[Lustre-discuss] Lustre and disk tuning

[Andreas Dilger]

On Jan 30, 2008  18:32 -0800, Dan wrote:
> I was a little uncertain of the stripe size calculation so here we go...
> My chunk size is 128k and there are 23 disks in RAID 6 (one hot spare
> leave 23).  That means 21 data disks?  Judging by your formula I take 23 *
> 128k whis is 2944.  Is this even close to what you intended?  This stripe
> size hangs at mount...

Hmm, I don't think the mballoc code can efficiently deal with a stripe size 
larger than the RPC size (which is 1MB) because this will always result in
a read-modify-write of the RAID stripe as not enough data can be collected
to fill a stripe.

> I've tried to test with the lustre-io kit but the tests (writes) fail on
> most OSTs.  That is the problem I'm having after all... frustrating.
> 
> Would it make sense to reconfigure the RAID controllers to have separate
> groups of disks in RAID 6?  For performance is there a recommended max
> size or number of disks for each OST?  Lastly, is it worth while to
> consider putting the ext3 journal on another device exported from the RAID
> controller?

Having 21 disks in the RAID set is probably too large to be practical
because of the high overhead of doing IO of such a large size.
Good configurations for such a system might be 2x 8+2 + spare = 21 disks
with 128kB chunk size, or 16+2 + spare = 19 disks with 64kB chunk size.
Both result in 1MB full stripe size, which is what mballoc and Lustre
are optimized to by default.

> > On Jan 18, 2008  16:45 -0800, Dan wrote:
> >>     I'm looking for some advice on improving disk performance and
> >> understanding what Lustre is doing with it.  Right now I have a ~28 TB
> >> OSS with 4 OSTs on it.  There are 4 clients using Lustre native - no
> >> NFS.  If I write to the lustre volume from the clients I get odd
> >> behavior.  Typically the writes have a long pause before any data
> >> starts hitting the disks.  Then 2 or 3 of the clients will write
> >> happily but one or two will not.  Eventually Lustre will pump out a
> >> number of I/O related errors such as "slow i_mutex 165 seconds, slow
> >> direct_io 32 seconds" and so on.  Next the clients that couldn't write
> >> will catch up and pass the clients that could write.  At some point (5
> >> minutes or so) the jobs start failing without any errors.  New jobs
> >> can be started after these fail and the pattern repeats.  Write speeds
> >> are low, around 22 MB/sec per client, the disks shouldn't have any
> >> problem handling 4 writes at this speed!!  This did work using NFS.
> >>
> >>     When these disks were formated with XFS I/O was fast.  No problems
> >> at
> >> all writing 475 MB/sec sustained per RAID controller (locally, not via
> >> NFS).  No delays.  After configuring for Lustre the peak sustained
> >> write (locally) is 230 MB/sec.  It will write for about 2 minutes
> >> before logging about slow I/O.  This is without any clients connected.
> >>
> >> So far I've done the following:
> >>
> >> 1.  Recompiled SCSI driver for RAID controller to use 1 MB blocks (from
> >> 256k).
> >> 2.  Adjusted MDS, OST threads
> >> 3.  Tried all I/O schedulers
> >> 4.  Tried all possible settings on RAID controllers for Caching and
> >> read-ahead.
> >> 5.  Some minor stuff I forgot about!
> >>
> >> Nothing makes a difference - same results under each configuration
> >> except
> >> for schedulers.  When running the deadline scheduler the writes fail
> >> faster and have delays around 30 seconds.  With all others the delays
> >> range from 100 to 500 seconds.
> >>
> >> The system has 4 cores and 4 GB of memory with 4 7 TB OSTs.  The disks
> >> are
> >> in RAID 6 split between two controllers with 2 GB cache each.  One
> >> controller has the MGS/MDT on it.  When running top it indicates 2/3 to
> >> 3/4 of memory utilized and 25% CPU utilization normally.
> >
> > Are you using Lustre 1.4 or 1.6?  Are you mounting your OSTs with
> > "-o extents,mballoc"?  We've had Lustre OSSs nodes running in excess
> > of 2GB/s with h/w RAID controllers.
> >
> > Are you using partitions on your RAID device?  You shouldn't - that causes
> > unaligned IO to the device and needless read-modify-write for each RAID
> > stripe.
> >
> > Is your RAID geometry efficient with 1MB IOs (e.g. 4+1 or 8+1)?  If not,
> > then you should consider mounting your OSTs with "-o
> > stripe={raid_stripe}",
> > where raid_stripe=N*raid_chunksize, N is the number of data disks for
> > RAID 5 N+1 or RAID 6 N+2.
> >
> > You should download the lustre-iokit and use sgpdd-survey,
> > obdfilter-survey,
> > and PIOS to determine what is causing the performance bottleneck.
> >
> > Cheers, Andreas
> > --
> > Andreas Dilger
> > Sr. Staff Engineer, Lustre Group
> > Sun Microsystems of Canada, Inc.
> >
> 
> 
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Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.