Re: raid5 to utilize upto 8 cores

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On 16/08/2012 07:58, Stan Hoeppner wrote:
On 8/15/2012 9:56 PM, vincent Ferrer wrote:

- My  storage server  has upto 8 cores  running linux kernel 2.6.32.27.
- I created  a raid5 device of  10  SSDs .
-  It seems  I only have single raid5 kernel thread,  limiting  my
WRITE  throughput  to single cpu  core/thread.

The single write threads of md/RAID5/6/10 are being addressed by patches
in development.  Read the list archives for progress/status.  There were
3 posts to the list today regarding the RAID5 patch.

Question :   What are my options to make  my raid5 thread use all the
CPU cores ?
                   My SSDs  can do much more but  single raid5 thread
from mdadm   is becoming the bottleneck.

To overcome above single-thread-raid5 limitation (for now)  I  re-configured.
      1)  I partitioned  all  my  10 SSDs into 8  partitions:
      2)  I created  8   raid5 threads. Each raid5 thread having
partition from each of the 8 SSDs
      3)  My WRITE performance   quadrupled  because I have 8 RAID5 threads.
Question: Is this workaround a   normal practice  or may give me
maintenance problems later on.

No it is not normal practice.  I 'preach' against it regularly when I
see OPs doing it.  It's quite insane.  The glaring maintenance problem
is that when one SSD fails, and at least one will, you'll have 8 arrays
to rebuild vs one.  This may be acceptable to you, but not to the
general population.  With rust drives, and real workloads, it tends to
hammer the drive heads prodigiously, increasing latency and killing
performance, and decreasing drive life.  That's not an issue with SSD,
but multiple rebuilds is.  That and simply keeping track of 80 partitions.


The rebuilds will, I believe, be done sequentially rather than in parallel. And each rebuild will take 1/8 of the time a full array rebuild would have done. So it really should not be much more time or wear-and-tear for a rebuild of this monster setup, compared to a single raid5 array rebuild. (With hard disks, it would be worse due to head seeks - but still not as bad as you imply, if I am right about the rebuilds being done sequentially.)

However, there was a recent thread here about someone with a similar setup (on hard disks) who had a failure during such a rebuild and had lots of trouble. That makes me sceptical to this sort of multiple array setup (in addition to Stan's other points).

And of course, all Stan's other points about maintenance, updates to later kernels with multiple raid5 threads, etc., still stand.

There are a couple of sane things you can do today to address your problem:

1.  Create a RAID50, a layered md/RAID0 over two 5 SSD md/RAID5 arrays.
  This will double your threads and your IOPS.  It won't be as fast as
your Frankenstein setup and you'll lose one SSD of capacity to
additional parity.  However, it's sane, stable, doubles your
performance, and you have only one array to rebuild after an SSD
failure.  Any filesystem will work well with it, including XFS if
aligned properly.  It gives you an easy upgrade path-- as soon as the
threaded patches hit, a simple kernel upgrade will give your two RAID5
arrays the extra threads, so you're simply out one SSD of capacity.  You
won't need to, and probably won't want to rebuild the entire thing after
the patch.  With the Frankenstein setup you'll be destroying and
rebuilding arrays.  And if these are consumer grade SSDs, you're much
better off having two drives worth of redundancy anyway, so a RAID50
makes good sense all around.

2.  Make 5 md/RAID1 mirrors and concatenate them with md/RAID linear.
You'll get one md write thread per RAID1 device utilizing 5 cores in
parallel.  The linear driver doesn't use threads, but passes offsets to
the block layer, allowing infinite core scaling.  Format the linear
device with XFS and mount with inode64.  XFS has been fully threaded for
15 years.  Its allocation group design along with the inode64 allocator
allows near linear parallel scaling across a concatenated device[1],
assuming your workload/directory layout is designed for parallel file
throughput.

#2, with a parallel write workload, may be competitive with your
Frankenstein setup in both IOPS and throughput, even with 3 fewer RAID
threads and 4 fewer SSD "spindles".  It will outrun the RAID50 setup
like it's standing still.  You'll lose half your capacity to redundancy
as with RAID10, but you'll have 5 write threads for md/RAID1, one per
SSD pair.  One core should be plenty to drive a single SSD mirror, with
plenty of cycles to spare for actual applications, while sparing 3 cores
for apps as well.  You'll get unlimited core scaling with both md/linear
and XFS.  This setup will yield the best balance of IOPS and throughput
performance for the amount of cycles burned on IO, compared to
Frankenstein and the RAID50.

For those that don't want to use XFS, or won't have balanced directories in their filesystem, or want greater throughput of larger files (rather than greater average throughput of multiple parallel accesses), you can also take your 5 raid1 mirror pairs and combine them with raid0. You should get similar scaling (the cpu does not limit raid0). For some applications (such as mail server, /home mount, etc.), the XFS over a linear concatenation is probably unbeatable. But for others (such as serving large media files), a raid0 over raid1 pairs could well be better. As always, it depends on your load - and you need to test with realistic loads or at least realistic simulations.


[1] If you are one of the uneducated masses who believe dd gives an
accurate measure of storage performance, then ignore option #2.  Such a
belief would indicate you thoroughly lack understanding of storage
workloads, and thus you will be greatly disappointed with the dd numbers
this configuration will give you.


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