Raid Configuration

[Matt Morgan]

On 07/29/2004 02:20 PM, Si Jones wrote:

> Hi All,
>
> Could someone tell me which one is faster out of mirror or striped 
> software raid in FC, in there experience?
>
> Cheers
>
> Simon Jones
>
A lot of people are saying mirrors are faster for read, and stripes are 
faster for write, which is mostly true. BUT, if you have, say, a 
database with zillions of tiny chunks of data, mirrors may be faster for 
write as well.

The issue is that seek time is generally faster with mirrors, because 
you just read the first copy you find; and stripes write smaller 
portions of the file simultaneously, so a stripe set of five disks, for 
example, can write a big file in roughly 1/5 the time (if your bus is 
fastest enough). But when you're writing a very small chunk of data, the 
seek time is proportionately more significant so for a tiny write job, a 
mirror set can outperform a stripe set.

So, at least theoretically, mirrors can be faster for writing in some 
cases. Whether that is often, rarely, or never true in the real world, I 
can't tell you. I have had Oracle gurus tell me they use mirrors 
specifically for this reason for the right kind of data. I don't know if 
they're right. All I know for sure is, we use RAID 5 for everything 
because it's safer, and pretty much any disk is faster than the network :-).

--Matt