Re: The performance is not as expected when used several disks on raid0.

[Calvin Walton <calvin.walton@kepstin.ca>]

On Mon, 2015-08-17 at 16:44 -0300, Eduardo Bach wrote:
> Based on previous testing with a smaller number of disk I'm
> suspecting
> that the 32 disks are not all being used. With 12 discs I got more
> speed with btrfs thanmdadm+xfs. With, btrfs, 12 disks and large files
> we got the entire theoretical speed, 12 x 200MB/s per disk. My hope
> was to get some light from you guys to debug the problem so the btrfs
> use the 32 discs (assuming this is the problem). Perhaps the debug
> this problem may be of interest to devs?

>From the sounds of this, you must be hitting some bottleneck in the
btrfs code. One thing I'm actually curious about: How is the CPU usage
during these tests?

Btrfs can more work on the CPU than mdadm+xfs - in particular, data che
cksums are enabled by default. If you have compression enabled, that
would obviously be a major hit as well. Make sure you don't have
compression enabled (it's off by default, or you can use the mount
option "compress=no"). You could try with the "nodatasum" option to see
if checksums make a difference.

It could be possible that you're saturating the CPU, and that's why
you're not seeing any additional gains over 3.5GB/s. Taking a look at
top output while the test is running might be informative.

On the other hand, if the CPU isn't saturated and the disk io isn't
saturated, then it's probably a scaling issue in btrfs, possibly
something like lock contention.

-- 
Calvin Walton <calvin.walton@kepstin.ca>