RAID 5 Drive Failures 6

[adamroof]

Hey, just a quicky, in a 4 drive(36gb per) Raid 5 array,
What would happen if one drive failed?

The current layout is two partitions on this array, the C: drive and an H: drive

drive C is an 8gb partition with the 2003 OS
drive H is a 94GB partition with user stuff and shares

how does that affect the failure?
there are no other drives on this system.

 

[ShackDaddy]

RAID 5 can run on a missing drive until one more drive fails. Then the whole array is toast.

The key to understanding this is the idea that data really gets written twice when written to a RAID5 array. Once to the drives and once to the parity stored across the array. It's a bit more complex than this, but that's the simple version. It's more difficult to read from parity, but when a drive is missing, that's where data is read from.

Hot spares are great, since when one kicks into action, you have a healed RAID5 on your hands and a bad drive to replace at your leisure, and when replaced, it simply becomes the hot spare waiting for a failure. The array spends very little time reading from parity. The only degraded time is during the period that the hot spare is being initiated into the array.

 

[dangermouse68]

ShackDaddy,

With a hotspare, wouldn't you be able to lose 2 drives? Just not at the same time. The second could be lost after the initial hot spare is spinning.

Can the Hot Spare be written too, in it's spare state? Would this depend on the controller and software? I'm just curious about this, as it would help in reducing the amount of time needed to add to the array.

 

[ShackDaddy]

Sure, Dangermouse, but there's no special RAID type called "RAID5+HS". If you had a three-drive RAID5 array with 10 hot spares, you could lose 11 drives and still be online...

Hot Spare's can't be written to in their spare state, since the parity written to each live drive is unique, it's a custom composite, and the spare has to be ready to fill in for that unique loss. You can't prepare hot spares ahead of time unless you really want to talk about RAID10*, which is building a RAID5 array out of mirror sets. So a "five drive" array would actually have 10 drives, and you'd only get a failure if you had four drives go out that happened to be all in two mirrors.

*Actually RAID 10 is a stripe made out of mirror sets (lacks parity), so that's not quite right, so I'll call it RAID10P. RAID50's already taken: it's a stripe of RAID5 arrays.

ShackDaddy