Re: Bad iostat numbers

People recommend LSI MegaRAID controllers on here regularly, but I have
found that they do not work that well. I have bonnie++ numbers that show
the controller is not performing anywhere near the disk's saturation level
in a simple RAID 1 on RedHat Linux EL4 on two seperate machines provided by
two different hosting companies. In one case I asked them to replace the
card, and the numbers got a bit better, but still not optimal.

LSI MegaRAID has proved to be a bit of a disapointment. I have seen better
numbers from the HP SmartArray 6i, and from 3ware cards with 7200RPM SATA
drives.

for the output: http://www.infoconinc.com/test/bonnie++.html (the first line
is a six drive RAID 10 on a 3ware 9500S, the next three are all RAID 1s on
LSI MegaRAID controllers, verified by lspci).

Alex.

On 12/4/06, Greg Smith <gsmith(at)gregsmith(dot)com> wrote:
>
> On Thu, 30 Nov 2006, Carlos H. Reimer wrote:
>
> > I would like to discover how much cache is present in
> > the controller, how can I find this value from Linux?
>
> As far as I know there is no cache on an Adaptec 39320. The write-back
> cache Linux was reporting on was the one in the drives, which is 8MB; see
>
> http://www.seagate.com/cda/products/discsales/enterprise/tech/1,1593,541,00.html
> Be warned that running your database with the combination of an uncached
> controller plus disks with write caching is dangerous to your database
> integrity.
>
> There is a common problem with the Linux driver for this card (aic7902)
> where it enters what's they're calling an "Infinite Interrupt Loop".
> That seems to match your readings:
>
> > Here is a typical iostat -x:
> > Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s rkB/s wkB/s
> > sda 0.00 7.80 0.40 6.40 41.60 113.60 20.80 56.80
> > avgrq-sz avgqu-sz await svctm %util
> > 22.82 570697.50 10.59 147.06 100.00
>
> An avgqu-sz of 570697.50 is extremely large. That explains why the
> utilization is 100%, because there's a massive number of I/O operations
> queued up that aren't getting flushed out. The read and write data says
> these drives are barely doing anything, as 20kB/s and 57KB/s are
> practically idle; they're not even remotely close to saturated.
>
> See http://lkml.org/lkml/2005/10/1/47 for a suggested workaround that may
> reduce the magnitude of this issue; lower the card's speed to U160 in the
> BIOS was also listed as a useful workaround. You might get better results
> by upgrading to a newer Linux kernel, and just rebooting to clear out the
> garbage might help if you haven't tried that yet.
>
> On the pessimistic side, other people reporting issues with this
> controller are:
>
> http://lkml.org/lkml/2005/12/17/55
> http://www.ussg.iu.edu/hypermail/linux/kernel/0512.2/0390.html
>
> http://www.linuxforums.org/forum/peripherals-hardware/59306-scsi-hangs-boot.html
> and even under FreeBSD at
> http://lists.freebsd.org/pipermail/aic7xxx/2003-August/003973.html
>
> This Adaptec card just barely works under Linux, which happens regularly
> with their controllers, and my guess is that you've run into one of the
> ways it goes crazy sometimes. I just chuckled when checking
> http://linux.adaptec.com/ again and noticing they can't even be bothered
> to keep that server up at all. According to
>
> http://www.adaptec.com/en-US/downloads/linux_source/linux_source_code?productId=ASC-39320-R&dn=Adaptec+SCSI+Card+39320-R
> the driver for your card is "*minimally tested* for Linux Kernel v2.6 on
> all platforms." Adaptec doesn't care about Linux support on their
> products; if you want a SCSI controller that actually works under Linux,
> get an LSI MegaRAID.
>
> If this were really a Postgres problem, I wouldn't expect %iowait=1.10.
> Were the database engine waiting to read/write data, that number would be
> dramatically higher. Whatever is generating all these I/O requests, it's
> not waiting for them to complete like the database would be. Besides the
> driver problems that I'm very suspicious of, I'd suspect a runaway process
> writing garbage to the disks might also cause this behavior.
>
> > Ive taken a look in the /var/log/messages and found some temperature
> > messages about the disk drives:
> > Nov 30 11:08:07 totall smartd[1620]: Device: /dev/sda, Temperature
> changed 2
> > Celsius to 51 Celsius since last report
> > Can this temperature influence in the performance?
>
> That's close to the upper tolerance for this drive (55 degrees), which
> means the drive is being cooked and will likely wear out quickly. But
> that won't slow it down, and you'd get much scarier messages out of smartd
> if the drives had a real problem. You should improve cooling in this case
> if you want to drives to have a healthy life, odds are low this is
> relevant to your performance issue though.
>
> --
> * Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD
>
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