Re: btrfs-progs 4.4 re-balance of RAID6 is very slow / limited to one cpu core?

[Christian Rohmann <crohmann@netcologne.de>]

Hey Chris,


sorry for the late reply.


On 01/27/2016 10:53 PM, Chris Murphy wrote:
> I can't exactly reproduce this. I'm using +C qcow2 on Btrfs on one SSD
> to back the drives in the VM.
> 
> 2x btrfs raid1 with files totalling 5G consistently takes ~1 minute
> [1]  to balance (no filters)
> 
> 4x btrfs raid6 with the same files *inconsistently* takes ~1m15s [2]
> to balance (no filters)
> iotop is all over the place, from 21MB/s writes to 527MB/s

To be honest, 5G is not really 21T spread across 12 spindles with LOTS
of data on them. On another box with 8x4TB spinning rust it's also very
slow.


> Do both of you get something like this:
> [root@f23m ~]# dmesg | grep -i raid
> [    1.518682] raid6: sse2x1   gen()  4531 MB/s
> [    1.535663] raid6: sse2x1   xor()  3783 MB/s
> [    1.552683] raid6: sse2x2   gen() 10140 MB/s
> [    1.569658] raid6: sse2x2   xor()  7306 MB/s
> [    1.586673] raid6: sse2x4   gen() 11261 MB/s
> [    1.603683] raid6: sse2x4   xor()  7009 MB/s
> [    1.603685] raid6: using algorithm sse2x4 gen() 11261 MB/s
> [    1.603686] raid6: .... xor() 7009 MB/s, rmw enabled
> [    1.603687] raid6: using ssse3x2 recovery algorithm


Yes:
--- cut ---
[    4.704396] raid6: sse2x1   gen()  4288 MB/s
[    4.772401] raid6: sse2x1   xor()  4036 MB/s
[    4.840403] raid6: sse2x2   gen()  7629 MB/s
[    4.908405] raid6: sse2x2   xor()  6247 MB/s
[    4.976404] raid6: sse2x4   gen() 10221 MB/s
[    5.044397] raid6: sse2x4   xor()  7620 MB/s
[    5.044525] raid6: using algorithm sse2x4 gen() 10221 MB/s
[    5.044641] raid6: .... xor() 7620 MB/s, rmw enabled
[    5.044767] raid6: using ssse3x2 recovery algorithm
--- cut ---






Would some sort of stracing or profiling of the process help to narrow
down where the time is currently spent and why the balancing is only
running single-threaded?




Regards

Christian