On 2017-04-10 12:54, Kai Krakow wrote:
Am Mon, 10 Apr 2017 18:44:44 +0200
schrieb Kai Krakow <hurikhan77@xxxxxxxxx>:
Am Mon, 10 Apr 2017 08:51:38 -0400
schrieb "Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>:
On 2017-04-10 08:45, Kai Krakow wrote:
Am Mon, 10 Apr 2017 08:39:23 -0400
schrieb "Austin S. Hemmelgarn" <ahferroin7@xxxxxxxxx>:
[...]
Does btrfs really support lazytime now?
It appears to, I do see fewer writes with it than without it. At
the very least, if it doesn't, then nothing complains about it.
Did you put it in /etc/fstab only for the rootfs? If yes, it probably
has no effect. You would need to give it as rootflags on the kernel
cmdline.
I did a "fgrep lazytime /usr/src/linux -ir" and it reveals only ext4
and f2fs know the flag. Kernel 4.10.
So probably you're seeing a placebo effect. If you put lazytime for
rootfs just only into fstab, it won't have an effect because on initial
mount this file cannot be opened (for obvious reasons), and on remount,
btrfs seems to happily accept lazytime but it has no effect. It won't
show up in /proc/mounts. Try using it in rootflags kernel cmdline and
you should see that the kernel won't accept the flag lazytime.
The command-line also rejects a number of perfectly legitimate arguments
that BTRFS does understand too though, so that's not much of a test.
I've just finished some quick testing though, and it looks like you're
right, BTRFS does not support this, which means I now need to figure out
what the hell was causing the IOPS counters in collectd to change in
rough correlation with remounting (especially since it appears to
happen mostly independent of the options being changed).
This is somewhat disappointing though, as supporting this would probably
help with the write-amplification issues inherent in COW filesystems.
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