RAID version performance comparisons

I've read this up on the web and want to clarify my ideas - can I run this
past you for comment, please?

This is RAID for use in a server where a UPS guarantees power and an orderly
shutdown. I assume a hardware RAID controller. Write performance I assume
to be asynchronous, i.e. totally managed by the controller so in general it
won't be affected by the number of disks or the RAID style.

What I'm interested in is read performance - I imagine this will have the
most effect on the performance as perceived by users.

Data is stored in stripes across all the disks. I don't know whether a
stripe contains a block from each disk, or a track, or a cylinder; but I
presume a cylinder. It follows that performance is in any case only
improved where the data volume read exceeds one stripe - for smaller volumes
the disk rotational latency will be the overriding consideration.

What I'm after is a general rule of thumb comparing RAID1 and RAID 5, as
follows:

RAID 1 improves disk performance by up to a factor of 2 since a stripe is
completely read after the rotational latency of the second disk.

RAID 5 improves disk performance roughly by a factor of the number of disks
less 1. As above, the stripe contains data from all the disks, and is again
read after the rotational latency of the last disk. But the descriptions
I've seen show that within the stripe, data from 1 disk is parity (so that
no further read operation is required to replace the data from a failing
disk with calculated data). On the assumption that disks don't fail very
often, the performance degradation of having to read data from a second
stripe to get the parity I would have thought would be acceptable - but
there ...

So RAID1 and 2 disks should compare with RAID 5 and 3 disks. Given that a
controller for RAID 5 is generally more expensive than one for RAID 1 it
would seem that RAID 1 represents good value unless you actually need 4 or 5
disks for RAID 5 - even then performance might be improved at better value
by choosing faster SAS disks rather than the slower SATA disks.

Any comments, please?

--
Graham

I'm reposting this because I see already I have a failure to understand the
performance improvement offered by RAID1 ...

I've read this up on the web and want to clarify my ideas - can I run this
past you for comment, please?

This is RAID for use in a server where a UPS guarantees power and an orderly
shutdown. I assume a hardware RAID controller. Write performance I assume
to be asynchronous, i.e. totally managed by the controller so in general it
won't be affected by the number of disks or the RAID style.

What I'm interested in is read performance - I imagine this will have the
most effect on the performance as perceived by users.

Data is stored in stripes across all the disks. I don't know whether a
stripe contains a block from each disk, or a track, or a cylinder; but I
presume a cylinder. It follows that performance is in any case only
improved where the data volume read exceeds one stripe - for smaller volumes
the disk rotational latency will be the overriding consideration.

What I'm after is a general rule of thumb comparing RAID1 and RAID 5, as
follows:

RAID 1 improves disk performance slightly. A stripe is stored on one disk.
The same stripe is stored on the second disk. The stripe data is available
after the rotational latency of the first disk. The virtue of RAID 1 is
reliability i.e. it will continue to work normally if one disk fails.

RAID 5 improves disk performance roughly by a factor of the number of disks
less 1. The stripe contains data from all the disks, and is again read
after the rotational latency of the last disk. But the descriptions I've
seen show that within the stripe, data from one disk is parity (so that no
further read operation is required to replace the data from a failing disk
with calculated data). On the assumption that disks don't fail very often,
the performance degradation of having to read data from another stripe to
get the parity I would have thought would be acceptable - but there ...

Given that a controller for RAID 5 is generally more expensive than one for
RAID 1 the question is what value to put on the increased performance of
RAID 5 ??

Any comments, please?

--
Graham J