Legacy HDS Forums

Difference between a normal and quick pairresync

Discussion created by Legacy HDS Forums on Jan 7, 2008
Latest reply on Dec 15, 2014 by Doug Tokle

Originally posted by: Optic



Hi,

The following explanation of the differences between a normal and quick pairresync is from the ShadowImage manual:

   Normal. The normal pairresync operation (see Figure 2.7) resynchronizes the S-VOL with
the P-VOL. The copy direction for a normal pairresync operation is P-VOL to S-VOL. The
pair status during a normal resync operation is COPY(RS), and the P-VOL remains
accessible to all hosts for both read and write operations during a normal pairresync.
   Quick. The quick pairresync operation (see Figure 2.7) speeds up the normal pairresync
operation by not copying the P-VOL data to the S-VOL right away. Instead, the S-VOL is
gradually resynched with the P-VOL as host updates are performed, when intermittent
copy is scheduled (TagmaStore internal), or when a user issues another pairsplit
command for the pair. The P-VOL remains accessible to all hosts for both read and write
operations during a quick pairresync (same as normal pairresync). The S-VOL becomes
inaccessible to all hosts during a quick pairresync operation (same as normal pairresync).

I have read this over and over, and it means absolutely nothing to me.  I must be very stupid. :)  Can someone explain what it means in English?

The thing I find really strange is that part of the explanation for the "quick" option is "speeds up the normal pairresync operations by *not* copying the P-VOL data to the S-VOL right away.  Instead, the S-VOL is gradually resynched with the P-VOL as host updates are performed".  Huh?

How can not copying the data right away "speed things up"?  What if you aren't doing any host updates?  I am really confused as to how/why this is quicker.  Can someone give a technical explanation of exactly what the difference is?

To further add to my confusion, this paragraph is also from the manual:

Note on Quick Resync/Restore/Split: The performing of the resync/restore/split depends
on the SVP mode setting. For example, the ShadowImage resync operation is performed as a
Quick Resync operation if the Quick Resync mode is enabled on the SVP. SVP mode 87 is used
to turn on Quick Resync, SVP mode 80 is used to turn off Quick Restore, and SVP mode 122 is
used to turn off Quick Split.

Once again - this paragraph means very little to me.  When I choose the "pairresync" option in the ShadowImage GUI both "Normal Copy" and "Quick Resync" are available options in the pull down menu.  Yet the paragraph above suggests that a Quick Resync operation is only performed if you have SVP mode 87 switched on.  I have checked the mode options that are switched on (on the SVP), and option 87 is not on.  Does that mean even if I choose "Quick Resync" when I do a pairresync that it doesn't really do a Quick Resync, because option 87 isn't on?  If it *doesn't* mean this, then what difference does it make whether option 87 is on or not?

Finally - there is a "Copy Pace" option.  Does anyone know how to set the copy pace when using the pairresync HORCM command?  I can't seem to find a command line equivalent to set the copy pace.

Thanks!
David

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