[ILUG] Why RAID
des at o2.ie
Mon Jul 19 23:14:34 IST 2004
Apologies for being a bit late to this thread but...
Must agree with the various posts on managing lots of servers and 100s of disks where RAID is a good/necessary thing! Especially after 3+ years when a batch of disks all die in a short period.
RAID 1+0, Paul - if you have 8 disks, RAID 0+1 means 4 striped mirrored to 4 striped. If disk 1 fails you're okay, you read/write off 4 remaining disks (5-8). If disk 2 then fails you're stuffed. RAID 1+0 means as John said disks 1+2, 3+4, 5+6, 7+8 individually mirrored, then striped together. If disks 1, 3, 5, 7 all fail I'm still in business. Is that not more resilient? :-)
RAID 5+1 - say you have 12 x 146GB disks in a hardware R5 array. You want two arrays for capacity and for resiliency in different racks/rooms/power supplies/HBAs. Create 10+1 raid 5 lun in each, with a hot spare in the array. Mirror in software between arrays. If you mirror 12 disks in software the hot spare usually ends up far bigger (e.g. 12x146GB in size). So sometimes this config has a place.
Hardware RAID controllers can have 2 characteristics:
1. offload RAID 5 XOR calculation from host CPU
2. have cache memory (to speed up small, random writes mainly)
- characteristic (2) can be more important.
I once replaced a HP disk with a Micropolis disk of same size for half the price. It failed within a day. It took 6 weeks to not get a replacement and get my (College's!) money back. As you all know, cheaper not always = better.
Other than that, disks are like washing machines, they all fail sooner or later.
Late breaking, repetitive additions...
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