslite.git

Go into the source directory and build it:

cd s3fslite

If there are no errors, then you are ready to install the binary:

sudo make install

This copies the executable into /usr/bin where it is ready to use.

I suggest also creating a directory to hold the attribute cache databases:

sudo mkdir -p /var/cache/s3fs

It is also convenient to put your Amazon credentials in a file. I use vim, so the command would be:

sudo vim /etc/passwd-s3fs

Substitute the name of your favorite editor (gedit is an easy choice if you do not know what else to use).

Inside this file, put your access key and your secret access key (log in to your Amazon S3 account to obtain these) in this format:


To protect your secret key, make the file only accessible by root:

sudo chmod 600 /etc/passwd-s3fs

Mounting a file system

You need a mount point for your file systems. This is just an empty directory that acts as a place to mount the file system:

sudo mkdir /mnt/myfilesystem

You only need to create this once. Put this directory where <mountpoint> is specified below.

Starting with an empty bucket (or one that you have used with other versions of s3fs already), mount it like this:

sudo s3fs <bucket> <mountpoint> -o attr_cache=/var/cache/s3fs -o allow_other

This mounts the file system with the attribute cache database in /var/cache/s3fs and allows all users of the local machine to use the mount.

You should now be able to use it like a normal file system, subject to some limitations discussed below.

To unmount it, make sure no terminal windows are open inside the file system, no applications have files in it open, etc., then issue:

sudo umount <mountpoint>

To simplify mounting in the future, add a line to /etc/fstab. Substituting your editor of choice for vim, do:

sudo vim /etc/fstab

and add a line of the form:

s3fs#<bucket> <mountpoint> fuse attr_cache=/var/cache/s3fs,allow_other 0 0

With that in place, you can mount it using:

sudo mount <mountpoint>

and unmount it using:

sudo umount <mountpoint>

This will also cause it to automatically mount at boot time.

Attribute cache

If the attribute cache ever gets out of sync, simply delete the database file. This is /var/cache/s3fs/<bucketname>.sqlite if you set things up as recommended. If you are accessing a single bucket from multiple machines, you must manage the cache yourself. You can either delete the file each time you switch machines, or you can copy it over. If you do the latter, you should unmount the bucket before copying the file, and before copying it into its new location. You should only have a bucket mounted from one place at a time.

The database file compresses very nicely; compressing it and copying it to another location (then decompressing it) is a viable solution.

When starting from a cold cache, you can just start using the system and it will gradually build the cache up. If you are using it interactively, it will be really slow at first, so I recommend priming the cache first. Just do:

find /mnt/myfilesystem

and go do something else while it runs. This will scan the entire mount and load the attributes for every file into the cache. From that point forward, using it interactively should be much more pleasant.

Using rsync

When using rsync to upload data, I recommend using the -a option (to sync file times, do recursive uploads, etc.) and the -W option. -W instructs it to always copy whole files. Without it, rsync will download the old version of a file and try to be clever about updating it. Since this all happens in the local cache, you do not save much, but you do incur the cost of downloading it. When it transfers a whole file, it just deletes the old version.

For example, I typically set it up so that the directory I want to upload has the same name as the mount point, say myname. If the source is ~/myname and the mount point is /mnt/myname, then I use a command like this:

rsync -avW --delete ~/myname /mnt/

The --delete option tells it to delete files in the target that are not in the source, so be careful with this option. An alternative is this:

rsync -avW --delete ~/myname/ /mnt/myname/

Beware that this means something slightly different. This syncs all of the files in /myname/, but does not sync the directory itself. As a result, files missing from ~/myname/ will not be deleted from /mnt/myname/.

S3's "eventually consistent" semantics can lead to some weird behavior. rsync will sometimes report a file vanishing and other problems. Wait 30 seconds or so and try again, and the problem will usually fix itself. As an example, sometimes when you delete a file it still shows up in directory listings, but reports a "File not Found" error when you try to access it. This also happens frequently when you do a rm -r over a medium to large directory structure. It correctly deletes the files, then fails to delete the directory. This is because S3 is still reporting the odd file as existing in a directory listing, but it has already been deleted. Wait for a few tens of seconds and it should fix itself.

rsync is nice because you can just repeat the command and it will only worry about the things that did not work the first time. With the attribute cache, only a few requests are necessary (to look up directory listings), so repeated rsync operations are pretty cheap.


The complete list of supported options is:

  • accessKeyId= specify the Amazon AWS access key (no default)

  • secretAccessKey= specify the Amazon AWS secret access key (no default)

  • acl= specify the access control level for files (default public-read for files with public read permissions, private for everything else).

  • retries= specify the maximum number of times a failed/timed out request should be retried in addition to the initial attempt (default 2)

  • connect_timeout= specify the timeout interval for request connections (default 2)

  • readwrite_timeout= specify the timeout interval for read and write operations (default 10)

  • url= specify the host to connect to (default http://%s.s3.amazonaws.com). If you want to use HTTPS instead of HTTP to get secure transfers, specify url=https://%s.s3.amazonaws.com as a mount option.

    The host URL should contain the bucket name as in a virtual host-style URL, or put %s in the host string and the bucket name will be substituted in for you.

  • attr_cache= specify the directory where the attribute cache database should be created and accessed (default current directory)

  • dir_cache= enable/disable directory caching. With this enabled, all metadata queries will be confined to the local cache if the file system believes it has up-to-date entries for every file in the directory. When creating a new file or trying to open a file that does not exist, this saves a round trip to the server. To decide if a directory is completely represented, it checks each time a readdir operation is invoked to see if every file the server names has a metadata cache entry. Future readdir operations are also satisfied by the cache. (default true).

  • dir_cache_reset= force the list of completely cached directories (see dir_cache= above) to be reset at file system mount time (default true).

  • writeback_cache= specify the directory where the write-back cache temporary files should be created (default /tmp). Files are unlinked as soon as they are created, so you will not generally see anything listed in the given directory, but the storage of that file system will still be used.

  • writeback_delay= specify the number of seconds a closed file should be cached before changes are uploaded to S3 (default 5).

Changes from s3fs

This fork has the following changes:

  • S3fslite has a write-back cache that holds open files and files that were closed within the last few seconds. This absorbs many of the requests that otherwise take a round-trip each to S3. For example, when rsync creates a file, it creates it with a temporary name, writes the data, sets the mode, sets the owner, sets the times, and then moves it over to its permanent name. Without the write-back cache, each of these operations requires a round trip to the server. With it, everything happens locally until the final version is uploaded with all of its metadata.

    To force a sync, do an ls in the directory of interest. The readdir call does a sync on every file in the directory before retrieving the directory listing.

  • File metadata is cached in a SQLite database for faster access. File systems do lots of getattr calls, and each one normally requires a HEAD request to S3. Caching them locally improves performance a lot and reduces the number (and hence cost) of requests to Amazon.

    The original s3fs has the beginnings of in-memory stat caching, but it does not persist across mounts. For large file systems, losing the entire cache on a restart is costly.

  • Directories can be renamed. This requires renaming all of the directory's children, grandchildren, etc., so it can be a slow operation, but it works. Files are all copied at the server, not by downloading them and re-uploading them, the same as for metadata updates, regular renames, and links.

    Directories with open files (this includes any descendents) cannot be renamed. Open files cannot be renamed either.

  • readdir requests do not send off file attribute requests. The original code effectively issues a getattr request to S3 for each file when directories are listed. The cache was not consulted, but the results were put in the cache.

    This behavior made listing directories ridiculously slow. It appears to have been an attempt to optimize (by priming the cache) that backfired. It wouldn't be the first time that a cache optimization has made things slower overall.

  • The MIME type of files is reset when files are renamed. This fixes a bug in s3fs that is particularly devastating for rsync users. rsync always writes to a temporary file, then renames it to the target name. Without this fix, MIME types were rarely correct, which confused browsers when looking at static content on an S3 archive.

  • By default, ACLs are set based on the file permission. If the file is publicly readable, the "public-read" ACL is used, which permits anyone to read the file (including web browsers). If not, it defaults to "private", which denies access to public browsers. Setting the default_acl option overrides this, and sets everything to the specified ACL.

  • MD5 sums are computed for all uploads and downloads. S3 provides MD5 hash values on downloads, and verifies them on the received data for uploads, ensuring that no data is corrupted in transit.

  • The use_cache option has been removed. An on-disk cache is not currently supported, except for the short-term write-back cache. For AFS-style caching (which is more-or-less what s3fs uses), a seperate caching layer would be more appropriate.


In order to compile s3fslite, you will need the following libraries:

  • Kernel-devel packages (or kernel source) installed that is the SAME version of your running kernel

  • LibXML2-devel packages

  • CURL-devel packages (or compile curl from sources at: curl.haxx.se/ use 7.15.X)

  • GCC, GCC-C++

  • pkgconfig

  • FUSE (2.7.x)

  • FUSE Kernel module installed and running (RHEL 4.x/CentOS 4.x users read below)

  • OpenSSL-devel (0.9.8)

  • SQLite 3

These packages may have additional dependencies. For Ubuntu users, the commands to install everything you need are given in the quick start guide. For other users, use your packaging system to install the necessary dependencies. Most compiler errors are due to missing libraries.


s3fslite logs error and status messages to /var/log/syslog. To make it display more messages, you can enable some debug flags:

  • DEBUG logs each VFS call that is made, e.g., getattr, readdir, open, read, write, etc.

  • DEBUG_WIRE logs each time it contacts S3. This can be useful for seeing how well the cache is working.

  • DEBUG_CACHE logs information about the write-back cache. This is fairly chatty output.

All of these messages go to /var/log/syslog, so open a terminal and run:

tail -f /var/log/syslog

To enable these flags, add the following to the CPPFLAGS line in the Makefile:


Then do a make clean and another make and make install to rebuild it with the caching options.

The write-back cache

When a file is opened, it is transferred from S3 to a local file. This is created in /tmp and immediately unlinked, so it is not visible in the file system and will automatically be deleted when closed (or if the program crashes).

All read/write operations take place on the cached copy of the file. It is held in the cache and not synchronized with the server (except in some special cases discussed below) until the file is closed and has not been touched for 5 seconds. Metadata updates reset the clock, so chmod and chown operations keep the item cached as well.

A file is normally only flushed to the server when it is closed and has been idle for 5 seconds. This covers file renames and deletes as well. This is designed for rsync, which writes the data to a temporary file, then sets its mode, ownership, and times, then renames it to its final name. All of this happens in the cache, and the final version of the file complete with metadata is pushed to the server in one transfer.

readdir operations are never cached, so they have the potential to pierce the abstraction and observe unsynced operations. To prevent this, all files in the cache are synced before a readdir operation. This is the simplest way to force a sync and make sure all of the data has been written out: just run ls in any directory inside the mounted file system. rmdir operations also force a sync for the same reason; non-empty directories cannot be removed, so a sync is performed first to make sure any recent deletes have been pushed to the server.

All of this means that operations (like rsync) will often be very fast for the first 5 seconds or so, and then suddenly slow down. The whole server uses a big ol' lock for synchronization, so only a single operation can be happening at once. When the thread that flushes the cache obtains the lock, it holds on to it until the cache has been cleared (to the 5 second limit). This means that while it is catching up, nothing else happens, preventing the backlog from getting too long.

As a result, the server is usually within about 10 or 20 seconds of being current with the cache. A sync is also forced when the file system shuts down normally.

To observe how all of this works, enable all of the debugging logs (see the "Debugging" section).

Known Issues:

s3fslite works fine with S3 storage. However, There are couple of limitations:

  • File permissions are not enforced. Files are always created as the user who mounts the file system (normally root), but anyone can change anything.

  • Hard links are faked. They are implemented by doing a simple (server-side) copy. This is great for most cases (notably when using hard links as a way to move a file to another directory), but it is not the same as a real hard link. If a file is open when it is linked, the two versions actually do share storage (and updates), but only until one of them is flushed from the cache. I do not recommend relying on this behavior.

  • This note comes from the original s3fs: CentOS 4.x/RHEL 4.x users: if you use the kernel that shipped with your distribution and didn't upgrade to the latest kernel RedHat/CentOS gives, you might have a problem loading the "fuse" kernel. Please upgrade to the latest kernel (2.6.16 or above) and make sure "fuse" kernel module is compiled and loadable since FUSE requires this kernel module and s3fs requires it as well.

  • S3fslite is mainly intended for publishing data to S3. It does not provide general local caching, nor services like encryption or compression. Some of these issues can be addressed with existing systems:

    • Encryption: if you want file-by-file encryption (as opposed to encrypting an entire block device), you can plug in EncFS with s3fslite. This acts as a layer on top of any other file system and provides encryption services.

    • Caching: Systems like FS-Cache and CacheFS promise to do the same thing for caching. You mount s3fslite, then you mount another layer on top of it that provides caching.

    • Compression: FuseCompress works on the same basic model as the others, compressing data for a file system that does not have direct support for compression.

    I have not tried these solutions (I just googled for them), and would welcome reports about whether or not (or how well) they work.

Source code tour

There are six main source files:

  1. common.cpp: utility functions and global variables.

  2. fileinfo.cpp: a simple class to hold file attributes.

  3. attrcache.cpp: the SQLite attribute caching. This cache is intended to reflect the current state of the server, and it knows nothing about the write-back cache.

  4. s3request.cpp: wire requests to Amazon S3 using libcurl. s3request is ignorant of any caching, and is purely concerned with forming requests and gathering responses.

  5. filecache.cpp: the write-back cache. This cache draws from and updates the attribute cache when necessary, and issues S3 requests when needed. In that sense, it sits right below the main file system operations layer.

  6. s3fs.cpp: the FUSE file system operations, along with startup and shutdown code. This code depends on and knows about everything else.


s3fslite retains the original GPL v2 license that s3fs uses. See the file COPYING for details.