topic Re: average I/O size ? in Operating System - OpenVMS

average I/O size ?

Charlie Powell — Fri, 26 Sep 2003 16:07:56 GMT

We are currently running OpenVMS v7.3-1 using HSZ80 disk controllers.

We can obtain much I/O info from the Monitor utility, but how does one determine the average disk I/O size?

Thanks,
Charlie

Re: average I/O size ?

Ian Miller. — Mon, 29 Sep 2003 04:05:47 GMT

there has been tools which intercepted disk I/O and recorded information such as size of request. See for example EPM on Freeeware V5. I'm not aware of a standard tool.

Re: average I/O size ?

Hein van den Heuvel — Mon, 29 Sep 2003 09:55:28 GMT

There really should be an MB/sec indicator somewhere in the standard tools no... but best I know there isn't. I would likely be maintained in the UCB (along with UCB$L_OPCNT), but I don't see it there. [ANAL/SYS... SHOW DEV DKxxx... READ IODEF ... FORMAT ]

Some performance tools (PSM, CA, PAWZ,... ?) will report this somehow.

Anyway, In 25 years of VMS performance work I have had very little use for it. The bottleneck has always been IO/sec, not MB/sec because the average IO size has always been low 4KB.

I believe theat the average IO size is drifting upwards 10KB? with RMS sequential
file buffers growing, RMS index file buckets being chosen larger, backups,... and I believe MB/sec can and will become an bottleneck.

It looks like the new and expanded SHOW MEMORY/CACHE/FULL also puts the focus on IO/sec. Hmmm... I'll ask the engineer over lunch for details.

You may be stuck using Hardware Monitor tools like VTDPY on the HSZ to get the KB/io numbers.

Cheers,
Hein.

Re: average I/O size ?

Hein van den Heuvel — Mon, 29 Sep 2003 15:09:39 GMT

Actually, while the base VMS does not keep track of IO sizes, the XFC does a great job in this space.

Check out
HELP SHOW MEM /CACHE
SHOW MEM/CACHE/FULL
SHOW MEM/CACHE/VOL=xxx

You'll even find nice histograms!

Hein

Re: average I/O size ?

Bruno Ganino — Tue, 30 Sep 2003 07:45:44 GMT

Try
ANALYZE /SYSTEM
(invokes the System Dump Analyzer SDA)
then see command SHOW.
Bruno

Re: average I/O size ?

Howard Taylor — Tue, 02 Dec 2003 00:16:14 GMT

There is a utility in the HSZ80 called DSTAT which will give you (among other things) the average I/O transfer size. Simply type RUN DSTAT at the HSZ80> prompt during a busy period. I have attached Ken Bates' manual.

There is a Windows analysis program (I think also written by Ken Bates) which can take the output of a long run of DSTAT and give you some meaningful stats. If your support staff won't send it to you, I can, but it's too big to post here.