Gilad Arnold wrote:
Thanks for your response, Roman.
On Fri, Jun 11, 2010 at 12:52:30AM +0600, Roman Mamedov wrote:
I have been using RAID5 on a desktop machine using a couple of 500
GB drives. My original intent was to grow the array by adding more
drives, as the need arises.
Do you currently run RAID5 with just 2 drives, in degraded mode, or
maybe you meant something else?
No, it is not degraded, it's a clean 2-drive RAID5. I know it doesn't
make much sense as it is ;-) the intent was to grow the array later,
relying on mdadm's grow feature. Right now, I'm guessing that it
operates like a RAID1 for all practical purposes.
I have the feeling that you will then get a lot of read, alter, rewrite
(RAW) operations on the parity chuck, where you would just get writes of
a copy for raid{1,10} configuration. The performance of raid5 is poor,
but with 4-5 drives it gives you some fault tolerance. With even three
drives the write performance is poor by algorithm, and the read
performance is poor by implementation (I see no reason for reading at
the speed of just one drive).
I think your upgrade will fail, but feel free to see if a two drive
raid5 will start with only one drive, I could be wrong.
If it were me, I would leave the raid up, install the new drive, and
just copy the data to it. That way you have error tolerance on the
original data, while your method doesn't. After doing that and reading
the entire new drive to be sure it's valid, configure the two original
drives as a degraded three drive raid10. After that's done AND TESTED to
see that the data are still all valid, then you add the new drive to the
raid10 and get full operation.
Actually, if it were me I'd have a backup, too. And be damn sure to run
on a UPS.
--
Bill Davidsen <davidsen@xxxxxxx>
"We can't solve today's problems by using the same thinking we
used in creating them." - Einstein
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