[Info-vax] analyze/disk errors

[Stephen Hoffman]

On 2014-02-07 15:01:37 +0000, tadamsmar said:

> ...in addtion to being available new, the 15K RPM would (I think) speed 
> up system response.

TL;DR: "It depends".

Tuning is an iterative process, and based on data; on finding and 
improving or replacing or tweaking the slowest component.

For instance, maxing out memory can often be an easier and cheaper win, 
as it can increase the (read) cache hit rates and reduces the I/O 
rates, and can (if enabled) reduce write rates with the (optional) 
write caching that can be performed by RMS.

Check the seek times and the transfer rates in the specs, not the RPMs, 
and check the I/O activity and queue depth, and see if you're pushing 
the I/O hard.  Faster seeks are better for smaller transfers and for 
fragmented storage, and higher-RPM disks are better for larger-sized 
transfers.

Switching to SSD and to host cache are usually much better than hitting 
any rotating rust.  Switching to larger I/Os and away from record 
access can be a win, too.

This performance and tuning stuff can rathole pretty easily, too.  For 
some I/O activity patterns, a slower disk with a bigger disk cache will 
beat a faster disk, for instance.

IIRC, the boxes involved are fossil-grade systems, and would have to be 
running fairly close to their respective I/O performance limits to see 
a wall-clock-relevant difference here between a 10K disk with decent 
seek times and a 15K disk with a similar seek time.  It's certainly 
possible you might see an improvement, but I wouldn't automatically 
assume it.  And I wouldn't buy something extra that's not going to be a 
benefit.  Wall clock relevant?  Even if the systems are close to their 
performance limits, I wouldn't spend a whole lot of time and effort to 
upgrade the hardware for an app that runs to completion in 60 minutes, 
versus an upgrade that allow the app to run in 40 minutes, unless that 
difference is business-relevant.

Now if you're considering swapping fossil-grade 15K disks for newer 10K 
disks, that could well be relevant in terns of stability and uptime.

> But, is there any problem with mixing 10K RPM and 15K RPM drives in a 
> shadowset?

You can shadow a RAM disk and a floppy disk, if you wanted to.

> (Except for the obvious fact that the slower disk would determine 
> system response.)

Not entirely true.  It'll determine the maximum write rates, certainly. 
 Reads only need to access multiple member volumes when there's an 
error.

I would encourage getting yourself a formal escalation path here; 
somebody that you can discuss this stuff with.


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