On 2018-09-19 04:43 AM, Tomasz Chmielewski wrote:
> 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.

Not related to the issue you are reporting, but I thought it's worth
mentioning, (since not many do), that using chattr +C on a BTRFS Raid 1
is a dangerous thing.  without COW, the 2 copies are never synchronized,
even if a scrub is executed.  So any kind of unclean shutdown that
interrupts writes (not to mention the extreme of a temporarily
disconnected drive.) will result in files that are inconsistent.  (ie,
depending on which disk happens to read at the time, the data will be
different on each read.)