ADSM-L

Re: Performance/striping/etc.

1998-09-04 20:41:42
Subject: Re: Performance/striping/etc.
From: Gene Mangum <gmangum AT UMICH DOT EDU>
Date: Fri, 4 Sep 1998 20:41:42 -0400
On Fri, 4 Sep 1998, Peter Gathercole wrote:
> Paul Hilchey wrote:
> > If you mirror your disk storage pool volumes, you want to use JFS.
> > Using AIX mirroring with a raw logical volume is bad because you
> > will clobber the mirror synchronization data that is kept in the
> > first bit of the logical volume.
>
> I'm not sure I agree about the mirror synchronisation being clobbered. I think
> that the mirror synchronisation is contained on the outer edge of the physical
> volume, not the logical volume itself. I do not believe that anything you
> write to the logical volume can damage the mirror synchronisation.

Paul is referring to the LVCB, the Logical Volume Control Block, which
occupies the first 512 bytes or so of the space within the LV.   It's
a shame that IBM chose to put this info inside the LV, but why isn't
ADSM (it *is* still IBM, after all) smart enough to skip over it like
databases usually do?

You can read the LVCB with "getlvcb -AT lvname".   The LVCB has the
eyecatcher "AIX LVCB" at the front, so that LVM can tell if it's been
overwritten.

Even if the LVCB is corrupted, AIX still mirrors the data, since the
ODM still has the number of copies.   You may get in trouble if you
export the volume group and import it on another system, though, since
the importing system will read the LVCBs to get this info.

--
Gene Mangum
Gene Mangum
University of Michigan Medical Center
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