[ale] SATA, iSCSI, and ESX

[Greg Freemyer]

On Nov 27, 2007 8:17 AM, James P. Kinney III
<jkinney at localnetsolutions.com> wrote:
> On Tue, 2007-11-27 at 01:07 -0500, Ashley Wilson wrote:
> > Forgive me if this is an absolutely stupid question... but is it possible to
> > get decent performance out of an iSCSI SAN full of SATA disks backing an ESX
> > server?  Has SATA advanced enough that it could be a reasonable (if somewhat
> > lesser-performing) alternative to SAS or SCSI disks in an iSCSI SAN?  The
> > VMs are dev only, but the SAN would have a built-in NAS feature hosting
> > users' home folders.  Am I trying to do too much with too little?
> > Thanks,
> > -Ash
> >
>
> Ashley,
> You can get some good performance from SATA drives if they are striped
> in the array and the network connection is not crowded. SATA 300will of
> course be faster. In fact, SATA 300 is about the same current sustained
> speed as SAS drives (which are a slight tad under the highest SCSI
> speeds). The absolute fastest performance will be in a NAS with a 10Gb
> connection feeding a dedicated 10Gb/1Gb switch with a bank of SATA 300
> drives in RAID0.

Sata-300 is readily available and there is not a cost penalty
associated with it.  But there is some Linux kernel compatibility
issues still.  I follow the kernel sata mailing list and they often
say, try limiting the speed to -150.  And it fixes the problem, so
don't get enamored with the -300 thing.

And, from what I've read SATA 300 is not useful in most situations. It
is simply faster than the rest of the infrastructure.  (ie. you will
not find it supported on PCI cards since it is faster than the PCI
bus,  PCI Express cards do the trick or pure MB setups.)  And with
typical disk seek loads, Sata-150 is as fast as the physical drive can
run anyway.

2 exceptions that _may_ take advantage of SATA-300:

1) Lots of extremely sequential disk activity.  (Possibly in a media
player backend?)  Even there I've done testing of "dd if=/dev/sda
of=/dev/null bs=4K" with a 6 month old MB with Sata-300 on the MB.  No
difference.between 150 and 300.

So only an very extremely fast MB will allow you to push past Sata-150
speeds for a single drive.

2) You have a multiplexer on the Sata line.  ie. One Sata Port
controlling multiple disks.  PMP support is just being added to the
Kernel.  OpenSUSE 10.3 has it.  It is part of the 2.6.24 kernel that
should be released soon.  I doubt if Fedora 8 has it.  (Novell's
people did the implementation, so that put in their distro a little
early.)

If you're doing a big raid with lots of disks, then an external
enclosure with PMP support and Sata-300 speeds _may_ be faster than
the same setup with Sata-150 drives.

Greg
-- 
Greg Freemyer
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http://www.linkedin.com/in/gregfreemyer
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