Raid Chunk Sizes for DSS type DB

On Tue, 30 Oct 2007 09:42:37 +0800
Ow Mun Heng <Ow.Mun.Heng@wdc.com> wrote:

It's not an optimal setup but since I only have 3x500G drives to play
with, I can't build a Raid10, so I'm going for Raid5 to test out
capability before I decide on Raid5 vs Raid1 tradeoff. (Raid1 = No
Fault tolerance since 3 drives)

Uhhh RAID 1 is your best bet. You get fault tolerance (mirrored) plus
you get a hot spare (3 drives).

RAID 5 on the other hand will be very expensive on writes.

Joshua D. Drake

Anyway.. I'm trying to figure out the chunk size for the raid. I'm
using 4k chunks since I'm reading that for DSS type queries, lots of
Large Reads, I should be using small chunks. [1] and I've aligned the
disks per [2]

and my stride will 3 for ext3

mdadm --create --verbose /dev/md1 --level=5 --raid-devices=3
--chunk=4 /dev/sdd1 /dev/sde1 /dev/sdf1 mkfs.ext3 -E stride=3 -O
dir_index /dev/md1 mount /dev/md1 /pgsql/ -o noatime,data=writeback

[1] http://wiki.centos.org/HowTos/Disk_Optimization
[2] http://www.pythian.com/blogs/411/aligning-asm-disks-on-linux

Just wondering if there's any suggestions/comments on this from the
PG ppl here.

Thanks for any/all comments.

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On 10/29/07, Joshua D. Drake <jd@commandprompt.com> wrote:

On Tue, 30 Oct 2007 09:42:37 +0800
Ow Mun Heng <Ow.Mun.Heng@wdc.com> wrote:

It's not an optimal setup but since I only have 3x500G drives to play
with, I can't build a Raid10, so I'm going for Raid5 to test out
capability before I decide on Raid5 vs Raid1 tradeoff. (Raid1 = No
Fault tolerance since 3 drives)

Uhhh RAID 1 is your best bet. You get fault tolerance (mirrored) plus
you get a hot spare (3 drives).

RAID 5 on the other hand will be very expensive on writes.

I agree. Note that at least in linux, you can have >2 disks in a
mirror. makes reads faster, writes usually not affected too negatvely

--- On Mon, 10/29/07, Ow Mun Heng <Ow.Mun.Heng@wdc.com> wrote:

(Raid1 = No Fault tolerance since 3 drives)

Raid1 with three drives will have fault tolerance. You will have three disks with the same image. This is triple redundancy. This could greatly improve select performance.

Having said this, I've used software raid5 and am currently using raid10 implemented from PCI ide cards but have had data loss errors occur with both setups. I am not sure if the problem is in the drives, the pci cards, or the software raid setup. (Thank goodness that this is my toy computer.)

However, I've used RAID1 with great success for my OS partitions and haven't had any problems of the last couple of years.

Regards,
Richard Broersma

On 30.10.2007 03:11, Joshua D. Drake wrote:

Ow Mun Heng <Ow.Mun.Heng@wdc.com> wrote:

It's not an optimal setup but since I only have 3x500G drives to play
with, I can't build a Raid10

Uhhh RAID 1 is your best bet. You get fault tolerance (mirrored) plus
you get a hot spare (3 drives).

This is not true with Linux MD RAID.

It might sound scary to most people, but you _can_ have a RAID 10 with
only 3 drives.

http://en.wikipedia.org/wiki/Non-standard_RAID_levels#Linux_MD_RAID_10

Another thing you want to do is to check if the MD device you created
supports barriers. I know MD RAID 1 does, MD RAID 5 does not, I don't
know about MD RAID 10.

If it does not, make sure you have an UPS.

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Regards,
Hannes Dorbath