Maybe some of the folks here can answer this...

Why doesn't RAID performance scale?

If a 7200RPM disk can write 20MB/s writes, why can't a stripe of 5
give 100MB/s?

If I use a controller with HW parity, surely it should be able to keep
up with a 6 disk RAID 5 volume and perform similar to a non-parity
stripe.

This is not just a single vendor...

I get about 60MB/s on a Sun 3511 array (big bucks)
I get around 45MB/s on a HP P400 (w/battery cache)
I've noticed similar problems on a Dell controller (don't remember the
model#)

In all cases, the controller write cache is enabled of course.

Shouldn't RAID scale?

Robert

Previously (E-Mail Removed) wrote:
> Maybe some of the folks here can answer this...

> Why doesn't RAID performance scale?

It does.

> If a 7200RPM disk can write 20MB/s writes, why can't a stripe of 5
> give 100MB/s?

If it is a RAID1 stripe and the rest of the system can handle 100MB/s
wor mixed device access, you will get that performance.

> If I use a controller with HW parity, surely it should be able to keep
> up with a 6 disk RAID 5 volume and perform similar to a non-parity
> stripe.

No. Some hardware controllers are surprisingly slow.

> This is not just a single vendor...

> I get about 60MB/s on a Sun 3511 array (big bucks)
> I get around 45MB/s on a HP P400 (w/battery cache)
> I've noticed similar problems on a Dell controller (don't remember the
> model#)

First, not only the controller is a bottleneck. Second
is this reading or writing? And what is the performance of the
individual disks, when connected to this contoller?

> In all cases, the controller write cache is enabled of course.

> Shouldn't RAID scale?

As I said, it does. But hardware has kimuts. It does not scale
past these limits.

Here is a speed sample from a 8-way Arcea RAID6:

Read speed 340MB/s. That is 56.6MB/s for the individual disks.
I believe the disks are in the 60MB/s class. Looks like good
scaling to me. Maybe your hardware is PCI or is just over-priced?

Arno

(E-Mail Removed) wrote in news:e3133f6c-32d3-4344-a770-(E-Mail Removed)
> Maybe some of the folks here can answer this...

> Why doesn't RAID performance scale?

It doesn't?

>
> If a 7200RPM disk can write 20MB/s writes, why can't a stripe of 5
> give 100MB/s?

It can't?

>
> If I use a controller with HW parity,

> surely it should be able to keep up

Whatever 'it' is supposed to mean.

> with a 6 disk RAID 5 volume

And why is that?

> and perform similar to a non-parity stripe.

You mean, to RAID0? Didn't you just say that 'RAID' doesn't scale?

RAID5 writes of incomplete strip(e)s (or whatever a full complement of
blocks is called here) also require a read in order to calculate the parity.
This should not showup though in static benchmarks.

>
> This is not just a single vendor...
>
> I get about 60MB/s on a Sun 3511 array (big bucks)
> I get around 45MB/s on a HP P400 (w/battery cache)
> I've noticed similar problems on a Dell controller (don't remember the
> model#)

> In all cases, the controller write cache is enabled

> of course.

Of course.
And now you are surprized that you are measuring the cache performance?

>
> Shouldn't RAID scale?

Doesn't it?

>
> Robert