On 12/10/2012 09:37 AM, Matt Garman wrote:
> In particular: (1) how
> to determine hardware requirements
That may be difficult at this point, because you really want to start by
measuring the number of IOPS. That's difficult to do if your
applications demand more than your hardware currently provices.
> -the users often experience a fair amount
> of lag (1--5 seconds) when doing anything on their home directories,
> including an “ls” or writing a small text file.
This might not be the result of your NFS server performance. You might
actually be seeing bad performance in your directory service. What are
you using for that service? LDAP? NIS? Are you running nscd or sssd
on the clients?
> There are eight 15k 2.5” 600 GB
> drives (Seagate ST3600057SS) configured in hardware RAID-6 with a
> single hot spare. RAID controller is a Dell PERC H700 w/512MB cache
> (Linux sees this as a LSI MegaSAS 9260).
RAID 6 is good for $/GB, but bad for performance. If you find that your
performance is bad, RAID10 will offer you a lot more IOPS.
Mixing 15k drives with RAID-6 is probably unusual. Typically 15k drives
are used when the system needs maximum IOPS, and RAID-6 is used when
storage capacity is more important than performance.
It's also unusual to see a RAID-6 array with a hot spare. You already
have two disks of parity. At this point, your available storage
capacity is only 600GB greater than a RAID-10 configuration, but your
performance is MUCH worse.
> OS is CentOS 5.6, home
> directory partition is ext3, with options “rw,data=journal,usrquota”.
data=journal actually offers better performance than the default in some
workloads, but not all. You should try the default and see which is
better. With a hardware RAID controller that has battery backed write
cache, data=journal should not perform any better than the default, but
probably not any worse.
> I have the HW RAID configured to present two virtual disks to the OS:
> /dev/sda for the OS (boot, root and swap partitions), and /dev/sdb for
> the home directories. I’m fairly certain I did not align the
> partitions optimally:
If your drives are really 4k sectors, rather than the reported 512B,
then they're not optimal and writes will suffer. The best policy is to
start your first partition at 1M offset. parted should be aligning
things well if it's updated, but if your partition sizes (in sectors)
are divisible by 8, you should be in good shape.
> Here is one iteration from the iostat process:
>
> Time: 09:37:28 AM
> Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz
> avgqu-sz await svctm %util
> sda 0.00 44.09 0.03 107.76 0.13 607.40
> 11.27 0.89 8.27 7.27 78.35
> sdb 0.00 2616.53 0.67 157.88 2.80 11098.83
> 140.04 8.57 54.08 4.21 66.68
If that's normal, you need a faster array configuration. That iteration
caught both disks with a very high % of maximum utilization. Consider
using RAID-10.
> What I observe, is that whenever sdb (home directory partition)
> becomes loaded, sda (OS) often does as well. Why is this?
Regardless of what you export to the OS, if the RAID controller really
only has one big RAID-6 array, you'd expect saturation of either OS disk
to affect both.
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