But unlike others, it is able to eat zfs-diff's output and apply the
changes to existing database.
-Why I wrote it? Indexing, just "find /big" can take a considerable
-amount of time, like an hour or so, with many I/O operations spent. But
-my home NAS has relatively few number of changes made every day. The
-only possible quick way to determine what exactly was modified is to
-traverse over ZFS'es Merkle trees to find a difference between
-snapshots. Fortunately zfs-diff command does exactly that, providing
-pretty machine-friendly output.
-
-Why this utility is so complicated? Initially it kept all database in
-memory, but that takes 2-3 GiBs of memory, that is huge amount. Moreover
-it fully loads it to perform any basic searches. So current
-implementation uses temporary files and heavy use of data streaming.
-
-Its storage format is trivial:
-
-* 16-bit BE size of the following name
-* entity (file, directory, symbolic link, etc) name itself.
- Directory has trailing "/"
-* single byte indicating current file's depth
-* 64-bit BE mtime seconds
-* 64-bit BE file or directory (sum of all files and directories) size
-
-Its indexing algorithm is following:
-
-* traverse over all filesystem hierarchy in a *sorted* order. All
- records are written to temporary file, without directory sizes,
- because they are not known in advance during the walking
-* during the walk, remember in memory each directory's total size
-* read all records from that temporary file, writing to another one, but
- replacing directory sizes with ones remembered
-
-Searching is trivial:
-
-* there is no actual searching, just a streaming through all the
- database file sequentially
-* if some root is specified, then the program will output only its
- hierarchy path, exiting after it is finished
-
-Updating algorithm is following:
-
-* read all [-+MR] actions from zfs-diff, validating the whole format
-* each file's "R" becomes "-" and "+" actions
-* if there are directory "R", then collect them and stream from current
- database to determine each path entity you have to "-" and "+"
-* each "+" adds an entry to the list of "M"s
-* sort all "-", "+" and "M" filenames in ascending order
-* get entity's information for each "M" (remembering its size and mtime)
-* stream current database records, writing them to temporary file
-* if record exists in "-"-list, then skip it
-* if any "+" exists in the *sorted* list, that has precedence over the
- record from database, then insert it into the stream, taking size and
- mtime information from "M"-list
-* if any "M" exists for the read record, then use it to alter it
-* all that time, directory size calculating algorithm also works, the
- same one used during indexing
-* create another temporary file to copy the records with actualized
- directory sizes
-
-How to use it?
-
- $ zfs snap big@snap1
- $ cd /big ; glocate -db /tmp/glocate.db -index
-
- $ glocate -db /tmp/glocate.db
- [list of all files]
-
- $ glocate -db /tmp/glocate.db -machine
- [machine parseable list of files with sizes and mtimes]
-
- $ glocate -db /tmp/glocate.db -tree
- [beauty tree-like list of files with sizes and mtimes]
-
- $ glocate -db /tmp/glocate.db some/sub/path
- [just a part of the whole hierarchy]
-
-and update it carefully:
-
- $ zfs snap big@snap2
- $ zfs diff -FH big@snap2 | glocate -db /tmp/glocate.db -strip /big/ -update
-
-glocate is copylefted free software: see the file COPYING for copying
-conditions.
+Why I wrote it? I have got ~18M files ZFS data storage, where even
+"find /storage" takes considerable amount of time, up to an hour.
+So I have to use separate indexed database and search against it.
+locate family of utilities does exactly that. But none of them are
+able to detect a few seldom made changes to the dataset, without
+traversing through the whole dataset anyway, taking much IO.
+
+Fortunately ZFS design with Merkle trees is able to show us the
+difference quickly and without notable IO. "zfs diff" command's
+output is very machine friendly. So locate-like utility has to be able
+to update its database with zfs-diff's output.
+
+Why this utility is so relatively complicated? Initially it kept all
+database in memory, but that took 2-3 GiBs of memory, that is huge
+amount. Moreover it fully loads it to perform any basic searches. So
+current implementation uses temporary files and heavy use of data
+streaming. Database in my case takes less than 128MiB of data. And
+searching takes only several seconds on my machine.
+
+It is free software: see the file COPYING for copying conditions.