Does DFSMS have a new facility for "dependent writes" that
somehow chains the writes together? Is it as simple as doing
all the writes in a single channel program with a guarantee
that the perceived updates of the backing store are congruent
with the order of the I/O requests?
As I use the term "dependant writes", it means that the application
issues writes in a specific sequence and does not initiate the next one
until the previous one is confirmed complete.
In database terms, such as DB2, this means that it
1) issues a write to the log to indicate that an update is going to take
place
2) issues one or more writes to the database files to accomplish the update
3) issues a write to the log to indicate that the update is complete
If this sequence is interrupted, such as by a system failure, power
outage, remote link failure, etc, then when DB2 is restarted it can tell
that the update was not completed from the log, and can back it out (or
sometimes complete it). I am no DB2 expert but I understand that this
is the way it works (those who can spell DB2 better than I, feel free to
chime in).
If you can capture your backups at a point of I/O consistency, then when
you restore those backups, the same sort of restart can take place.
I/O consistency means that there is no possibility that you have backups
of the log and database voliumes where #3 was completed, but #2 was not
completed.
With normal backups, and even with normal "instant replication", I/O
consistency cannot be guaranteed. However, EMC, HDS, and now IBM have
mechanisms for guaranteeing I/O consistency when you use "instant
replication" on a set of volumes. In all cases it involves a way of
suspending I/O to all the volumes involved until the replication is
complete (a few seconds).
--
Bruce A. Black
Senior Software Developer for FDR
Innovation Data Processing 973-890-7300
personal: [EMAIL PROTECTED]
sales info: [EMAIL PROTECTED]
tech support: [EMAIL PROTECTED]
web: www.innovationdp.fdr.com
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