HSM in a Virtual Tape Environment - Part 2

Yet another entry regarding HSM in a Virtual Tape Environment.

This time we will be adressing the impact of PtP implementations in backup and migrate (ML2) processing.

First of all, let's take a look at a timeline for creating a single-file-single-volume dataset on a PtP volume with copymode settings to imemdiate. In this case an HSM migrate level 2 dataset, but you this is also true for other single-file-single-volume datasets.





The data is first written at the local site. Before completing the O/C EOV (rewind unload) the volume is copied (in compressed format) to the remote VTS. Penalty for creating this 'synchronous' copy is the time required to perform this copy (on average between 40% and 60% of the 'original' job I/O time).

Now what happens if an HSM migrate task results in a dataset spanning multiple (lets assume 2) volumes?

At the point in time where HSM needs to get the second volume a rewind-unload is issued. Analogous to the first example this is followed (due to the immediate copy mode) by the copy to the remote site before this rewind-unload is completed. The time it takes to complete this copy causes a delay for the HSM task. As soon as the copy has completed, the second volume is mounted. At completion of writing to the ML2 dataset another rewind-unload will be issued, followed by another copy process to the remote site.

This results in the 'penalty' for creating the immediate copy is yet again roughly 40%-60% of I/O time.

For non-HSM multivolume datasets specifying unit=(,,2) will create an improvement in elapsetime because the PtP-copy can then be made in parallel to the local write to the second volume (see figure below). Unfortunately HSM allocates only one unit to each migrate tasks. Maybe we will see a SETSYS MAXCONCURRENTML2UNITS in the future giving us the option to fully exploit the VTS capabilities.

As reference a figure displaying the manner in which single-file-multiple-volume tape datasets are being created in a VTS-PtP environment with unit=(,,2) specified:

For the sake of completeness, all the three figures in one handy A4-sized picture. This image can be freely pinned in your cubicle.