Re: Disk upgrade with no downtime - possible?

[Charles Wilt <charles.wilt@xxxxxxxxx>]

Also something to consider.

With different size disks in the same ASP, the system will try to
balance the % used.  Which means that the larger drives will be
written to (and thus read from) more often than the smaller ones.

This can lead to performance issues.

If for example, your current disks average about 20-30% busy and to
are replacing half of them with disks that are twice the size, you
might find yourself hurt as the larger disks exceed the 40% busy
guideline IBM recommends.

I've seen a system with mismatched disk sizes have two larger disks
running at 100% busy and the other 8 smaller ones at 5%.  It was dog
slow!

IBM does provide a way to rebalance (STRASPBAL) the disk according to
usage instead of capacity, but depending on how much you add/delete
you may have to run it periodically.

Have fun!
Charles


On Wed, Mar 11, 2009 at 10:51 AM, Charles Wilt <charles.wilt@xxxxxxxxx> wrote:
> Disks in a RAID set have to be all the same size.
>
> Theoretically, you could stop RAID. (which gives you 1 drive's worth
> of space back).
> Remove a drive, Add a drive
> repeat 4 more times.
> Start RAID.
>
> However, as far as I know you have to do a manual IPL to DST to stop RAID.
>
> So you could limit your downtime, but not eliminate it.
>
> Charles
>
> On Wed, Mar 11, 2009 at 10:29 AM, Pete Helgren <Pete@xxxxxxxxxx> wrote:
> > Seems like I have either asked or seen this before but I am planning to
> > upgrade 5 out of 10 disks allocated to an LPAR which is currently at
> > 86%.  My goal is to hopefully replace the drives while the partition is
> > active so the question is: Can I replace the drives one at a time and
> > just allow RAID-5 to rebuild the drives?
> >
> > The info center for V5R4 says this:
> >
> > "A disk unit that is running with device parity protection can be
> > exchanged only if it has failed. A disk unit running with device parity
> > protection cannot be replaced with a non-configured disk even if it has
> > failed."
> >
> > So it sounds like I *can't* do it.
> >
> > Any pointers here?  Seems like I should be able to just replace each
> > disk one at a time and let then it rebuild before replacing the next
> > one, but having only replaced truly failed drives, I am in uncharted
> > territory.
> >
> > The existing drives are model 4326  type 072 (30GB) and I plan to
> > replace them with 4327 (70GB).  V5R4M0 9406 520
> >
> > Pete
> >
> >
> >
> >
> > --
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