Re: Disk cache with shadow sets

From: Bill Todd (billtodd@metrocast.net)
Date: 04/02/03 

From: "Bill Todd" <billtodd@metrocast.net>
Date: Tue, 1 Apr 2003 18:07:06 -0500

"Rob Young" <young_r@encompasserve.org> wrote in message
news:eX5m7ZTgb98z@eisner.encompasserve.org...
> In article <DsScnRlKOOSocBSjXTWcpQ@metrocast.net>, "Bill Todd"
<billtodd@metrocast.net> writes:
> >
>
> >
> > My guess would be that at the point where you're ready to break the
mirror
> > set (after having quiesced applications and flushed dirty data at the
RMS
> > level) you'd want to signal the controller to destage any cached dirty
data
> > to disk.
>
> This step isn't necessary (this was kicked around in the last
> 2 years and yes, I found it). When it goes to read those LBNs it
> doesn't matter if those LBNs are in writeback cache or platter.

It does if you're about to separate some of the disks from that cache and
use them independently (e.g., 'break the mirror' so as to use the broken-out
disks to perform a stable backup) - unless the controller is smart enough to
allow you to break the mirror and then still use the exactly the same set of
blocks of dirty data in the cache both for the remaining (e.g., production)
set of disks and the separated (e.g., backup) set of disks (which gets
especially interesting if one or the other set then wants to update it).

Of course, if there are separate write-back caches for each mirror copy then
the broken-out set of disks carries with it its own copy of the write-back
cache and there's no problem. But not all environments work that way (e.g.,
Symmetrix didn't, though they may have fixed that recently).

- bill

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