On Wed, 07.01.15 15:10, Josef Bacik (jba...@fb.com) wrote: > On 01/07/2015 12:43 PM, Lennart Poettering wrote: > >Heya! > > > >Currently, systemd-journald's disk access patterns (appending to the > >end of files, then updating a few pointers in the front) result in > >awfully fragmented journal files on btrfs, which has a pretty > >negative effect on performance when accessing them. > > I've been wondering if mount -o autodefrag would deal with this problem but > I haven't had the chance to look into it.
Hmm, I am kinda interested in a solution that I can just implement in systemd/journald now and that will then just make things work for people suffering by the problem. I mean, I can hardly make systemd patch the mount options of btrfs just because I place a journal file on some fs... Is "autodefrag" supposed to become a default one day? Anyway, given the pros and cons I have now changed journald to set the nocow bit on newly created journal files. When files are rotated (and we hence know we will never ever write again to them) the bit is tried to be unset again, and a defrag ioctl will be invoked right after. btrfs currently silently ignores that we unset the bit, and leaves it set, but I figure i should try to unset it anyway, in case it learns that one day. After all, after rotating the files there's no reason to treat the files special anymore... I'll keep an eye on this, and see if I still get user complaints about it. Should autodefrag become default eventually we can get rid of this code in journald again. One question regarding the btrfs defrag ioctl: playing around with it it appears to be asynchronous, the defrag request is simply queued and the ioctl returns immediately. Which is great for my usecase. However I was wondering if it always was async like this? I googled a bit, and found reports that defrag might take a while, but I am not sure if those reports were about the ioctl taking so long, or the effect of defrag actually hitting the disk... Lennart -- Lennart Poettering, Red Hat -- 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
