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Re: udev, MD, and stripes of mirrors



Jussi Silvennoinen wrote:
Out of curiosity, why this raid5 over mirrors instead of raid10? Would't that give you more speed, capacity and fault-tolerance?

speed and performance, yes. fault-tolerance: definitely not.


RAID15 (raid5 composed of raid1s) is probably for the "truly paranoid"
or for storage that simply must *never* fail.

RAID10 would give 25% more capacity, probably about a similar
performance boost over raid15. Writes to raid15 would be 25% larger than
raid10 (X/4 vs X/5) and reads from raid10 would have 25% more spindles
to read the same data from.

However, assuming ten drives, raid10 can't survive the failure of 2
drives 11% of the time (if both drives in a mirror set die, the whole
dataset is gone) and would never survive a failure of more than 5
drives.  raid15, OTOH, can always survive any conceivable 3 drive
failure, and something like 75% of potential 4-drive failures (I'm too
lazy to do the math), and if you're really lucky, might be able to
survive a 6-drive failure.

raid15 and raid51 are quite unusual to see, though.  Amazingly fault
tolerant, high performance (even if not quite as good as raid10 and
raid01)...  rather expensive, though.   Sometimes you'll see a similar
setup with 2 hardware raid5s mirrored via software (hybrid raid51),
which has very similar performance and fault-tolerance (it has lower
odds of surviving a 4-disk failure but the same 3-disk guarantee).
--
Eric Eisenhart <eric eisenhart sonoma edu>
Linux/Unix Systems Administrator
AIM: ericeisenhart, ICQ: 156218985
Sonoma State University, IT


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