Re: RAID-1 to RAID-5 online migration?
On Fri, Sep 03, 2004 at 09:28:02AM +0100, Gavin Hamill wrote:
> On Friday 03 September 2004 06:28, Dave Watkins wrote:
> > >After that is done you can delete the old raid1 completly and add the now
> > >free disk to the raid5...
>
> > Ralph Pa?gang wrote: I've actually done this exact thing before and it
> > worked flawlessly.
>
> Ooh you lovely people - thank you for the good news :)
i've done it too, and it works.....but the catch is that it takes a lot longer
to do it this way than to just backup your data, create the new raid from
scratch, and restore.
making a raid array is a very quick operation. so to do it from scratch takes:
whatever time to backup your data, less than a minute to mkraid and mkfs. then
however long it takes to restore your data.
if you have significantly less than 200GB of data (i.e. the size of each disk
in the array), then this will be much quicker than hot-adding a third drive
into a degraded-mode raid-5 array. doing it this way will take: a minute to
mkraid and mkfs the new raid-5, however long to copy your data, and then a long
time to hot-add the third drive.
of course, the advantage is that even though it takes a long time for the
hot-add to complete, it is running in the background so the machine can be up
and running as normal (but slower for the duration).
actually, downtime for both ways of doing it is about the same (time to
mkraid/mkfs and either copy or restore your data). the difference is that
doing it from scratch, the job will be finished as soon as you've restored, but
with array juggling it won't be finished until the entire 200GB drive is synced
with the rest of the array.
FYI, on one of my boxes (P3-933, 512MB RAM) it took about 9 or 10 hours to
hot-add an 80GB drive (seagate barracuda 7200rpm, 8MB cache) in the background.
the machine was running as normal but was quite slow until it finished. the
entire operation worked perfectly.
it wouldn't have taken anywhere near that long to just copy 80GB of data from
one drive to another, so the parity calculations must be really slowing it
down.
craig
PS: i wouldn't recommend software raid 5 if you care about performance. i am
going to convert one of my raid-5 machines (4 x 80GB barracudas) to raid-1 (2 x
200GB barracudas) very soon because i'm unhappy with the performance(*)...if i
had a spare approx $600AUD, i'd buy an IDE raid card with at least 32MB
non-volatile cache memory and that would give me raid-5 with decent
performance, but it's just not worth that much to me for a workstation.
(*) also because it gives me the 4 x 80GB drives to use in other machines :)
--
craig sanders <cas@taz.net.au>
The next time you vote, remember that "Regime change begins at home"
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