Re: Poor raid 1 performance?

"Antoine Leca" <root@xxxxxxxxxxxxxxxxx> wrote in message news:439d7aa3$0$29626$636a15ce@xxxxxxxxxxxx
> To make a long story short:

You wish.

>
> In news:43970b2c$1$3384$892e7fe2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx, Folkert Rienstra va escriure:
> > > And my (perhaps faulty) conclusion was that this represented a small
> > > penalty (with respect to e.g. RAID 0,
> >
> > No, you said "non-Raid".
>
> Okay, I found the origin of the misunderstanding.
>
> I was specifically referring to RAID variations:
> ] The difference between RAID 0 and RAID 1 is the distance between one
> stripe
> ] and the next on disk.

There are no stripes on RAID1. You are obsessed with stripes. Get rid of it.

>
> I made a little mistake while replying: I wrote
> ] [...] which should give a small penalty with respect to a non-RAID
> ] configuration (assuming media access is the bottleneck, that is).

> I meant "non-RAID1" but did not make it clear.

Non-RAID1 is not any clearer either.

> It clearly should have been ``contiguous reading'' (or plain ``RAID-0''
> if you prefer), would have been much clearer.

Still makes no sense to me.
There is a huge difference between contiguous reading from a single
physical (ie non-RAIDed) drive and a RAIDed (logical) drive, whether
that's RAID0 or RAID1 with split reads.

> I am sorry for the mistake.
>
> OTOH, I basically assumed a 1-sector stripe (which is not real.)

Right.

> Or that the stripe size is lower than available free cache on the RAID
> controller (and we hope so.)

Cache has nothing got to do with it either except for when speaking of
RAID on the same channel and long sequential reads. Cache-ahead in that
case allows reading data without the read command being issued yet.
When the delayed read command for the second drive finally arrives the
data comes from cache and no extra accesstime is incurred because of that
necessarily delayed command.

>
>
> > Read half of them, get half the throughput.
>
> My point, exactly.

I doubt it. The comment was about RAID1 read in stripe fashion ...

> Since there is two drives, we end up at twice this throughput when data are
> delivered to the main system by the RAID controller, and cannot make better
> while reading sequencially.

.... where this comment is about RAID0. And the RAID controller doesn't do
anything magical, it is a standard controller augmented with special firmware
and drivers. Except for hardware generated checksums everything a RAID
controller can do can be done in software also.

>
> If you compare to a non-RAID setup, RAID-1 might result in a better
> throughput, for example if the connection link between the controller

> and the drive is a limiting factor (ATA33 IIRC).

Nope. Has nothing got to do with it.

>
> However, in the common case where the limiting factor is media access,
> RAID-1 will result in about the same figures as non-RAID when seen from
> outside

Has nothing got to do with that.

> (as you explained long and wide, thanks.)

Only for the continuous benchmark example. Not for the occasional single file read.

>
> OTOH, RAID-0 may deliver an higher throughput, because it can use more of
> the available bandwidth on the connection link.

Nonsense. So can RAID1.
The interface is never a bottleneck, bandwidth wise, unless there is a deliberate
interface mismatch between drives and host interface.

>
>
> Antoine
.

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