Re: [PERFORM] Quad processor options - summary

I see you've got an LSI Megaraid card with oodles of Cache.  However, don't underestimate the power of the software RAID implementation that Red Hat Linux comes with.

We're using RHE 2.1 and I can recommend Red Hat Enterprise Linux if you want an excellent implementation of software RAID.  In fact we have found the software implementation more flexible than that of some expensive hardware controllers.  In addition there are also tools to enhance the base implementation even further, making setup and maintenance even easier.  An advantage of the software implementation is being able to RAID by partition, not necessarily entire disks.

To answer question 1, if you use software raid the chunk size is part of the /etc/raidtab file that is used on initial container creation. 4KB is the standard and a LARGE chunk size of 1MB may affect performance if you're not writing down to blocks in that size continuously.  If you make it to big and you're constantly needing to write out smaller chunks of information, then you will find the disk "always" working and would be an inefficient use of the blocks. There is some free info around about calculating the ideal chunk size. Looking for "Calculating chunk size for RAID" through google.

In the software implementation, after setup the raidtab is uncessary as the superblocks of the disks now contain their relevant information.
As for the application knowing any of this, no, the application layers are entirely unaware of the lower implementation.  They simply function as normal by writing to directories that are now mounted a different way.  The kernel takes care of the underlying RAID writes and syncs.
3 is easy to implement with software raid under linux.  You simply partition the drive like normal, mark the partitions you want to "raid" as 'fd' 'linux raid autodetect', then configure the /etc/raidtab and do a mkraid /dev/mdxx where mdxx is the matching partition for the raid setup.  You can map them anyway you want, but it can get confusing if you're mapping /dev/sda6 > /dev/sdb8 and calling it /dev/md7.
We've found it easier to make them all line up,  /dev/sda6 > /dev/sdb6 > /dev/md6

FYI, if you want better performance, use 15K SCSI disks, and make sure you've got more than 8MB of cache per disk.  Also, you're correct in splitting the drives across the channel, that's a trap for young players ;-)

Bjoern is right to recommend an LVM, it will allow you to dynamically allocate new size to the RAID volume when you add more disks.  However I've no experience in implementation with an LVM under the software RAID for Linux, though I believe it can be done.

The software RAID implementation allows you to stop and start software RAID devices as desired, add new hot spare disks to the containers as needed and rebuild containers on the fly. You can even change kernel options to speed up or slow down the sync speed when rebuilding the container.

Anyway, have fun, cause striping is the hot rod of the RAID implementations ;-)

Regards.
    Hadley


On Fri, 2004-05-14 at 09:53, Bjoern Metzdorf wrote:

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" 
> (http://otn.oracle.com/deploy/availability/pdf/oow2000_same.pdf). 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 
numbers?

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?

Regards,
Bjoern

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