Re: How safe is software RAID compared to how safe hardware RAID is!?

["Eric Wood" <eric@interplas.com>]

3tcdgwg3 wrote:
> Did I do anything wrong?
>
> I didn't see RAID 1 hits multiple drives concurrently, specially for
> reading.
>
> I set up RAID 5, RAID 1 and single disk partitions on x86 machine,
> kernel 2.4-18.
>
> For 800 MB breading, RAID 5 take 2/3 of time used by single disk,
> but RAID 1 take 1.1 of time used by single disk.

Doing anything (reading or writing) with RAID 1 set will always be slower
than with a single drive.  Because even with reading, you're still updating
the atime on both drives.  As a test, mount that partition with noatime and
see if RAID-1 reading test nears to 1.

Here's a nice message from while back:

http://www.cs.helsinki.fi/linux/linux-kernel/2002-17/0313.html
<snip> Jaime Medrano wrote:
> I have taken a look at the read balancing code at raid1.c and I have found
> that when a sequential read happens no balancing is done, and so all the
> reading is done from only one of the mirrors while the others are iddle
<snip>

So RAID-1 sets don't stripe read.  RAID 5 is forced to stripe seek because
of its very nature of having XORed data across the drives.

-eric wood