Optimum cluster size for EVA ?

[Hoff]

The internal disk track and sector geometries are now typically synthetic, between the drive's own controller and the particular bus controller.

The cluster size recommendation was received directly from Brian Allison during various of our conversations, and Brian is one of the senior technical folks in OpenVMS I/O Engineering, and charged with getting bits onto and off of disks. Quickly.

Minimally 16, or 32, or better, for the cluster size.

He had indicated it was derived from several issues and considerations, not the least of which were "knees" in the controller performance curves when transfer sizes and transfer rates were compared.

Not all controllers had these "knees", but it was likely enough that your disks would end up connected to one at some point in its existence.

I had gathered that aligned transfers allowed transfer optimization(s) in some of the newer controllers, but did not confirm that. Alpha and particularly Itanium processors are very sensitive to data alignment, so it didn't surprise me that the controllers are (also?) sprouting similar behaviors.

[Jon Pinkley]

I will agree that larger transfers are generally better than small ones, but there is no direct relationship between clustersize and transfer size. There can be secondary effects, for example, clustersize can affect RMS indexed file bucket sizes, and bucket size will affect the QIO transfer size requested by RMS. Also a disk with a small clustersize is likely to get fragmented more quickly, and therefore lead to more split I/O transfers, but the number of blocks transferred by a VMS QIO is not affected by the cluster size; i.e. a QIO to a contiguous file on a disk with a clustersize of 1 will result in the same transfer size as the same QIO to a disk with a cluster size of 16380.

I agree with what Hein wrote in his note from Mar 26, 2008 12:56:49 GMT "Clustersize defines first-block-in-file LBN alignment. No more, no less."

Unless given more than an appeal to authority, I am not convinced that clustersize alone makes much difference at all, at least on a freshly initialized disk. There are good reasons for large cluster sizes, but there are also good reasons for small ones. The blanket statement that the clustersize should be a minimum of 16 for even better 32 should be qualified. As with most things, "it depends" applies.

Jon

[Guinaudeau]

hi,

i am surprised by that "system" :

- the ITRC forum itself

- the VMS community

i do know some of this, and appreciate, in general, and ever already positive surprised.

but today a bit more surprised :

one can add after thread had been closed add-ons. they are eventually worth. although i only overview today these, but hoff especially since he was the speaker two years who brings me to look at the issue.

=> thanks for those add-ons. i ack here the great force of such a community. yes it is.