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Re: RAID Suggestion for webserver

> so a 4 x 40GB drive raid-5 array would give either 120GB (no hot-spare)
> or 80GB (1 hot-spare)

Yeap... thats why I said both RAID 10 and RAID 5 in this case would give
80G usable.

> > > The question becomes... which provides more performance and is more
> > > reliable?

> raid 10 is slightly more reliable in theory (but only if you're lucky).
> in practice...who knows? probably about the same.

Seems about the same... but RAID 5 will allow any disk to die the 2nd time
because the hot spare was used (only during reconstruction will the array
not tolerate a second failure), while with RAID 10 you have 25% chance
that the 2nd disk dying is the one you cannot have die (the one in the
same stripe, if i am not mistaken).

> raid5 is faster than raid10, which should be obvious when you think
> about it.  raid0 (stripe) is fast, but raid1 (mirror) is very slow...it
> is certainly much slower to write the same data to two drives than it is
> to write to the data to one drive plus a parity calc & write.  with
> raid10, you end up with a striped (double-speed) mirror (half-speed),
> for no net speed gain over a single drive.  it doesn't get any faster if
> you add more disks, either.
> raid5 on the other hand stripes data and parity over all disks, so the
> more disks in the array the faster it gets.

Okay, so performance wise RAID 5 will be faster than RAID 10.

> i did some benchmarks last year with an 8 drive scsi hardware raid
> system to determine what configuration would provide the best
> performance.   i tested different raid configurations, different
> filesystems (xfs, reiserfs, ext2), and different usage patterns
> (simulations of file-server, mail-server, database-server etc type
> loads)
> like you, i expected raid10 to be the fastest because that's what most
> of the info on the net says.  nope.  i was surprised to find that raid5
> was significantly faster than raid10.
> FWIW, i don't think it's possible to generalise about raid performance
> and be at all accurate.  it depends very much on whether you have
> hardware or software raid AND on whether your raid controller has a
> large *non-volatile* write-cache.

3ware hardware IDE raid. Their performance benchmarks show them to have
some of the highest RAID 5 performance, but they didn't compare RAID 10
performance with other brands so I don't know how they stack up against

> without the non-volatile write-cache, raid5 is very slow.  with it, it
> is very fast.

I would assume in this case that the 3ware card would have at least
comparable performance. 3ware boasts that it has very good RAID 5
read/write algorithms, but I won't really know till i actually use it ;-)

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