Tom Marchant wrote:
On Thu, 31 Aug 2006 22:23:19 -0500, Russell Witt <[EMAIL PROTECTED]> wrote:
.... For those that
never went to virtual-devices, it's a good way to encrypt data. But since
IBM, STK, EMC and CA have all been pushing the advantages of virtual-tape
(and there are so many advantages, its hard not to use it), this offering
leaves a lot to be desired.
IMHO, the only advantage of virtual tape is that it allows ancient
applications to continue to write to tape long after the economics of it
make sense.
...
Since we will never completely eliminate human error (entering bad data,
submitting bad jobs, software bugs, errors in application specification
and design), there will always be a need for point-in-time backups to
allow recovery of databases to multiple prior states. One of the
problems with trying to do this entirely on DASD is the relative ease
with which a file on DASD can be erased by a finger check, versus one on
removable media, plus the high cost of bandwidth to a remote site for DR
coverage (assuming you have the luxury of a hot site for DR). For most
installations, I don't see any thing other than removable media (tape)
being practical for many point-in-time backups.
The major advantage of virtual tape is that it allows you to separate
the issues and economics of upgrading to newer tape technologies with
radically higher tape capacities from issues of application design.
Analyzing large numbers of existing applications to accurately determine
what usages of tape are required to be on removable media for
point-in-time recovery and which were dictated by obsolete size
considerations is an expensive and potentially error-prone task; and
unless you have a static environment that analysis may need to be repeated.
Unless one gets way behind on tape technology, it is invariably the case
that newer tape media is more expensive than the old. A large part of
the economics behind a move to newer tape technology requires that you
be able to effectively use the larger capacity of new media. You can do
this with either a costly analysis and rework of tape applications or by
restricting most usage of the new technology to virtual tape and DFHSM,
which can utilize the new capacity with almost no effort. We find there
are many cases where DFHSM is a poor match for handling
application-level point-in-time backups, which leaves a requirement for
some kind of virtual tape.
While YMMV, the major usage and major growth of our virtual tape usage
(CA-VTape software) has been in support of newer DB2 applications, not
ancient applications. Point-in-time backups of many small related
tables used to require great manual effort by our DBAs to analyze the
size of tables and stack datasets even when we were on 3490E drives, and
invariably changes in retention requirements would cause files to get
inappropriately stacked with improper retention of some files. When we
went to VTape, all these small DB2-related tape datasets, and newly
created ones, were unstacked, substantially increasing the number of
virtual volumes, but saving many man-hours and improving the reliability
of the process. We currently create (and scratch) somewhere in the
neighborhood of 3000 - 4000 virtual volumes daily, and I would estimate
over 80% of these are DB2 related.
One of our larger collections of retained data is DB2 archive log data.
For DB2 dumps of archive logs to effectively utilize a 3590E
cartridge, one would have to allocate a minimum of four, most likely
six, 30 GB DASD archive logs. We much prefer a shorter and more
frequent offload process, which we get with a larger number of smaller
DASD logs (but much less than 180 GB) sized for 3490E with compression.
The on-site copies we put on physical 3490E. The off-site copies,
which are unlikely to ever actually be read, we put to virtual tape, and
with Vtape we aggregate just those datasets together and put the day's
offload to 1 or 2 3590 carts to save off-site vault space and to
eliminate any need to recycle this data as it expires (since all virtual
volumes on a physical tape expire within a day of each other).
Admittedly software virtual tape tends to be more flexible than hardware
solutions, but virtual tape is not just for ancient tape applications.
--
Joel C. Ewing, Fort Smith, AR [EMAIL PROTECTED]
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