Re: Q: Is RAID/striping good with Oracle???
Date: 1996/08/28
Message-ID: <01bb9529$449a9600$100410ac_at_david>#1/1
R Glenn Stauffer <stauffer_at_crabapple.cc.swarthmore.edu> wrote in article
<Random header information snipped>
> Common wisdom has it that there is a significant write penaltly with
Raid5.
> I've also heard some suggest that you will lose the entire raid volume if
you
> lose two drives - I'm not sure that is any more likely than losing both
drives
> in a mirrored set.
>
To write to any point on the disk, the system must write to both the data
disk that the data is bein placed on, and the parity disk. The effect of
this is that you cannot write to two disks (data disks) at once.
As far as the loss of a drive, yes, a RAID 5 array cannot recover from a
two drive failure, as far as how unlikely this is?, well that depends on
how many drives you are using. For example If you use three drives in a
disk array, the probability of an unrecoverable loss is (d^2)/3 where d is
the probability of loss on one drive, by contrast if you are using 4 drives
mirrored (the same amount of data) the probability is approx. (d^2)/2.
Since it can happen on either of two drives. Thus the likelyhood of an
unrecoverable failure is 3/2 times more likely in a 3 drive raid 5 array.
However, for many applications, this probability is so small as to be
insignificant, or at least bearable. However, there are applications where
you want the increased security.
> If you run RAID5 on a transaction system without a caching controller
with a
> fairly large cache, you will see a performance hit. If your system is a
read
> intensive DSS, that shouldn't be as true (of course, Oracle is still
doing
> writes for sorts, etc). I read something when I was configuring our
server
> that you need something on the order of a 40mb write cache on the
controller
> to eliminate most of the RAID5 write penalty. Of course, then you have
to
> weigh the cost of more drives (RAID0+1) vs more costly controller
hardware
> for Raid5.