Re: Disk Mangement Advice

From: Phillip Renouf (PhillipRenouf_at_discussions.microsoft.com)
Date: 01/06/05 

Date: Thu, 6 Jan 2005 08:27:02 -0800

I got that backwards, my appology: that'll teach me to make posts right after
coming back from vacation ;)

Writing to a RAID5 set is a much bigger performance hit than writing to a
RAID1 set. In a RAID1 set you are only writing to 2 drives, but in a RAID5
set you are writing to 3+ drives and it also has to calculate the parity
information.

Also, the n-1 does not denote that one drive holds the parity data:
effectively with a RAID5 set the available drive space is effectively equal
to n-1. The parity data is actually spread across all the drives.

Phil

"m.marien" wrote:

>
> "Phillip Renouf" <PhillipRenouf@discussions.microsoft.com> wrote in message
> news:5DA17D2B-6E6D-4CD2-839F-5BCF8527F8DE@microsoft.com...
> > Understanding that there really is no wrong way to do things, I think that
> > this is a bad idea. Running your system drive on a RAID5 array isn't a
> > great
> > option because it offers a slower Read time than a RAID1 array since in
> > RAID1
> > you are reading from 1 disk and in RAID5 you are reading from 3+ drives.
> >
>
> I think that is the whole idea. Reading from multiple drives in parallel
> moves the data n-1 times faster where n is the number of drives in the
> RAID5. It's n-1 because one of the drives just holds the parity.
>
> http://www.pcguide.com/ref/hdd/perf/raid/levels/singleLevel5-c.html
>
> You generally get better then RAID0 reading when you have more than three
> drives.
>
> RAID1 is the slowest read and write. It reads from one drive which isn't any
> better then having a single drive and has to write the data twice. Once to
> each drive. The only thing it offers is redundancy.
>
> The only problem is that you can't boot from a software RAID5, whereas you
> can from a software RAID1. With software RAID1 you are just reading from one
> drive. If you have hardware RAID, then you can boot from any RAID level, as
> the RAID looks like a single drive to the o/s.
>
>
>
>

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