Re: SAN vs Internal Disks - Mailing list pgsql-performance

On 6-9-2007 14:35 Harsh Azad wrote:
> 2x Quad Xeon 2.4 Ghz (4-way only 2 populated right now)

I don't understand this sentence. You seem to imply you might be able to
fit more processors in your system?
Currently the only Quad Core's you can buy are dual-processor
processors, unless you already got a quote for a system that yields the
new Intel "Tigerton" processors.
I.e. if they are clovertown's they are indeed Intel Core-architecture
processors, but you won't be able to fit more than 2 in the system and
get 8 cores in a system.
If they are Tigerton, I'm a bit surprised you got a quote for that,
although HP seems to offer a system for those. If they are the old
dual-core MP's (70xx or 71xx), you don't want those...

> 32 GB RAM
> OS Only storage - 2x SCSI 146 GB 15k RPM on RAID-1
> (Data Storage mentioned below)

I doubt you need 15k-rpm drives for OS... But that won't matter much on
the total cost.

> HELP 1: Does something look wrong with above configuration, I know there
> will be small differences b/w opetron/xeon. But do you think there is
> something against going for 2.4Ghz Quad Xeons (clovertown i think)?

Apart from your implication that you may be able to stick more
processors in it: no, not to me. Two Quad Core Xeons were even faster
than 8 dual core opterons in our benchmarks, although that might also
indicate limited OS-, postgres or underlying I/O-scaling.
Obviously the new AMD Barcelona-line of processors (coming next week
orso) and the new Intel Quad Core's DP (Penryn?) and MP (Tigerton) may
be interesting to look at, I don't know how soon systems will be
available with those processors (HP seems to offer a tigerton-server).

> B: Go for Internal of DAS based storage. Here for each server we should
> be able to have: 2x disks on RAID-1 for logs, 6x disks on RAID-10 for
> tablespace1 and 6x disks on RAID-10 for tablespace2. Or maybe 12x disks
> on RAID-10 single table-space.

You don't necessarily need to use internal disks for DAS, since you can
also link an external SAS-enclosure either with or without an integrated
raid-controller (IBM, Sun, Dell, HP and others have options for that),
and those are able to be expanded to either multiple enclosures tied to
eachother or to a controller in the server.
Those may also be usable in a warm-standby-scenario and may be quite a
bit cheaper than FC-hardware.

> But for a moment keeping these aside, i wanted to discuss, purely on
> performance side which one is a winner? It feels like internal-disks
> will perform better, but need to understand a rough magnitude of
> difference in performance to see if its worth loosing the manageability
> features.

As said, you don't necessarily need real internal disks, since SAS can
be used with external enclosures as well, still being DAS. I have no
idea what difference you will or may see between those in terms of
performance. It probably largely depends on the raid-controller
available, afaik the disks will be mostly the same. And it might depend
on your available bandwidth, external SAS offers you a 4port-connection
allowing for a 12Gbit-connection between a disk-enclosure and a
controller. While - as I understand it - even expensive SAN-controllers
only offer dual-ported, 8Gbit connections?
What's more important is probably the amount of disks and raid-cache you
can buy in the SAN vs DAS-scenario. If you can buy 24 disks when going
for DAS vs only 12 whith SAN...

But then again, I'm no real storage expert, we only have two Dell MD1000
DAS-units at our site.

Best regards and good luck,

Arjen