RE: Re[2]: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS

[This is a second re-send to see if we've fixed the weird line-break = problem
I've been having with this list for the past few weeks.]

The "4W" part of the RAID 5 formula is actually true only for RAID 5 = groups
larger than 4 disks. On a 3-disk RAID 5 array, for example, both large writes and small writes will consume only 3W I/O calls (one for each = disk).
The complete formula is actually R + min(4,G)*W. Since most of you have = G>3,
the simplified formula that uses '4' as a constant yields the correct answer.

In the spirit of my prior note, if you use RAID 1, the formula is still = R +
min(4,G)*W (yes, the same one, because RAID 1+0 is actually the same = thing
as RAID 5 with G=3D2). Because for RAID 1+0, the value of G is G=3D2, = the
formula simplifies to R + 2W. My original post neglected to mention the = '2'
in this formula. My apologies.

Note, however, that R + 4W > R + 2W for all values W > 0, which was the point that I had intended to convey originally.

Cary Millsap
Hotsos Enterprises, Ltd.
http://www.hotsos.com
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