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Chunker Split-chunking overlay remote 2019-08-30

Chunker (BETA)

The chunker overlay transparently splits large files into smaller chunks during upload to wrapped remote and transparently assembles them back when the file is downloaded. This allows to effectively overcome size limits imposed by storage providers.

To use it, first set up the underlying remote following the configuration instructions for that remote. You can also use a local pathname instead of a remote.

First check your chosen remote is working - we'll call it remote:path here. Note that anything inside remote:path will be chunked and anything outside won't. This means that if you are using a bucket based remote (eg S3, B2, swift) then you should probably put the bucket in the remote s3:bucket.

Now configure chunker using rclone config. We will call this one overlay to separate it from the remote.

No remotes found - make a new one
n) New remote
s) Set configuration password
q) Quit config
n/s/q> n
name> overlay
Type of storage to configure.
Choose a number from below, or type in your own value
[snip]
XX / Transparently chunk/split large files
   \ "chunker"
[snip]
Storage> chunker
Remote to chunk/unchunk.
Normally should contain a ':' and a path, eg "myremote:path/to/dir",
"myremote:bucket" or maybe "myremote:" (not recommended).
Enter a string value. Press Enter for the default ("").
remote> remote:path
Files larger than chunk size will be split in chunks.
Enter a size with suffix k,M,G,T. Press Enter for the default ("2G").
chunk_size> 100M
Choose how chunker handles hash sums.
Enter a string value. Press Enter for the default ("md5").
Choose a number from below, or type in your own value
   / Chunker can pass any hash supported by wrapped remote
 1 | for a single-chunk file but returns nothing otherwise.
   \ "none"
 2 / MD5 for multi-chunk files. Requires "simplejson".
   \ "md5"
 3 / SHA1 for multi-chunk files. Requires "simplejson".
   \ "sha1"
   / Copying a file to chunker will request MD5 from the source
 4 | falling back to SHA1 if unsupported. Requires "simplejson".
   \ "md5quick"
 5 / Similar to "md5quick" but prefers SHA1 over MD5. Requires "simplejson".
   \ "sha1quick"
hash_type> md5
Edit advanced config? (y/n)
y) Yes
n) No
y/n> n
Remote config
--------------------
[overlay]
type = chunker
remote = remote:bucket
chunk_size = 100M
hash_type = md5
--------------------
y) Yes this is OK
e) Edit this remote
d) Delete this remote
y/e/d> y

Specifying the remote

In normal use, make sure the remote has a : in. If you specify the remote without a : then rclone will use a local directory of that name. So if you use a remote of /path/to/secret/files then rclone will chunk stuff in that directory. If you use a remote of name then rclone will put files in a directory called name in the current directory.

Chunking

When rclone starts a file upload, chunker checks the file size. If it doesn't exceed the configured chunk size, chunker will just pass the file to the wrapped remote. If a file is large, chunker will transparently cut data in pieces with temporary names and stream them one by one, on the fly. Each chunk will contain the specified number of data byts, except for the last one which may have less data. If file size is unknown in advance (this is called a streaming upload), chunker will internally create a temporary copy, record its size and repeat the above process. When upload completes, temporary chunk files are finally renamed. This scheme guarantees that operations look from outside as atomic. A similar method with hidden temporary chunks is used for other operations (copy/move/rename etc). If an operation fails, hidden chunks are normally destroyed, and the destination composite file stays intact.

Chunk names

By default chunk names are BIG_FILE_NAME.rclone-chunk.001, BIG_FILE_NAME.rclone-chunk.002 etc, because the default chunk name format is *.rclone-chunk.###. You can configure another name format using the --chunker-name-format option. The format uses asterisk * as a placeholder for the base file name and one or more consecutive hash characters # as a placeholder for sequential chunk number. There must be one and only one asterisk. The number of consecutive hash characters defines the minimum length of a string representing a chunk number. If decimal chunk number has less digits than the number of hashes, it is left-padded by zeros. If the number stringis longer, it is left intact. By default numbering starts from 1 but there is another option that allows user to start from 0, eg. for compatibility with legacy software.

For example, if name format is big_*-##.part and original file name is data.txt and numbering starts from 0, then the first chunk will be named big_data.txt-00.part, the 99th chunk will be big_data.txt-98.part and the 302nd chunk will become big_data.txt-301.part.

When the list rclone command scans a directory on wrapped remote, the potential chunk files are accounted for and merged into composite directory entries only if their names match the configured format. All other files are ignored, including temporary chunks. The list command might encounter composite files with missing or invalid chunks. If chunker detects a missing chunk it will by default silently ignore the whole group. You can use the --chunker-fail-on-bad-chunks command line flag to make list fail with an error message.

Metadata

By default when a file is large enough, chunker will create a metadata object besides data chunks. The object is named after the original file. Chunker allows user to disable metadata completely (the none format). Please note that currently metadata is not created for files smaller than configured chunk size. This may change in future as new formats are developed.

Simple JSON metadata format

This is the default format. It supports hash sums and chunk validation for composite files. Meta objects carry the following fields:

  • ver - version of format, currently 1
  • size - total size of composite file
  • nchunks - number of chunks in the file
  • md5 - MD5 hashsum of composite file (if present)
  • sha1 - SHA1 hashsum (if present)

There is no field for composite file name as it's simply equal to the name of meta object on the wrapped remote. Please refer to respective sections for detils on hashsums and handling of modified time.

No metadata

You can disable meta objects by setting the meta format option to none. In this mode chunker will scan directory for all files that follow configured chunk name format, group them by detecting chunks with the same base name and show group names as virtual composite files. When a download is requested, chunker will transparently assemble compound files by merging chunks in order. This method is more prone to missing chunk errors (especially missing last chunk) than metadata-enabled formats.

Hashsums

Chunker supports hashsums only when a compatible metadata is present. Thus, if you choose metadata format of none, chunker will return UNSUPPORTED as hashsum.

Please note that metadata is stored only for composite files. If a file is small (smaller than configured chunk size), chunker will transparently redirect hash requests to wrapped remote, so support depends on that. You will see the empty string as a hashsum of requested type for small files if the wrapped remote doesn't support it.

Many storage backends support MD5 and SHA1 hash types, so does chunker. Currently you can choose one or another but not both. MD5 is set by default as the most supported type. Since chunker keeps hashes for composite files and falls back to the wrapped remote hash for small ones, we advise you to choose the same hash type as wrapped remote so that your file listings look coherent.

Normally, when a file is copied to a chunker controlled remote, chunker will ask the file source for compatible file hash and revert to on-the-fly calculation if none is found. This involves some CPU overhead but provides a guarantee that given hashsum is available. Also, chunker will reject a server-side copy or move operation if source and destination hashsum types are different, resulting in the extra network bandwidth, too. In some rare cases this may be undesired, so chunker provides two optional choices: sha1quick and md5quick. If the source does not support primary hash type and the quick mode is enabled, chunker will try to fall back to the secondary type. This will save CPU and bandwidth but can result in empty hashsums at destination. Beware of consequences: the sync command will revert (sometimes silently) to time/size comparison if compatible hashsums between source and target are not found.

Modified time

Chunker stores modification times using the wrapped remote so support depends on that. For a small non-chunked file the chunker overlay simply manipulates modification time of the wrapped remote file. If file is large and metadata is present, then chunker will get and set modification time of the metadata object on the wrapped remote. If file is chunked but metadata format is none then chunker will use modification time of the first chunk.

Migrations

The idiomatic way to migrate to a different chunk size, hash type or chunk naming scheme is to:

  • Collect all your chunked files under a directory and have your chunker remote point to it.
  • Create another directory (possibly on the same cloud storage) and configure a new remote with desired metadata format, hash type, chunk naming etc.
  • Now run rclone sync oldchunks: newchunks: and all your data will be transparently converted at transfer. This may take some time, yet chunker will try server-side copy if possible.
  • After checking data integrity you may remove configuration section of the old remote.

If rclone gets killed during a long operation on a big composite file, hidden temporary chunks may stay in the directory. They will not be shown by the list command but will eat up your account quota. Please note that the deletefile rclone command deletes only active chunks of a file. As a workaround, you can use remote of the wrapped file system to see them. An easy way to get rid of hidden garbage is to copy littered directory somewhere using the chunker remote and purge original directory. The copy command will copy only active chunks while the purge will remove everything including garbage.

Caveats and Limitations

Chunker requires wrapped remote to support server side move (or copy + delete) operations, otherwise it will explicitly refuse to start. This is because it internally renames temporary chunk files to their final names when an operation completes successfully.

Note that a move implemented using the copy-and-delete method may incur double charging with some cloud storage providers.

Chunker will not automatically rename existing chunks when you run rclone config on a live remote and change the chunk name format. Beware that in result of this some files which have been treated as chunks before the change can pop up in directory listings as normal files and vice versa. The same warning holds for the chunk size. If you desperately need to change critical chunking setings, you should run data migration as described in a dedicated section.

If wrapped remote is case insensitive, the chunker overlay will inherit that property (so you can't have a file called "Hello.doc" and "hello.doc" in the same directory).

Standard Options

Here are the standard options specific to chunker.

--chunker-remote

Remote to chunk/unchunk. Normally should contain a ':' and a path, eg "myremote:path/to/dir", "myremote:bucket" or maybe "myremote:" (not recommended).

  • Config: remote
  • Env Var: RCLONE_CHUNKER_REMOTE
  • Type: string
  • Default: ""

--chunker-chunk-size

Files larger than chunk size will be split in chunks.

  • Config: chunk_size
  • Env Var: RCLONE_CHUNKER_CHUNK_SIZE
  • Type: SizeSuffix
  • Default: 2G

--chunker-hash-type

Choose how chunker handles hash sums.

  • Config: hash_type
  • Env Var: RCLONE_CHUNKER_HASH_TYPE
  • Type: string
  • Default: "md5"
  • Examples:
    • "none"
      • Chunker can pass any hash supported by wrapped remote
      • for a single-chunk file but returns nothing otherwise.
    • "md5"
      • MD5 for multi-chunk files. Requires "simplejson".
    • "sha1"
      • SHA1 for multi-chunk files. Requires "simplejson".
    • "md5quick"
      • Copying a file to chunker will request MD5 from the source
      • falling back to SHA1 if unsupported. Requires "simplejson".
    • "sha1quick"
      • Similar to "md5quick" but prefers SHA1 over MD5. Requires "simplejson".

Advanced Options

Here are the advanced options specific to chunker (Transparently chunk/split large files).

--chunker-name-format

String format of chunk file names. The two placeholders are: base file name (*) and chunk number (#...). There must be one and only one asterisk and one or more consecutive hash characters. If chunk number has less digits than the number of hashes, it is left-padded by zeros. If there are more digits in the number, they are left as is. Possible chunk files are ignored if their name does not match given format.

  • Config: name_format
  • Env Var: RCLONE_CHUNKER_NAME_FORMAT
  • Type: string
  • Default: "*.rclone_chunk.###"

--chunker-start-from

Minimum valid chunk number. Usually 0 or 1. By default chunk numbers start from 1.

  • Config: start_from
  • Env Var: RCLONE_CHUNKER_START_FROM
  • Type: int
  • Default: 1

--chunker-meta-format

Format of the metadata object or "none". By default "simplejson". Metadata is a small JSON file named after the composite file.

  • Config: meta_format
  • Env Var: RCLONE_CHUNKER_META_FORMAT
  • Type: string
  • Default: "simplejson"
  • Examples:
    • "none"
      • Do not use metadata files at all. Requires hash type "none".
    • "simplejson"
      • Simple JSON supports hash sums and chunk validation.
      • It has the following fields: size, nchunks, md5, sha1.

--chunker-fail-on-bad-chunks

The list command might encounter files with missinng or invalid chunks. This boolean flag tells what rclone should do in such cases.

  • Config: fail_on_bad_chunks
  • Env Var: RCLONE_CHUNKER_FAIL_ON_BAD_CHUNKS
  • Type: bool
  • Default: false
  • Examples:
    • "true"
      • Fail with error.
    • "false"
      • Silently ignore invalid object.