RE: Re[2]: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS
- From: "Cary Millsap" <cary.millsap@xxxxxxxxxx>
- To: <Oracle-L@xxxxxxxxxxxxx>
- Date: Sun, 12 Sep 2004 19:59:01 -0500
[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.
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