Re: Performance question, RAID5

["Keld Jørn Simonsen" <keld@keldix.com>]

On Sun, Jan 30, 2011 at 01:54:42AM +0000, Mathias Burén wrote:
> On 30 January 2011 01:52, Keld Jørn Simonsen <keld@keldix.com> wrote:
> > On Sat, Jan 29, 2011 at 11:44:01PM +0000, Mathias Burén wrote:
> >> On 29 January 2011 22:53, Roman Mamedov <rm@romanrm.ru> wrote:
> >> > On Sat, 29 Jan 2011 22:48:06 +0000
> >> > Mathias Burén <mathias.buren@gmail.com> wrote:
> >> >
> >> >> Hi,
> >> >>
> >> >> I'm wondering if the performance I'm getting is OK or if there's
> >> >> something I can do about it. Also, where the potential bottlenecks
> >> >> are.
> >> >
> >> > How are your disks plugged in? Which controller model(s), which bus.
> >> > But generally, on an Atom 1.6 Ghz those seem like good results.
> >> >
> >> > --
> >> > With respect,
> >> > Roman
> >> >
> >>
> >>
> >> Hi,
> >>
> >> Sorry, of course I should've included that. Here's the info:
> >>
> >> ~/bin $ sudo ./drivescan.sh
> >> Controller device @ pci0000:00/0000:00:0b.0 [ahci]
> >>   SATA controller: nVidia Corporation MCP79 AHCI Controller (rev b1)
> >>     host0: /dev/sda ATA Corsair CSSD-F60 {SN: 10326505580009990027}
> >>     host1: /dev/sdb ATA WDC WD20EARS-00M {SN: WD-WCAZA1022443}
> >>     host2: /dev/sdc ATA WDC WD20EARS-00M {SN: WD-WMAZ20152590}
> >>     host3: /dev/sdd ATA WDC WD20EARS-00M {SN: WD-WMAZ20188479}
> >>     host4: [Empty]
> >>     host5: [Empty]
> >
> >
> > Hmm, it seems like you have 2 empty slots on the on-board  SATA
> > controller. Try to move 2 of the disks from the other controller to the
> > on-board controller.
> >
> > And I would also avoid LVM. I think LVM affects striping.
> >
> > best regards
> > Keld
> >
> 
> Sadly the 2 empty slots are not to be found on the motherboard, I
> guess they're in the chipset only.

maybe then use the 5th drive on the sata on-board controller in the
raid5 - the sda drive. If the raid5 is where you want performance from. 

what do you use sda for? Your OS? It is a lot of space to use just for
the OS. It could easily go into the raid5 too. And you culd use a raid
for the system too, to secure you from bad things happening to your
system.

Or you could have a few 5 to 10 GB partitions in the beginning of
each drive, for experimenting with raid layout and performance. 
This should be outside any LVM to exclude LVM having an impact on 
the tests. 

Maybe your PCI-e cannot do more than 2.5 Gbit - then 2 of your disks
would be enough to fill that connection. You could try out
a raid0 on the 3 drives. If you cannot get more than about 300 MB/s, then 
the PCI-E is a bottleneck. 

If that is so, then having 3 drives from the PCI-E could slow down the
whole raid5, and using only 2 drives could speed up the full raid5.

The on-board sata controller is normally much faster, having a direct
connection to the southbridge - and typically a speed in the neighbourhood of
20 Gbit - or 2500 MB/s - which would be enough for many systems to not
be the bottleneck. It can often pay off to have a motherboard with two
on-board sata controllers with in total 8 SATA ports or more,
instead of bying an extra PCI-E controller.

Looking forward to hear what you find out.

best regards
keld
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