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Re: [PERFORM] Quad processor options - summary

From: Bjoern Metzdorf <bm(at)turtle-entertainment(dot)de>
To: James Thornton <james(at)jamesthornton(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org, pgsql-admin(at)postgresql(dot)org
Subject: Re: [PERFORM] Quad processor options - summary
Date: 2004-05-13 21:53:31
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Lists: pgsql-adminpgsql-performance
James Thornton wrote:

>> This is what I am considering the ultimate platform for postgresql:
>> Hardware:
>> Tyan Thunder K8QS board
>> 2-4 x Opteron 848 in NUMA mode
>> 4-8 GB RAM (DDR400 ECC Registered 1 GB modules, 2 for each processor)
>> LSI Megaraid 320-2 with 256 MB cache ram and battery backup
>> 6 x 36GB SCSI 10K drives + 1 spare running in RAID 10, split over both 
>> channels (3 + 4) for pgdata including indexes and wal.
> You might also consider configuring the Postgres data drives for a RAID 
> 10 SAME configuration as described in the Oracle paper "Optimal Storage 
> Configuration Made Easy" 
> ( Has 
> anyone delved into this before?

Ok, if I understand it correctly the papers recommends the following:

1. Get many drives and stripe them into a RAID0 with a stripe width of 
1MB. I am not quite sure if this stripe width is to be controlled at the 
application level (does postgres support this?) or if e.g. the "chunk 
size" of the linux software driver is meant. Normally a chunk size of 
4KB is recommended, so 1MB sounds fairly large.

2. Mirror your RAID0 and get a RAID10.

3. Use primarily the fast, outer regions of your disks. In practice this 
might be achieved by putting only half of the disk (the outer half) into 
your stripe set. E.g. put only the outer 18GB of your 36GB disks into 
the stripe set. Btw, is it common for all drives that the outer region 
is on the higher block numbers? Or is it sometimes on the lower block 

4. Subset data by partition, not disk. If you have 8 disks, then don't 
take a 4 disk RAID10 for data and the other one for log or indexes, but 
make a global 8 drive RAID10 and have it partitioned the way that data 
and log + indexes are located on all drives.

They say, which is very interesting, as it is really contrary to what is 
normally recommended, that it is good or better to have one big stripe 
set over all disks available, than to put log + indexes on a separated 
stripe set. Having one big stripe set means that the speed of this big 
stripe set is available to all data. In practice this setup is as fast 
as or even faster than the "old" approach.


Bottom line for a normal, less than 10 disk setup:

Get many disks (8 + spare), create a RAID0 with 4 disks and mirror it to 
the other 4 disks for a RAID10. Make sure to create the RAID on the 
outer half of the disks (setup may depend on the disk model and raid 
controller used), leaving the inner half empty.
Use a logical volume manager (LVM), which always helps when adding disk 
space, and create 2 partitions on your RAID10. One for data and one for 
log + indexes. This should look like this:

----- ----- ----- -----
| 1 | | 1 | | 1 | | 1 |
----- ----- ----- -----  <- outer, faster half of the disk
| 2 | | 2 | | 2 | | 2 |     part of the RAID10
----- ----- ----- -----
|   | |   | |   | |   |
|   | |   | |   | |   |  <- inner, slower half of the disk
|   | |   | |   | |   |     not used at all
----- ----- ----- -----

Partition 1 for data, partition 2 for log + indexes. All mirrored to the 
other 4 disks not shown.

If you take 36GB disks, this should end up like this:

RAID10 has size of 36 / 2 * 4 = 72GB
Partition 1 is 36 GB
Partition 2 is 36 GB

If 36GB is not enough for your pgdata set, you might consider moving to 
72GB disks, or (even better) make a 16 drive RAID10 out of 36GB disks, 
which both will end up in a size of 72GB for your data (but the 16 drive 
version will be faster).

Any comments?


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