On Thu, Jun 12, 2014 at 12:37:13PM +0000, Duncan wrote: > Goffredo Baroncelli <kreij...@libero.it> posted on Thu, 12 Jun 2014 > 13:13:26 +0200 as excerpted: > > >>systemd has a very stupid journal write pattern. It checks if there is > >>space in the file for the write, and if not it fallocates the small > >>amount of space it needs (it does *4 byte* fallocate calls!) and then > >>does the write to it. All this does is fragment the crap out of the log > >>files because the filesystems cannot optimise the allocation patterns. > > > > I checked the code, and to me it seems that the fallocate() are done in > > FILE_SIZE_INCREASE unit (actually 8MB). > > FWIW, either 4 byte or 8 MiB fallocate calls would be bad, I think > actually pretty much equally bad without NOCOW set on the file.
So maybe it's been fixed in systemd since the last time I looked. Yup: http://cgit.freedesktop.org/systemd/systemd/commit/src/journal/journal-file.c?id=eda4b58b50509dc8ad0428a46e20f6c5cf516d58 The reason it was changed? To "save a syscall per append", not to prevent fragmentation of the file, which was the problem everyone was complaining about... > Why? Because btrfs data blocks are 4 KiB. With COW, the effect for > either 4 byte or 8 MiB file allocations is going to end up being the > same, forcing (repeated until full) rewrite of each 4 KiB block into its > own extent. And that's now a btrfs problem.... :/ Cheers, Dave. -- Dave Chinner da...@fromorbit.com -- 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
