ADSM-L

Re: Performance/striping/etc.

1998-09-08 09:18:43
Subject: Re: Performance/striping/etc.
From: Anthony Jones <Jones4A AT KOCHIND DOT COM>
Date: Tue, 8 Sep 1998 08:18:43 -0500
I had the misfortune of dealing with this very issue two weeks ago.  I
inherited a system built by AIX novices, and they made raw logical volumes
for the
dB's as well as some storage pool volumes.  While trying to move LV's around
for performance improvements, I was hit with a big surprise.

Since, the LVCB had been overwritten by ADSM, after my "cplv" commands made
each LV was unusable.  I couldn't even move the data from within ADSM, since
the bad LVCB would not let me bring the volume online.   I had to destroy
the LV's and restore the data to them from tape.

In short, if your going to use raw LV's, make sure you skip the first 512
bytes of each LV.

--- Anthony Jones
    Koch Industries, Inc.
    Koch Industries, Inc.

> -----Original Message-----
> From: Gene Mangum [SMTP:gmangum AT UMICH DOT EDU]
> Sent: Friday, September 04, 1998 7:42 PM
> To:   ADSM-L AT VM.MARIST DOT EDU
> Subject:      Re: Performance/striping/etc.
>
> 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
> University of Michigan Medical Center
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