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Re: RAID 0+1



On Tue, 21 Mar 2006, Gordon Messmer wrote:

Jeff Vian wrote:
Why not raid 5?  The available space is n-1 for the number of drives
used.

Read and write performance are both significantly better under RAID 10. The only reason to use RAID 5 over RAID 10 is cost. If you can't afford (or justify) RAID 10, then RAID 5 is a lower-cost, lower-performance option.

A single disk problem takes out the entire mirror
copy (3 disks) because of striping.

If you strip sets of RAID 1 devices, then you'll only have to rebuild the disk that fails, when it's replaced.

A data error on one copy and any
other error (even in a different location) on the second copy will take
everything out (again because of the striping effect of raid 0).

Simultaneous errors on multiple drives will take out pretty much any RAID setup.

RAID10 has a statistical resistance to multiple simultaneous failures.

With six drives, RAID10 can withstand 1 failed drive guaranteed, 2 failed drives with an 80% probability and 3 simultaneously failed drives with a probability of 40%. Even with only 4 drives in a RAID10 array, you still get a 67% chance of surviving a two drive failure situation. RAID10 is noticably more robust than RAID5 - which can never withstand more than one simultaneous drive failure.

The same 6 drives in raid 5 would give you 5*73 or 365gb of space and a
single drive failure would not in any way harm your data. The redundant
drive feature would keep everything working while the one drive is
replaced.

But a data error on one drive, and any other error on another drive will prevent you from rebuilding the array, possibly destroying the entire thing. Alarmist, isn't it?

RAID is not a replacement for backups.

Very true. On and offsite.

--
Benjamin Franz

If you can't handle reality, it *will* handle you.


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