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authorLars Wirzenius <>2022-08-14 11:12:54 +0300
committerLars Wirzenius <>2022-08-14 11:12:54 +0300
commitbb5be8fdc3b18a295a4b014b53993335dcb86005 (patch)
parentcb9d343780c8cfb6a48a48ab8189a71e989256cc (diff)
docs: add thoughts about chunk splitting approached to
Sponsored-by: author
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@@ -587,6 +587,153 @@ using configuration management, and if necessary, backups can be
triggered on each host by having the server reach out and run the
Obnam client.
+## On splitting file data into chunks
+A backup program needs to split the data it backs up into chunks. This
+can be done in various ways.
+### A complete file as a single chunk
+This is a very simple approach, where the whole file is considered to
+be a chunk, regardless of its size.
+Using complete files is often impractical, since they need to be
+stored and transferred as a unit. If a file is enormous, transferring
+it completely can be a challenge: if there's a one-bit error in the
+transfer, the whole thing needs to be transferred again.
+There is no de-duplication except possibly of entire files.
+### Fixed size chunks
+Split a file into chunks of a fixed size. For example, if the chunk
+size is 1 MiB, a 1 GiB file is 1024 chunks. All chunks are of the same
+size, unless a file size is not a multiple of the chunk size.
+Fixed size chunks are very easy to implement and make de-duplication
+of partial files possible. However, that de-duplication tends to only
+work for the beginnings of file: inserting data in the file tends to
+result in chunks after the insertion not matching anymore.
+### Splitting based on a formula using content
+A rolling checksum function is computed on a sliding window of bytes
+from the input file. The window has a fixed size. The function is
+extremely efficient to compute when bytes are moved into or out of the
+window. When the value of the function, the checksum, matches a
+certain bit pattern, it is considered a chunk boundary. Such a pattern
+might for example be that the lowest N bits are zero. Any data that
+is pushed out of the sliding window also forms a chunk.
+The code to split into chunks may set minimum and maximum sizes of
+chunks, whether from checksum patterns or overflowed bytes. This
+prevents pathological input data, where the checksum has the boundary
+bit pattern after every byte, to not result in each input byte being
+its own chunk.
+This finds chunks efficiently even new data is inserted into the input
+data, with some caveats.
+Example: assume a sliding window of four bytes, and a 17-byte input
+file where there are four copies of the same 4-byte sequence with a
+random byte in between.
+|data | a| b| c| d| a| b| c| d|ff| a| b| c| d| a| b| c| d|
+Bytes 1-4 are the same as bytes 5-8, 10-13, and 14-17. When we compute
+the checksum for each input byte, we get the results below, for a
+moving sum function.
+| offset | byte | checksum | chunk boundary? |
+| 0 | a | a | no |
+| 1 | b | a+b | no |
+| 2 | c | a+b+c | no |
+| 3 | d | a+b+c+ | yes |
+| 4 | a | a | no |
+| 5 | b | a+b | no |
+| 6 | c | a+b+c | no |
+| 7 | d | a+b+c+ | yes |
+| 9 | ff | ff | no |
+| 10 | a | ff+a | no |
+| 11 | b | ff+a+b | no |
+| 12 | c | ff+a+b+c | no |
+| 13 | d | a+b+c+d | yes |
+| 14 | a | a | no |
+| 15 | b | a+b | no |
+| 16 | c | a+b+c | no |
+| 17 | d | a+b+c+ | yes |
+Note that in this example, the byte at offset 9 (0xff) slides out of
+the window then byte at offset 13 slides in, and in results in bytes
+at offsets 10-13 being recognized as a chunk by the checksum function.
+This example is carefully constructed for that happy co-incidence. In
+a more realistic scenario, the data after the inserted bytes might not
+notice a chunk boundary until after the sliding window has been filled
+By choosing a suitable window size and checksum value pattern for
+chunk boundaries, the chunk splitting can find smaller or large
+chunks, balancing the possibility for more detailed de-duplication
+versus the overhead of storing many chunks.
+### Varying splitting method based on file type
+The data may contain files of different types, and this can be used to
+vary the way data is split into chunks. For example, compressed video
+files may use one way of chunking, while software source code may use
+For example:
+* emails in mbox or Maildir formats could be split based on headers,
+ body, and attachment and each of those into chunks
+* SQL dumps of databases tend to contain very large numbers containing
+ the same structure
+* video files are often split into frames, possibly those can be used
+ for intelligent chunking?
+* uncompressed tar files have a definite structure (header, followed
+ by file data) that can probably be used for splitting into chunks
+* ZIP files compress each file separately, which could be used to
+ split them into chunks: this way, two ZIP files with the same file
+ inside them might share the compressed file as a chunk
+* disk images often contain long sequences of zeroes, which could be
+ used for splitting into chunks
+### Next actions
+Obnam currently splits data using fixed size chunks. This can and will
+be improved, and the changes will only affect the client. Help is
+### Thanks
+Thank you to Daniel Silverstone who explained some of the mathematics
+about this to me.
# File metadata
Files in a file system contain data and have metadata: data about the