I stand corrected, and yet stand firmly. I was wrong, but I'm still right. ;)
I looked at the volume manager manual (I've got a 2.5 version from 1998),
and you are indeed correct. They have been using the snapshot term for a
while, and have been using it to mean an additional copy of a volume that
can be split off for backup. At the same time, they have what they call
file system snapshots, which are standard, copy on write, snapshots.
I would submit that their use of the term snapshot for volume copies is
confusing and should be dropped. The rest of the industry uses snapshots
to imply copy on write type setups. Even vendors from diverse industries
use this term in this way. Compaq (a SAN vendor), NetApp (a NAS vendor),
BMC (a sofware vendor), all have products that are copy-on-write based, and
they refer to them as snapshots. (Who did it first is irrelevent. It's
what the rest of the industry is doing that's important.) Due to all of
this information, I defined a snapshot in my book, Unix Backup and
Recovery, as copy-on-write. The reason that snapshot works so well for
this is that a snapshot ISN'T a copy of the volume, and more than a
snapshot of your face is a copy of your face -- but it looks just like your
face did when the snapshot was taken.
When the rest of the industry is using snapshot to mean one thing, AND
VERITAS is using snapshot to mean one thing (when they're talking about
file system snapshots), then using it in another sense in another one of
their products is confusing. They should drop the use of the term snapshot
when referring to full copies. The question is what to call this thing
that they're now calling a volume snapshot.
I'm writing a second book now that will include a discussion on the
difference between snapshots and split mirror type backups. There doesn't
seem to be a good, industry-standard, term for the additional mirror. I've
been using "third mirror," but that doesn't fit everyone. Hitachi, for
example, doesn't require RAID 1+0 or 0+1 on the primary side. They can use
RAID 5. So calling the additional mirror that gets split off a "third"
mirror is not very accurate. However, everyone seems to know what a third
mirror is when you say that term. I therefore may be forced to use it as
the generic term.
At 10:34 AM 7/10/2001 -0700, Roger B.A. Klorese wrote:
>On Tue, 10 Jul 2001, W. Curtis Preston wrote:
> > All snapshots are copy on right (or act that way), or they're not a
> > snapshot. That's the definition of a snapshot. If it's a full copy, then
> > it's a copy.
>
>Trust me -- the feature called "snapshot" in Volume Manager is *not*
>copy-on-write. It is merely an altered semantic for mirror-breaking.
>
>The "definition" of a snapshot is "any point-in-time feature you choose
>to call a snapshot." Volume Manager used the term for this
>non-copy-on-write semantic before BCV's existed and before NetApp was
>NetApp.
---
W. Curtis Preston
Principal Consultant for Storage Designs, your storage experts
Webmaster: http://www.backupcentral.com Phone: 760 710 7017
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