Re: Sequential writing to degraded RAID6 causing a lot of reading

[NeilBrown <neilb@suse.de>]

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On Thu, 15 May 2014 09:04:27 +0200 Patrik Horník <patrik@dsl.sk> wrote:

> Hello Neil,
> 
> did you make some progress on this issue by any chance?

No I haven't - sorry.
After 2 year, I guess I really should.

I'll make another note for first thing next week.

NeilBrown


> 
> I am hitting the same problem again on degraded RAID 6 missing two
> drives, kernel Debian 3.13.10-1, mdadm v3.2.5.
> 
> Thanks.
> 
> Patrik
> 
> 2012-05-28 3:31 GMT+02:00 NeilBrown <neilb@suse.de>:
> >
> > On Thu, 24 May 2012 14:37:28 +0200 Patrik Horník <patrik@dsl.sk> wrote:
> >
> > > On Thu, May 24, 2012 at 6:48 AM, NeilBrown <neilb@suse.de> wrote:
> >
> > > > Firstly, degraded RAID6 with a left-symmetric layout is quite different from
> > > > an optimal RAID5 because there are Q blocks sprinkled around and some D
> > > > blocks missing.  So there will always be more work to do.
> > > >
> > > > Degraded left-symmetric-6 is quite similar to optimal RAID5 as the same data
> > > > is stored in the same place - so reading should be exactly the same.
> > > > However writing is generally different and the code doesn't make any attempt
> > > > to notice and optimise cases that happen to be similar to RAID5.
> > >
> > > Actually I have left-symmetric-6 without one of the "regular" drives
> > > not the one with only Qs on it, so it should be similar to degraded
> > > RAID6 with a left-symmetric in this regard.
> >
> > Yes, it should - I had assumed wrongly ;-)
> >
> > >
> > > > A particular issue is that while RAID5 does read-modify-write when updating a
> > > > single block in an array with 5 or more devices (i.e. it reads the old data
> > > > block and the parity block, subtracts the old from parity and adds the new,
> > > > then writes both back), RAID6 does not. It always does a reconstruct-write,
> > > > so on a 6-device RAID6 it will read the other 4 data blocks, compute P and Q,
> > > > and write them out with the new data.
> > > > If it did read-modify-write it might be able to get away with reading just P,
> > > > Q, and the old data block - 3 reads instead of 4.  However subtracting from
> > > > the Q block is more complicated that subtracting from the P block and has not
> > > > been implemented.
> > >
> > > OK, I did not know that. In my case I have 8 drives RAID6 degraded to
> > > 7 drives, so it would be plus to have it implemented the RAID5 way.
> > > But anyway I was thinking the whole-stripe detection should work in
> > > this case.
> > >
> > > > But that might not be the issue you are hitting - it simply shows that RAID6
> > > > is different from RAID5 in important but non-obvious ways.
> > > >
> > > > Yes, RAID5 and RAID6 do try to detect whole-stripe write and write them out
> > > > without reading.  This is not always possible though.
> > > > Maybe if you told us how many devices were in your arrays (which may be
> > > > import to understand exactly what is happening), what the chunk size is, and
> > > > exactly what command you use to write "lots of data".  That might help
> > > > understand what is happening.
> > >
> > > The RAID5 is 5 drives, the RAID6 arrays are 7 of 8 drives, chunk size
> > > is 64K. I am using command dd if=/dev/zero of=file bs=X count=Y, it
> > > behaves the same for bs between 64K to 1 MB. Actually internal read
> > > speed from every drive is slightly higher that write speed, about cca
> > > 10%. The ratio between write speed to the array and write speed to
> > > individual drive is cca 5.5 - 5.7.
> >
> > I cannot really picture how the read speed can be higher than the write
> > speed.  The spindle doesn't speed up for reads and slow down for writes does
> > it?  But that's not really relevant.
> >
> > A 'dd' with large block size should be a good test.  I just did a simple
> > experiment.  With a 4-drive non-degraded RAID6 I get about a 1:100 ratio for
> > reads to writes for an extended write to the filesystem.
> > If I fail one device it becomes 1:1.  Something certainly seems wrong there.
> >
> > RAID5 behaves more as you would expect - many more writes than reads.
> >
> > I've made a note to look into this when I get a chance.
> >
> > Thanks for the report.
> >
> > NeilBrown
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