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Home -> Community -> Mailing Lists -> Oracle-L -> RE: Raid5 Vs Raid0+1 -- Raw Vs Solaris 9 Concurrent Direct IO UFS
> 1. Is 4W figure (in formula above) constant in context of RAID 5 array =
and
> not depend on spindles count? I suspect that it can be constant in any =
> RAID5 implementation. In case of 6 spindles block will be distributed =
as:
> Disk1 =3D> Block1
> Disk2 =3D> Block2
> Disk3 =3D> Block3
> Disk4 =3D> Checksum 123
> Disk5 =3D> Block4
> Disk6 =3D> Block5
> Disk1 =3D> Block6
> Disk2 =3D> Checksum 456
> Disk..
> Is my assumption correct?
First, it's not "checksum", it's "parity", defined as the XOR sum of the
corresponding bits on all the other devices in the group (array). =
Second, if
your group size is G=3D6, then you might see a stripe that looks like =
this
(below), but not the one you show above:
d1 =3D> b1 d2 =3D> b2 d3 =3D> xor(b1,b2,b4,b5,b6) d4 =3D> b4 d5 =3D> b5 d6 =3D> b6
> 2. If we need to change one of 3 data blocks belonging to one RAID5 =
set,=20
> then controller/array/we need to make one write to this particular =
block +
> read 2 remaining block for check sum calculation + write check sum to =
4=20
> block. Do I understand correctly? So for writing one block into RAID5 =
we=20
> need 2W+2R. Or I am wrong?
If you have a G=3D3 RAID 5 array (not many will, but I'll go with the
assumption), then the physical cost of one small write will be 3 I/Os (3
writes). If you have G=3D4, then the cost of one small write will be 4 =
I/Os
(either 4 writes, or 2 reads and 2 writes, whichever the controller
decides). If you have G=3D5, then the cost of one small write will be 2 =
reads
and 2 writes.
To be more thorough than before, if your average I/O (read/write) cost =
is C,
and if you have a read requirement of R reads/sec and a [small] write
requirement of W writes/sec, then the cost of your G-disk RAID 5 array =
is
going to be C * (R + (min(G,4)*W).
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