On Fri, Sep 07, 2007 at 02:10:32PM -0700, david@lang.hm wrote:
> >3. Easy to set up "warm standby" functionality. (Then again, if the
> >postgres server fails miserably, it's likely to be due to a disk
> >crash).
>
> and if postgres dies for some other reason the image on disk needs repair,
> unless you script stopping postgres when the SAN does it's snapshots,
> those snapshots are not going to be that good. the problems are useually
> repairable, but that makes starting your warm spare harder.

Uh, the "image" you get from a PITR backup "needs repair" too. There's
absolutely nothing wrong with using a SAN or filesystem snapshot as a
backup mechanism, as long as it's a true snapshot, and it includes *all*
PostgreSQL data (everything under $PGDATA as well as all tablespaces).

Also, to reply to someone else's email... there is one big reason to use
a SAN over direct storage: you can do HA that results in 0 data loss.
Good SANs are engineered to be highly redundant, with multiple
controllers, PSUs, etc, so that the odds of losing the SAN itself are
very, very low. The same isn't true with DAS.

But unless you need that kind of HA recovery, I'd tend to stay away from
SANs.

BTW, if you need *serious* bandwidth, look at things like Sun's
"thumper" (I know there's at least one other company that makes
something similar). 40-48 drives in a single 4U chassis == lots of
throughput.

Finally, if you do get a SAN, make sure and benchmark it. I've seen more
than one case of a SAN that wasn't getting anywhere near the performance
it should be, even with a simple dd test.
--
Decibel!, aka Jim Nasby decibel@decibel.org
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)

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On Tue, Sep 11, 2007 at 03:55:51PM -0500, Decibel! wrote:
>Also, to reply to someone else's email... there is one big reason to use
>a SAN over direct storage: you can do HA that results in 0 data loss.
>Good SANs are engineered to be highly redundant, with multiple
>controllers, PSUs, etc, so that the odds of losing the SAN itself are
>very, very low. The same isn't true with DAS.

You can get DAS arrays with multiple controllers, PSUs, etc. DAS !=
single disk.

Mike Stone


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On Tue, Sep 11, 2007 at 05:09:00PM -0400, Michael Stone wrote:
> On Tue, Sep 11, 2007 at 03:55:51PM -0500, Decibel! wrote:
> >Also, to reply to someone else's email... there is one big reason to use
> >a SAN over direct storage: you can do HA that results in 0 data loss.
> >Good SANs are engineered to be highly redundant, with multiple
> >controllers, PSUs, etc, so that the odds of losing the SAN itself are
> >very, very low. The same isn't true with DAS.
>
> You can get DAS arrays with multiple controllers, PSUs, etc. DAS !=
> single disk.

It's still in the same chassis, though, which means if you lose memory
or mobo you're still screwed. In a SAN setup for redundancy, there's
very little in the way of a single point of failure; generally only the
backplane, and because there's very little that's on there it's
extremely rare for one to fail.
--
Decibel!, aka Jim Nasby decibel@decibel.org
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)

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On Tue, 11 Sep 2007, Decibel! wrote:

> On Tue, Sep 11, 2007 at 05:09:00PM -0400, Michael Stone wrote:
>> On Tue, Sep 11, 2007 at 03:55:51PM -0500, Decibel! wrote:
>>> Also, to reply to someone else's email... there is one big reason to use
>>> a SAN over direct storage: you can do HA that results in 0 data loss.
>>> Good SANs are engineered to be highly redundant, with multiple
>>> controllers, PSUs, etc, so that the odds of losing the SAN itself are
>>> very, very low. The same isn't true with DAS.
>>
>> You can get DAS arrays with multiple controllers, PSUs, etc. DAS !=
>> single disk.
>
> It's still in the same chassis, though, which means if you lose memory
> or mobo you're still screwed. In a SAN setup for redundancy, there's
> very little in the way of a single point of failure; generally only the
> backplane, and because there's very little that's on there it's
> extremely rare for one to fail.

not nessasarily. direct attached doesn't mean in the same chassis,
external drive shelves attached via SCSI are still DAS

you can even have DAS attached to a pair of machines, with the second box
configured to mount the drives only if the first one dies.

David Lang

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Yeah, the DAS we are considering is Dell MD3000, it has redundant hot
swappable raid controllers in active-active mode. Provision for hot spare
hard-disk. And it can take upto 15 disks in 3U, you can attach two more
MD1000 to it, giving a total of 45 disks in total.

-- Harsh

On 9/12/07, david@lang.hm <david@lang.hm> wrote:
>
> On Tue, 11 Sep 2007, Decibel! wrote:
>
> > On Tue, Sep 11, 2007 at 05:09:00PM -0400, Michael Stone wrote:
> >> On Tue, Sep 11, 2007 at 03:55:51PM -0500, Decibel! wrote:
> >>> Also, to reply to someone else's email... there is one big reason to
> use
> >>> a SAN over direct storage: you can do HA that results in 0 data loss.
> >>> Good SANs are engineered to be highly redundant, with multiple
> >>> controllers, PSUs, etc, so that the odds of losing the SAN itself are
> >>> very, very low. The same isn't true with DAS.
> >>
> >> You can get DAS arrays with multiple controllers, PSUs, etc. DAS !=
> >> single disk.
> >
> > It's still in the same chassis, though, which means if you lose memory
> > or mobo you're still screwed. In a SAN setup for redundancy, there's
> > very little in the way of a single point of failure; generally only the
> > backplane, and because there's very little that's on there it's
> > extremely rare for one to fail.
>
> not nessasarily. direct attached doesn't mean in the same chassis,
> external drive shelves attached via SCSI are still DAS
>
> you can even have DAS attached to a pair of machines, with the second box
> configured to mount the drives only if the first one dies.
>
> David Lang
>
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>



--
Harsh Azad
=======================
Harsh.Azad@gmail.com

[Decibel! - Tue at 06:07:44PM -0500]
> It's still in the same chassis, though, which means if you lose memory
> or mobo you're still screwed. In a SAN setup for redundancy, there's
> very little in the way of a single point of failure; generally only the
> backplane, and because there's very little that's on there it's
> extremely rare for one to fail.

Funny, the only time we lost a database server was due to a backplane
failure ...

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On Tue, Sep 11, 2007 at 06:07:44PM -0500, Decibel! wrote:
>On Tue, Sep 11, 2007 at 05:09:00PM -0400, Michael Stone wrote:
>> You can get DAS arrays with multiple controllers, PSUs, etc. DAS !=
>> single disk.
>
>It's still in the same chassis, though,

I think you're confusing DAS and internal storage.

Mike Stone

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