Re: suggestions for postgresql setup on Dell 2950 , PERC6i controller

On Wed, Feb 4, 2009 at 11:45 AM, Rajesh Kumar Mallah
<mallah(dot)rajesh(at)gmail(dot)com> wrote:
> Hi,
>
> I am going to get a Dell 2950 with PERC6i with
> 8 * 73 15K SAS drives +
> 300 GB EMC SATA SAN STORAGE,
>
> I seek suggestions from users sharing their experience with
> similar hardware if any. I have following specific concerns.
>
> 1. On list i read that RAID10 function in PERC5 is not really
> striping but spanning and does not give performance boost
> is it still true in case of PERC6i ?

I have little experience with the 6i. I do have experience with all
the Percs from the 3i/3c series to the 5e series. My experience has
taught me that a brand new, latest model $700 Dell RAID controller is
about as good as a $150 LSI, Areca, or Escalade/3Ware controller.
I.e. a four or five year old design. And that's being generous.

> 2. I am planning for RAID10 array of 8 drives for entrire database
> ( including pg_xlog) , the controller has a write back cache (256MB)
> is it a good idea ?
> or is it better to have 6 drives in HW RAID1 and RAID0 of 3 mirrors
> in s/w and leave 2 drives (raid1) for OS ?

Hard to say without testing. Some controllers work fine with all the
drives in one big RAID 10 array, some don't. What I'd do is install
the OS on a separate drive from the RAID controller, and start
benchmarking the performance of your RAID controller with various
configurations, like RAID-10, RAID-5 and RAID-6 (assuming it supports
all three) and how it behaves when the array is degraded.

You may well find that your machine is faster if you either run the
controller in JBOD mode and do all the RAID in the kernel, or with a
mix, with the RAID controller running a bunch of RAID-1 mirrors and
the OS building a RAI(D)-0 on top of that.

With larger arrays and busy dbs I usually always put the OS and
pg_xlog on either a single mirror set or two different mirrorsets.
Whether or not this will be faster for you depends greatly on your
usage scenario, which I don't think you've mentioned. For
transactional databases it's almost always a win to split out the
pg_xlog from the main array. Unless you have a LOT of disks, a single
RAID-1 pair is usually sufficient.

> 3. Is there any preferred Stripe Size for RAID0 for postgresql usage ?

You'll really have to test that with your controller, as on some it
makes a difference to change it and on others, the default setting is
as good as it ever gets.

> 4. Although i would benchmark (with bonnie++) how would the EMC
> SATA SAN storage compare with locally attached SAS storage for the
> purpose of hosting the data , i am hiring the storage primarily for
> storing base base backups and log archives for PITR implementation.
> as retal of separate machine was higher than SATA SAN.

That really depends on how the SAN is implemented I'd think. I only
have a bit of experience with storage arrays, and that experience
hasn't been all that great in terms of performance.

--
When fascism comes to America, it will be the intolerant selling it as
diversity.