[Lustre-discuss] Lustre and disk tuning

Dan dan at nerp.net
Thu Jan 31 08:25:58 PST 2008


Thanks Andreas.  I'll reconfigure the RAID and give it another shot
today.  Would it be reasonable to credit the stalled writes with this
I/O mismatch I have?

Dan


On Thu, 2008-01-31 at 01:40 -0700, Andreas Dilger wrote:

> 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.
> > >
> > 
> > 
> > _______________________________________________
> > Lustre-discuss mailing list
> > Lustre-discuss at lists.lustre.org
> > http://lists.lustre.org/mailman/listinfo/lustre-discuss
> 
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
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