Zygo Blaxell posted on Mon, 13 Apr 2015 00:04:36 -0400 as excerpted: > A database ends up maxing out at about a factor of two space usage > because it tends to write short uniform-sized bursts of pages randomly, > so we get a pattern a bit like bricks in a wall: > > 0 MB AA BB CC DD EE FF GG HH II JJ KK 1 MB half the extents 0 MB > LL MM NN OO PP QQ RR SS TT UU V 1 MB the other half > > 0 MB ALLBMMCNNDOOEPPFQQGRRHSSITTJUUKV 1 MB what the file looks > like > > Fixing this is non-trivial (it may require an incompatible disk format > change). Until this is fixed, the most space-efficient approach seems > to be to force compression (so the maximum extent is 128K instead of > 1GB) and never defragment database files ever.
... Or set the database file nocow at creation, and don't snapshot it, so overwrites are always in-place. (Btrfs compression and checksumming get turned off with nocow, but as we've seen, compression isn't all that effective on random-rewrite-pattern files anyway, and databases generally have their own data integrity handling, so neither one is a huge loss, and the in-place rewrite makes for better performance and a more predictable steady-state.) -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
