When is size of RAID-5 set too large

[Victor Semaska_3]

Tried posting this in Storage -> Disk Array but got no response. Since this involves Tru64 I figured I try posting it here. Hope that's OK.

Greetings,

It has been awhile since I had to set up a RAID-5 set since we're mainly using an EVA 3000 for storage. Anyway, the server is a DS25 with Smart Array 5304A running Tru64 V5.1B. There currently is a 5 disk RAID-5 set and we're going to buy another 5 disks.

Question is, is it better to rebuild the RAID-5 set as a 10 disk set or just set up another 5 disk RAID-5 set? Is there a threshold for RAID-5 sets that shouldn't be exceeded? The server is an Oracle database server for report generation so the I/O is mostly reads.

Thanks,
Vic

[Hein van den Heuvel · Accepted solution]

As you add disks to a raid5 set, the risk level increases (double disk failures!), the relative performance increment per disk for reads diminishes and the overhead for writes increases. No worries under 6 or so. Above 20 surely is a problem. The 10 proposed should still be fine.
The EVA actually makes smaller raid-5 redundancy groups out of the large disk groups to deal with this (8 members RSS?).

You may also want to take you shelf configuration into consideration. If you restrict the raid-5 disks to all come from individual shelfs, then you can withstand loosing a shelf and maintain data access.

Google for raid5 for several interesting articles. The wikipedia article seemed only so-so

Good luck,
Hein.

http://www.tomshardware.com/2004/07/23/raid_5_scaling_tests_with_up_to_eight_drives/page4.html

http://www.miracleas.com/BAARF/RAID5_versus_RAID10.txt

http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=PSD_CN0496W&prodTypeId=12169&prodSeriesId=321347&locale=en_US

[Victor Semaska_3]

Hein,

Thanks for the opinion, we'll probably go with the 10 disk RAID-5 set. Also thanks for the links, they're very informative.

Vic