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Oh, c'mon - *all we're really good at* is suggesting hyperoptimized
solutions that you can't actually do! :)<br>
<br>
It's funny you should mention that because I was just working with a
Dell PE 2950 matching your description. <br>
<br>
One thing I noticed: on the 2950 in question, the drives in slots 4
and 5 were on a different controller channel than the ones in slots
0 through 3; that's a compartmentalization you may be able to
exploit. <br>
<br>
First, if no one really cares how fast this PostgreSQL server is,
then leaving it as a 6-drive RAID5 as your /dev/sda is Good Enough
(tm); it is the least cumbersome and most futureproofed path you
could take. But if you want to improve matters, I think the
organizing principle you need to start from is "Keep heavy write
away from RAID5."<br>
<br>
Given that some write on RAID5 may be inevitable, if you want to
improve that, you want more drives, not fewer. With n drives, every
nth write is a parity write, so the bigger n is, the less write
bandwidth difference there is between n and n-1. So I'd look at
your PostgreSQL and OS disk footprints, where the heavy write takes
place, and the size of your drives into consideration, determine
just what it is you want to improve, and go from there. <br>
<br>
If it turns out that the heaviest write is to the archive logs, I
would probably RAID1 the pair of drives on their own controller
channel in slots 4 and 5 and put the logs there, leaving the DB
data and the OS on the RAID5, rationale being that OS operations
aren't going to be tying up disk very much taking care of its own
business. <br>
<br>
And if you want to grease the wheels past that, well, you need to go
off into 3xRAID1, RAID1 plus RAID10/01, etc., disk space willing, or
add drive enclosures or other things you say you don't want to do.<br>
<br>
- Jeff<br>
<br>
On 10/27/11 11:54 AM, Lightner, Jeff wrote:
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<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Question:<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">For large (~1 TB) Postgresql DB is it
better to have archive logs on same RAID set as the DB to
maximize spindles for the DB or to setup two separate RAID
sets thereby reducing spindles for the DB?<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">We currently have a Dell PowerEdge with
PERC (LSI OEM) RAID Controller. Initially it was setup
with all 6 internal disks in the same RAID5 set.
<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Our DBAs believe that having archive
logs on same disks as DB is a problem so want us to redo
the layout so that we have 1 x 2 disk mirror for the OS
and archive longs and a 1 x 4 disk RAID 5 for the DB.
This effectively reduces the spindles for DB from 6 to
4. I’m wondering if the performance implications of such
a reduction would outweigh the data integrity setup.<o:p></o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"><o:p> </o:p></span></font></p>
<p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Please do NOT suggest installing more
disks, or attaching to an external array. These aren’t
options for this system. Also please don’t talk about
the importance of doing backups I already know that.
Finally don’t talk about whether RAID 5 is a good idea – I
already know the pitfalls. My questions is not to talk
about how it could be done better with more/different
equipment but rather what is best with the equipment
described above.<o:p></o:p></span></font></p>
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