Re: Odd CPU usage in the last stage of a raid5 reshape

[NeilBrown <neilb@suse.de>]

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On Fri, 19 Apr 2013 09:54:04 +0200 Goswin von Brederlow <goswin-v-b@web.de>
wrote:

> Hi,
> 
> I'm still reshaping a 2 disk raid5 to 3 disks. It has now progressed
> past the 50% mark so all data has been reshaped. So now the kernel
> simply writes zeroes (I assume) to all 3 disks. There are no more
> reads, only writes.
> 
> Now what is odd is the cpu usage:
> 
> /proc/mdsata:
> md0 : active raid5 sdd1[3] sdc1[2] sda1[0]
>       3907015168 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU]
>       [============>........]  reshape = 62.0% (2425537964/3907015168) finish=22
> 0.0min speed=112230K/sec
> 
> iostat -k 10:
> Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
> sda             834.40         0.00    103372.60          0    1033726
> md0               0.00         0.00         0.00          0          0
> sdc             813.90         0.00    104601.80          0    1046018
> sdd             718.50         0.00    104499.40          0    1044994
> 
> top:
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND           
>  2058 root      20   0     0    0    0 R   96  0.0   1106:40 md0_raid5
> 18379 root      20   0     0    0    0 R   50  0.0 324:11.19 md0_reshape
> 
> Is the kernel zero filling the raid device and computing the XOR of
> zeroes for the parity blocks? Wouldn't it be less cpu consuming to
> insert zero filled stripes directly into the stripe cache?
> 

Yes, the kernel is computing an XOR of the zeros to determine the parity
block.  This certainly could be optimised, but it is hardly a priority.

NeilBrown

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