May 08, 2011

Backup of 5 million tiny files and paths

I think in ideal world this shouldn't be happening, it really is a job for a proper database engine.
Some filesystems (reiserfs, pomegranate) are fairly good at dealing with such use-cases though, but not the usual tools for working with fs-based data, which generally suck all the time and resources on such a mess.
In my particular case, there's a (mostly) legacy system, which keeps such tons-of-files db with ~5M files, taking about 5G of space, which have to be backed-up somehow. Every file can be changed, added or unlinked, total consistency between parts (like snapshotting the same point in time for every file) is not necessary. Contents are (typically) php serializations (yuck!).
Tar and rsync are prime example of tools that aren't quite fit for the task - both eat huge amounts of RAM (gigs) and time to do this, especially when you have to make these backups incremental, and ideally this path should be backed-up every single day.
Both seem to build some large and not very efficient list of existing files in memory and then do a backup against that. Both aren't really good at capturing the state - increments either take a huge amounts of space/inodes (with rsync --link-dest) or loose info on removed entries (tar).
Nice off-the-shelf alternatives are dar, which is not a fs-to-stream packer, but rather squashfs-like image builder with the ability to make proper incremental backups, and of course mksquashfs itself, which supports append these days.
These sound nice, but somehow I failed to check for "append" support in squashfs (although I remember hearing about it before), plus there's still doesn't seem to be a way to remove paths.
dar seem to be good enough solution, and I'll probably get back to it, but as I was investigating "the right way" to do such backups, first thing that naturally came to mind (probably because even fs developers suggest that), is to cram all this mess into a single db, and I wanted to test it via straightforward fs - berkdb (btree) implementation.
Results turned out to be really good - 40min to back all this stuff up from scratch and under 20min to do an incremental update (mostly comparing the timestamps plus adding/removing new/unlinked keys). Implementation on top of berkdb also turned out to be fairly straightorward (just 150 lines total!) with just a little bit of optimization magic to put higter-level paths before the ones nested inside (by adding \0 and \1 bytes before basename for file/dir).
I still need to test it against dar and squashfs when I'll have more time (as if that will ever happen) on my hands, but even such makeshift python implementation (which includes full "extract" and "list" functionality though) proven to be sufficient and ended up in a daily crontab.
So much for the infamous "don't keep the files in a database!" argument, btw, wonder if original developers of this "db" used this hype to justify this mess...

Obligatory proof-of-concept code link.

Update:tried mksquashfs, but quickly pulled a plug as it started to eat more than 3G of RAM - sadly unfit for the task as well. dar also ate ~1G and been at it for a few hours, guess no tool cares about such fs use-cases at all.

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