Thank you for the advice!
I have done Direct IO dd tests writing the same 20 GB files. The results are
an eye opener! bs=1GB, count=2
Single instance repeats of 830, 800 MB/s, compared to >100 to under 300 MB/s
Two instances aggregate of 304 MB/s, six instances aggregate of 587 MB/s.
System drive /home RAID 1 of 130 MB/s compared to 51 MB/s buffered.
So the problem is with the buffered writes.
> Youἀd have to get all the fixes from 2.6.30 to 2.6.32,
> and the
> backport would be very difficult to get right. Better
> be طust to upgrade the kernel to 2.6.32 ;)
If I change the kernel, I would have no support from Novell. I would try my
luck and convince them.
> > > I'd suggest that you might need to look at
> increasing the
> > > maximum IO
> > > size for the block device
> > > (/sys/block/sdb/queue/max_sectors_kb),
> > > maybe the request queue depth as well to get
> larger IOs to
> > > be pushed
> > > to the raid controller. if you can, at least get
> it to the
> > > stripe
> > > width of 1536k....
> > Could you give a good reference for performance tuning
> of these
> > parameters? I am at a total loss here.
> Welcome to the black art of storage subsystem tuning ;)
> I'm not sure there is a good reference for tuning the block
> parameters - most of what I know was handed down by word of
> from gurus on high mountains.
> The overriding principle, though, is to try to ensure that
> stripe width sized IOs can be issued right through the IO
> stack to
> the hardware, and that those IOs are correctly aligned to
> stripes. You've got the filesystem configuration and layout
> correct, now it's just tuning the block layer to pass the
Can I confirm that
(/sys/block/sdb/queue/max_sectors_kb)=stripe width 1536 kB
Which parameter is "request queue depth"? What should be the value?
> FWIW, your tests are not timing how longit takes for all
> data to hit the disk, only how long it takes to get into
Thank you! I do know that XFS buffers writes extensively. The drive LEDs
remain lighted long after the OS says the writes are completed. Plus some
timings are physically impossible.
> That sounds wrong - it sounds like NCQ is not functioning
> as with NCQ enabled, disabling the drive cache should not
> throughput at all....
I do not remember clearly if NCQ is available for that motherboard, it is an
Ubuntu 32-bit, but I do remember seeing queue depth in the kernel. I will
check it out next week.
But what I read is that NCQ hurts single write performance. That is also what
I found with another Areca SATA RAID in Windows XP.
What I found with all the drives we tested was that disabling the cache badly
hurt sequential write performance (no file system, write data directly to
> I'd suggest trying to find another distributor that will
> bring them
> in for you. Putting that many drives in a single chassis is
> certainly going to cause vibration problems, especially if
> you get
> all the disk heads moving in close synchronisation (which
> is what
> happens when you get all your IO sizing and alignment
I am working on changing to the WD Caviar RE4 drives. Not sure if I can pull
Chin Gim Leong
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