Comments on: Host based Striping of LUNs

By: twometrebill

twometrebill — Sat, 25 Mar 2006 11:09:21 +0000

To expand on Tom’s excellent response, the same reasoning applies to reads. Depending on the number of drives in the raid set, you have reduced the number of I/O’s per second by that number of drives.

For example, if you have 4 drives in each raid set then each drive could be performing I/O separately if they were configured as JBOD or RAID 1/10. Configured as RAID 5, they could perform only a single I/O at a time.

While Tom talked about writes, the same is true of reads as well; all the drives are tied up for each single read. Since a typical operational database is 90-99% read intensive, it is easy to see why you would have poor performance with your configuration.

Furthermore, if you choose RAID 1 or 10, some storage subsystems will give you the further advantage of performing reads to the mirrored devices in parallel. You might have two separate transactions trying to read data from the same logical volume. With this feature, the reads could be performed simultaneously, one against each side of the mirror.

Taking this latest example (again with 4 drives in the RAID set), you could theoretically perform 8 times as many I/Os per second with this configuration vs. the one you have deployed.

In conclusion, you should NEVER use RAID 5 for an operational database unless the database is very lightly used and the cost of the storage is more important than the value of better performance.

Bill

By: vijayprasads

vijayprasads — Fri, 24 Mar 2006 11:22:52 +0000

I believe Microsoft suggests RAID 10 for Exchange Setup for better performance. Our benchmarking tests also shows better performance for MSExchange on RAID 10 than RAID 5.

I would like to provide you some more links hoping that you didnt get a chance to have a look at them.

http://www.microsoft.com/technet/prodtechnol/exchange/guides/E2k3Perf_ScalGuide/c1af1369-c3cd-41ca-90f3-372e02ea30e4.mspx?mfr=true
http://www.microsoft.com/technet/prodtechnol/exchange/guides/StoragePerformance/fa839f7d-f876-42c4-a335-338a1eb04d89.mspx?mfr=true
http://www.microsoft.com/whdc/device/storage/subsys_perf.mspx
http://www.dell.com/downloads/global/solutions/Dell_Exchange_SMB.pdf

HTH.

Cheers,
VP

By: vijayprasads

vijayprasads — Fri, 24 Mar 2006 11:22:52 +0000

I believe Microsoft suggests RAID 10 for Exchange Setup for better performance. Our benchmarking tests also shows better performance for MSExchange on RAID 10 than RAID 5.

I would like to provide you some more links hoping that you didnt get a chance to have a look at them.

HTH.

Cheers,
VP

By: tomtreadway

tomtreadway — Fri, 24 Mar 2006 09:42:20 +0000

Ah, I see. From your original mail I incorrectly throught that you were putting both RAID-5′s on the same disks. If they’re on different disks, then you can pretty much ignore everything I said. And putting the RAID-0 in the host is ok.

However, a RAID-1 or 10 will still have drastically better random write performance than RAID-5 or 50.

By: geddesb

geddesb — Fri, 24 Mar 2006 09:09:12 +0000

Thanks for the quick reply Tom, makes sense. I have a question though.

Our LUNS are designed so that: 1 LUN= 1Raid set. i.e if 2 LUNS were striped from a host level the 2 underlying LUNS would be on different disks anyway. (A raid set never contains more than 1 LUN)

Would this change the fact that you prefer raid 10 over host based raid 50 for random access databases?

Thanks