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https://github.com/archiecobbs/s3backer

FUSE/NBD single file backing store via Amazon S3
https://github.com/archiecobbs/s3backer

disk nbd s3

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FUSE/NBD single file backing store via Amazon S3

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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.



------------------------------------------------------------------------



Home page: https://github.com/archiecobbs/s3backer



------------------------------------------------------------------------



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!