Brackup::DigestCache - cache digests of file and chunk contents
The brackup DigestCache caches the digests (currently SHA1) of files and file chunks, to prevent untouched files from needing to be re-read on subsequent, iterative backups.
The digest cache is purely a cache. It has no critical data in it, so if you lose it, subsequent backups will just take longer while the digest cache is re-built.
Note that you don't need the digest cache to do a restore.
The digest cache makes use of Dictionary modules (Brackup::Dict::*) to handle the storage of the cache. The default dictionary used is Brackup::Dict::SQLite, which stores the cache as an SQLite database in a single file. The schema is created automatically as needed... no database maintenance is required.
The dictionary type can be specified in the [SOURCE] declaration in your brackup.conf file, using the 'digestdb_type' property e.g.:
[SOURCE:home] path = /home/bradfitz/ # specify the lighter/slower Brackup::Dict::SQLite2 instead of the default digestdb_type = SQLite2
The cache database file is stored in either the location specified in a Brackup::Root's [SOURCE] declaration in ~/.brackup.conf, as:
[SOURCE:home] path = /home/bradfitz/ # be explicit if you want: digestdb_file = /var/cache/brackup-brad/digest-cache-bradhome.db
Or, more commonly (and recommended), is to not specify it and accept the default location, which is ".brackup-digest.db" in the root's root directory.
[SOURCE:home] path = /home/bradfitz/ # this is the default: # digestdb_file = /home/bradfitz/.brackup-digest.db
Files digests keys (see Brackup::File)
Chunk digests keys (see Brackup::PositionedChunk)
In both cases, the values are the digest of the chunk/file, in form: