Ubuntu Manpage: s3backer — FUSE-based single file backing store via Amazon S3

Provided by: s3backer_1.5.4-2build1_amd64

NAME

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

SYNOPSIS

     s3backer [options] bucket /mount/point

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

     s3backer --erase [options] bucket

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

DESCRIPTION

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 complex 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 critical 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 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
Amazon S3 makes relatively weak guarantees relating to the timing and consistency of reads
vs. writes (collectively known as ``eventual consistency''). 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-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.

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-interpretation of an existing
filesystem.

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 ``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 verified via MD5 checksum with Amazon S3 before reuse.

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

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 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 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 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.

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

Compression is configured via the --compress flag. Compression 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 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.) 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 statistics can be
reset to zero by attempting to remove the file.

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

OPTIONS

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 specified directly in /etc/fstab and passed
seamlessly to s3backer by FUSE.

--accessFile=FILE
Specify a file containing `accessID:accessKey' pairs, one per-line. Blank lines and
lines beginning with a `#' are ignored. If no --accessKey is specified, this file
will be searched for the entry matching the access ID specified via --accessId; if
neither --accessKey nor --accessId is specified, the first entry in this file will
be used. Default value is $HOME/.s3backer_passwd.

--accessId=ID
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 publicly available filesystems will work.

--accessKey=KEY
Specify Amazon S3 access key. To avoid publicizing this secret via the command line,
use --accessFile instead of this flag.

--accessType=TYPE
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'.

--accessEC2IAM=ROLE
Download access credentials and security token in JSON document form from
http://169.254.169.254/latest/meta-data/iam/security-credentials/ROLE 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.

--authVersion=TYPE
Specify how to authenticate requests. There are two supported authentication
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.

--baseURL=URL
Specify the base URL, which must end in a forward slash. Default is
`http://s3.amazonaws.com/'.

--blockCacheFile=FILE
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 guarantee 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 possible.

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 --blockCacheWriteDelay.

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.

--blockCacheMaxDirty=NUM
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.

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

--blockCacheSize=SIZE
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.

--blockCacheThreads=NUM
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.

--blockCacheTimeout=MILLIS
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).

--blockCacheWriteDelay=MILLIS
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.

--blockCacheSync
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 completed. This flag is a stricter
requirement than --blockCacheWriteDelay=0, which merely causes the writes to be
initiated 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.

--blockCacheRecoverDirtyBlocks
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.

--blockHashPrefix
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, this flag must be used consistently once a disk image is
established.

--blockSize=SIZE
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.

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 disagrees 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.

--cacert=FILE
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[=LEVEL]
Compress blocks before sending them over the network. This should result in less
network traffic (in both directions) and lower storage costs.

The compression level is optional; if given, it must be between 1 (fast compression)
and 9 (most compression), inclusive. If omitted, the default compression level is
used.

This flag only enables compression of newly written blocks; decompression 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 automatically enabled when --encrypt is used, though you may also
specify --compress=LEVEL to set a non-default compression level.

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

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

--debug
Enable logging of debug messages. Note that this flag is different from -d, which
is a flag to FUSE; however, the -d FUSE flag implies this flag.

--debug-http
Enable printing of HTTP headers to standard output.

--defaultContentEncoding=VALUE
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 substituted.

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

--encrypt[=CIPHER]
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.

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. Therefore, encrypted data cannot be
ported to different buckets or prefixes.

This flag implies --compress.

--erase
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 option implies --listBlocks.

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

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

--force
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 confirmation.

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

--initialRetryPause=MILLIS
Specify the initial pause time in milliseconds before the first retry attempt after
failed HTTP operations. 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, starting with this initial
retry pause time. Default value is 200ms. See also --maxRetryPause.

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

--keyLength
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, causing 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 64.

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.

--listBlocks
Perform a query at startup 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.

This flag will slow down startup in direct proportion to the number of blocks that
already exist.

--maxUploadSpeed=BITSPERSEC

--maxDownloadSpeed=BITSPERSEC
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.

--maxRetryPause=MILLIS
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.

--minWriteDelay=MILLIS
Specify a minimum time in milliseconds between the successful completion 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.

--md5CacheSize=SIZE
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. Alternately, a value equal to the number of blocks in the filesystem
eliminates 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 1000.

--md5CacheTime=MILLIS
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; otherwise
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.

--noAutoDetect
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 required.

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

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

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

The default prefix is the empty string.

--quiet
Suppress progress output during initial startup.

--readAhead=NUM
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.

--readAheadTrigger=NUM
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 sequentially. This option has no effect if the
block cache is disabled. Default value is 2.

--readOnly
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.

--region=REGION
Specify an AWS region. This flag changes the default base URL to include the region
name and automatically sets the --vhost flag.

--reset-mounted-flag
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.

--rrs Deprecated; equivalent to --storageClass=REDUCED_REDUNDANCY.

--size=SIZE
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, or 'Y' for
yottabytes. 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 disagrees with the auto-detected
value, s3backer will exit with an error unless --force is also given.

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

Currently the only supported encryption algorithm is AES256.

--ssl Equivalent to --baseURL https://s3.amazonaws.com/

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

--storageClass=TYPE
Specify storage class.

Valid values are: STANDARD, STANDARD_IA, and REDUCED_REDUNDANCY.

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.

--timeout=SECONDS
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 threads.

FILES

     $HOME/.s3backer_passwd
             Contains Amazon S3 `accessID:accessKey' pairs.

BUGS

Due to a design flaw in FUSE, an unmount of the s3backer filesystem will complete
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.

s3backer should really be implemented as a device rather than a filesystem. However, this
would require writing a kernel module instead of a simple user-space daemon, because Linux
does not provide a user-space API for devices like it does for filesystems with FUSE.
Implementing s3backer as a filesystem and then using the loopback mount is a simple
workaround.

On Mac OS X, the kernel imposes its own timeout (600 seconds) on FUSE operations, and
automatically unmounts the filesystem when this limit is reached. This can happen when a
combination of --maxRetryPause and/or --timeout settings 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 perspective.
Typically, users will want a larger value to maximize compression and minimize transaction
costs, e.g., 1m.

AUTHOR

     Archie L. Cobbs <archie@dellroad.org>