Wow, I never figured how many different RAID configurations one could think
After reading lots of material, forums and of course, this mailing-list, I
think I am going for a RAID5 configuration of 6 disks (18Gb, 15.000 rpm
each), one of those six disks will be a 'hot spare'. I will just put the OS,
the WAL and the data one one volume. RAID10 is way to expensive :)
If I understand correctly, this will give great read-performance, but less
write-performance. But since this server will be equipped with an embedded
RAID controller featuring 128Mb of battery-backed cache, I figure that this
controller will negate that (at least somewhat). I will need to find out
whether this cache can be configured so that it will ONLY cache WRITES, not
Also because of this battery backed cache controller, I will go for the ext2
file system, mounted with 'noatime'. I will use a UPS, so I don't think I
need the journaling of ext3. XFS is not natively supported by RedHat and I
will go for the easy way here :)
1 Gb of RAM should be enough, I think. That is about the only point that
almost everyone agrees on :) Do you think ECC is very important? The
server I have in mind does not support it. Another one does, but is is about
1.000 euros more expensive :(
One CPU should also be enough.
As for postgresql.conf settings, I think I will start with the following :
max_connections = 128
superuser_reserved_connections = 1
shared_buffers = 8192
max_fsm_relations = 1000
max_fsm_pages = 100000
wal_buffers = 32
sort_mem = 2048
vacuum_mem = 32768
effective_cache_size = 28672 (this one I'm not sure about, maybe this one
needs to be higher)
random_page_cost = 2
geq0_threshold = 20
This pretty much sums it up. What do you think about this config? It may not
be the fastest, but a server like this will cost about 4750 euros, and that
is including an Intel Xeon 2.4GHz cpu, redundant power supply, WITHOUT the
UPS. Seems very reasonable to me...
----- Original Message -----
From: "Vincent van Leeuwen" <pgsql(dot)spam(at)vinz(dot)nl>
Sent: Tuesday, July 22, 2003 11:40 AM
Subject: Re: [PERFORM] Tuning PostgreSQL
> On 2003-07-22 09:04:42 +0200, Alexander Priem wrote:
> > Hi all,
> > Vincent, You said that using RAID1, you don't have real redundancy. But
> > RAID1 is mirroring, right? So if one of the two disks should fail, there
> > should be no data lost, right?
> Right. But the proposal was a single disk for WAL, without redundancy, and
> argued that wasn't really safe. RAID1 by itself is extremely safe,
> even the safest RAID type there is.
> > I have been thinking some more. 18Gb drives are cheaper than 36 or 72Gb
> > drives. I don't know if I can get the money for this, but how would the
> > following setup sound?
> > Two 18Gb (15.000rpm) disks in RAID1 array for Operating System + WAL.
> > Four 18Gb (15.000rpm) disks in RAID5 array for data.
> Our own testing has shown that a 6 disk RAID-10 array is faster than what
> describe. Of course, this is very much dependant on how much
> you generate (which taxes your WAL more), so your mileage may vary.
> > For the same amount of money, I could also get:
> > Two 36Gb (10.000rpm) disks in RAID1 array for Operating System + WAL.
> > Five/Six 36Gb (10.000rpm) disks in RAID5 array for data.
> It is said that a higher RPM is particularly useful for a WAL disk. So you
> might consider using two 18GB 15K rpm drives for a RAID-1 WAL disk (+OS
> swap), and using 36GB 10K rpm disks in a RAID-5 array if you need that
> > Which would be the best of the above? The one with four 15k-rpm disks or
> > one with five/six 10k-rpm disks?
> > Would these configs be better than all disks in one huge RAID5 array?
> > are so many possible configs with RAID.......
> 15K rpm disks are significantly faster than 10K rpm disks. If your only
> concern is performance, buy 15K rpm disks. If you want more diskspace for
> money, fall back to larger 10K rpm disks.
> I personally think seperate WAL disks are vastly overrated, since they
> shown a big performance gain in our own tests. But as I have said, this is
> extremely dependant on the type of load you generate, so only your own
> can tell you what you should do in this respect.
> About RAID types: the fastest RAID type by far is RAID-10. However, this
> cost you a lot of useable diskspace, so it isn't for everyone. You need at
> least 4 disks for a RAID-10 array. RAID-5 is a nice compromise if you want
> much useable diskspace as possible and still want to be redundant. RAID-1
> very useful for small (2-disk) arrays.
> If you have the time and are settled on buying 6 disks, I'd test the
> - 6-disk RAID-10 array (should perform best)
> - 4-disk RAID-10 array containing data, 2-disk RAID-1 array for WAL, OS,
> - 4-disk RAID-5 array containing data, 2-disk RAID-1 array for WAL, OS,
> - 6-disk RAID-5 array (will probably perform worst)
> Hope this helps.
> Vincent van Leeuwen
> Media Design - http://www.mediadesign.nl/
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster
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