topic Re: LUNs in Operating System - HP-UX

LUNs

lanke_1 — Tue, 14 Jan 2003 18:50:31 GMT

Hi,

What is the best configuration of LUN splits
from a 73G drive either RAID1/0 or RAID5 for an OLTP database.

Any ideas would be greatly appreciated.

Thanks in advance,
Lanke

Re: LUNs

Eugeny Brychkov — Tue, 14 Jan 2003 19:20:20 GMT

RAID 0/1
Simultaneous access to multiple mirrored pairs increases I/O performance. In general, the greater the number of mirrored pairs, the greater the increase in performance.
RAID 5
??? Provides high read throughput for small block-size requests (2 KB to 8 KB);
??? Write performance is limited by the need to perform four I/O operations per write request;
??? Because some I/O operations occur simultaneously, performance depends on the number of disks in the LUN. Additional disks may improve performance;
??? The I/O performance of RAID 5 benefits significantly from write caching.
RAID 0/1 is good for medium/large block size write transfers. RAID 5 is good for small/medium block size read transfers.
So in your situation I would choose RAID5.
You would forget about all these is you use Autoraid-based arrays (VA, EVA) because all these things are managed and optimized by their firmware
Eugeny

Re: LUNs

Tim D Fulford — Tue, 14 Jan 2003 22:56:00 GMT

I would choose RAID1/0.... In stark contrast to the above!!!!

Why...

Well OLTP generally means reading & writing from DB. Furthur than this the writes are "generally" small. updating customer details, placing orders etc. Thus most writes will be RANDOM & SMALL.

A RAID5 or RAID5DP/RAID6 stragtey will mean 1 small write within the stripe will cause the whole stripe to be read & the parity re-calculated then the small write will happen PLUS a parity update/write (or two if it is RAID5DP/RAID6). This will cause very slow write performance as it will cause multiple reads & writes.

IF you can protect against small writes, or are doing mostly reads then RAID5, RAID5DP or RAID6 will be fine.

Large writes on RAID5, RAID5DP or RAID6 are also OK (by large I mean writes that are of a similar size to the stripe, so 8kB stripe width, across 10 disks would mean ~ 80kB). Reads are also OK (small or large) as they will not trigger any parity calculations or

Basically does your OLTP database do small writes? If so avoid RAID5, RAID5DP or RAID6.

One last thisng IF you have VA74xx you can use AutoRAID which SHOULD configure RAID5DP & RAID1/0 to make sure the small writes are less of a problem, but maximise you storage.

Regards

Tim