RAID-1 - suboptimal write performance?

[Tomasz Chmielewski <tch@virtall.com>]

While doing rsyncs of large archives from one RAID-1 btrfs filesystem
to another RAID-1 btrfs filesystem:

btrfs filesystem 1: sda + sdb (RAID-1), being copied to:
btrfs filesystem 2: sdc + sdd (RAID-1)
Server has 32 GB RAM


I can observe the following:


>From time to time, rsync "freezes", while there is high IO on only *one*
of write drives.


To reproduce:

dd if=/dev/urandom of=/mnt/btrfs1/bigfile.img bs=1M count=10000; sync
# cp should work, too, but won't show copy speed/progress
rsync -a -v --progress /mnt/btrfs1/bigfile.img /mnt/btrfs2/

In another terminal, run iostat -m 1:

1) a few seconds of writes to only one RAID-1 member:

Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sdc             355.00         0.00       177.50          0        177
sdb               0.00         0.00         0.00          0          0
sdd               0.00         0.00         0.00          0          0
sda               0.00         0.00         0.00          0          0


2) then, a few seconds of writes to the other RAID-1 member:

Device:            tps    MB_read/s    MB_wrtn/s    MB_read    MB_wrtn
sdc               0.00         0.00         0.00          0          0
sdb               0.00         0.00         0.00          0          0
sdd             351.00         0.00       175.50          0        175
sda               0.00         0.00         0.00          0          0


Is it optimal behaviour? With software RAID-1, I'm seeing writes to
both devices at the same time.

Also, what happens when the system crashes, and one drive has several
hundred megabytes data more than the other one?

-- 
Tomasz Chmielewski
http://wpkg.org