Re: hardware raid 1, 5 question
- From: Paul <nospam@xxxxxxxxxx>
- Date: Thu, 10 Jan 2008 19:23:24 -0500
ToddAndMargo wrote:
Question: in general, when using hardware raid,
is raid 1 any faster, slower or the same as raid 5?
Many thanks,
-T
There are two performance sections written in this
article. A typical motherboard RAID1, would mirror
one drive with a second drive. Only one drive is
read at a time, so performance should be similar
to a single drive. This article mentions a more
complicated mode of operation, but a soft RAID1
may not be using that.
http://en.wikipedia.org/wiki/RAID5
RAID5 performance really depends on a couple things.
Software versus hardware controller. A hardware controller card
has a cache DIMM, an XOR chip (as an IOP perhaps) for parity
calculation, and a battery backup for the cache DIMM contents.
(Better still, put the computer on a UPS, so the RAID5 can
go through an orderly shutdown during a power failure.)
The hardware controller card has the advantage, that when a
short write is done, the read-modify-write needed to
correctly write it out, can be queued for later, when
an idle moment is available. The existence of the cache
DIMM makes that possible. I/O operations can be reordered
for best disk usage.
If the hardware controller card has an IOP that can compute
the parity block to be written, that reduces the work that the
host processor must do.
In the case of an Nvidia chipset based motherboard, someone
made an interesting discovery. They got a big speedup in
performance, by realigning where the first data is written.
Now, I don't believe this, in a way, that the Nvidia software
wouldn't have done a proper layout in the first place, but
the author of the posting did some benchmarks to back up his
claim. So, it is possible, if using certain chipset (soft)
RAID5 setups, to get better performance, but you'd have to be
pretty geeky to pull it off. I'd sooner just buy a proper
hardware controller card, if doing RAID5. It costs a few
more bucks, but is less likely to let you down in an emergency.
I have run into a few posters, who had problems with RAID5
made from three disks. If you are going to do RAID5, I would
recommend using four disks. For some reason, a degraded
RAID5 with three disks, would not boot. Rather than take
a chance, I'd try four disks in the RAID5 instead (at least,
if the RAID was also being used to boot the computer).
Ideally, a RAID user should test that the hardware plus software
really works. You should learn how to replace a failed RAID1
mirror, or replace a bad RAID5 drive, *before* the array has
real data on it. I don't know how many times a frustrated user
posts, wanting an *exact* recipe as to what to do. A person
helping here, is not in a very good position to give a
guaranteed correct recipe to a hapless user. You should buy
one extra drive while buying the others, so you have a spare
of exactly the right capacity, to do maintenance in the future.
The extra drive will come in handy while doing your maintenance
experiments. Don't copy real data onto the array, until your
education is complete.
Neither RAID1 nor RAID5 is a backup scheme. Consider the following
scenario. Your ATX power supply fails. It puts +15V onto the 12V
rail used by the disk drive motors. All disk drive motors are
burned. Now, your redundant array is no longer redundant. All
data is lost. You've been warned... You still need to do backups,
to yet more backup disk disks or to tape storage. Unplug the backup
disks and put them in a safe place.
Paul
.
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