I have a mysql slave which writes to a RAID-1 btrfs filesystem (with
4.17.14 kernel) on 3 x ~1.9 TB SSD disks; filesystem is around 40% full.
The slave receives around 0.5-1 MB/s of data from the master over the
network, which is then saved to MySQL's relay log and executed. In ideal
conditions (i.e. no filesystem overhead) we should expect some 1-3 MB/s
of data written to disk.
MySQL directory and files in it are chattr +C (since the directory was
created, so all files are really +C); there are no snapshots.
Now, an interesting thing.
When the filesystem is mounted with these options in fstab:
defaults,noatime,discard
We can see a *constant* write of 25-100 MB/s to each disk. The system is
generally unresponsive and it sometimes takes long seconds for a simple
command executed in bash to return.
However, as soon as we remount the filesystem with space_cache=v2 -
writes drop to just around 3-10 MB/s to each disk. If we remount to
space_cache - lots of writes, system unresponsive. Again remount to
space_cache=v2 - low writes, system responsive.
That's a huuge, 10x overhead! Is it expected? Especially that
space_cache=v1 is still the default mount option?
Tomasz Chmielewski
https://lxadm.com
