Re: BUG REPORT: md RAID5 write throughput will drop for 1~2s every 16s (under 1Hz sample rate)

Hello Neil:

Sorry for the late reponse (busy at other stuff)

If I'm interpreting your numbers correctly, it is just an occasional single
write that is slow - not a series of writes during a one second interval that
are each slow.  It would help if you could confirm that.

No.

"output.txt" interpretation
 - "max rw": max per-512KB-block write time in one second
 - "speed": average write throughput in one second

"speed" reveals: It is an occasional single write-throughput-per-second is slow

I don't know where you got 16 from

Sorry, I do the wrong math

For some reason raid5 sometimes decides that it needs to pre-read the
 'other' block to calculate parity rather than waiting for the other block
 to be written

Verifying using blktrace...
(Now I can reproduce the problem on "virtual machine + virtual disk",
with kernel 2.6.34)

I'd be keen to hear about any further discoveries you make.

OK.

BTW, can you reproduce the problem and help debug on this ?
My debugging process will very slow (not familiar with RAID code)


Thanks
Eddy