Re: partition question for new server setup

>>
>> server information:
>> Dell PowerEdge 2970, 8 core Opteron 2384
>> 6 1TB hard drives with a PERC 6i
>> 64GB of ram
>
> We're running a similar configuration: PowerEdge 8 core, PERC 6i, but we have
> 8 of the 2.5" 10K 384GB disks.
>
> When I asked the same question on this forum, I was advised to just put all 8
> disks into a single RAID 10, and forget about separating things. The
> performance of a battery-backed PERC 6i (you did get a battery-backed cache,
> right?) with 8 disks is quite good.
>
> In order to separate the logs, OS and data, I'd have to split off at least two
> of the 8 disks, leaving only six for the RAID 10 array. But then my xlogs
> would be on a single disk, which might not be safe. A more robust approach
> would be to split off four of the disks, put the OS on a RAID 1, the xlog on a
> RAID 1, and the database data on a 4-disk RAID 10. Now I've separated the
> data, but my primary partition has lost half its disks.
>
> So, I took the advice, and just made one giant 8-disk RAID 10, and I'm very
> happy with it. It has everything: Postgres, OS and logs. But since the RAID
> array is 8 disks instead of 4, the net performance seems to quite good.
>

If you go this route, there are a few risks:
1. If everything is on the same partition/file system, fsyncs from the
xlogs may cross-pollute to the data. Ext3 is notorious for this, though
data=writeback limits the effect you especially might not want
data=writeback on your OS partition. I would recommend that the OS, Data,
and xlogs + etc live on three different partitions regardless of the number
of logical RAID volumes.
2. Cheap raid controllers (PERC, others) will see fsync for an array and
flush everything that is dirty (not just the partition or file data), which
is a concern if you aren't using it in write-back with battery backed cache,
even for a very read heavy db that doesn't need high fsync speed for
transactions.

> But ... your mileage may vary. My box has just one thing running on it:
> Postgres. There is almost no other disk activity to interfere with the
> file-system caching. If your server is going to have a bunch of other
> activity that generate a lot of non-Postgres disk activity, then this advice
> might not apply.
>
> Craig
>

6 and 8 disk counts are tough. My biggest single piece of advise is to have
the xlogs in a partition separate from the data (not necessarily a different
raid logical volume), with file system and mount options tuned for each case
separately. I've seen this alone improve performance by a factor of 2.5 on
some file system / storage combinations.

>
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