Testing devices for discard support properly

[Ric Wheeler <ricwheeler@gmail.com>]

(repost without the html spam, sorry!)

Last week at LSF/MM, I suggested we can provide a tool or test suite to 
test discard performance.

Put in the most positive light, it will be useful for drive vendors to 
use to qualify their offerings before sending them out to the world. For 
customers that care, they can use the same set of tests to help during 
selection to weed out any real issues.

Also, community users can run the same tools of course and share the 
results.

Down to the questions part:

 * Do we just need to figure out a workload to feed our existing tools 
like blkdiscard and fio?

* What workloads are key?

Thoughts about what I would start getting timings for:

* Whole device discard at the block level both for a device that has 
been completely written and for one that had already been trimmed

* Discard performance at the block level for 4k discards for a device 
that has been completely written and again the same test for a device 
that has been completely discarded.

* Same test for large discards - say at a megabyte and/or gigabyte size?

* Same test done at the device optimal discard chunk size and alignment

Should the discard pattern be done with a random pattern? Or just 
sequential?

I think the above would give us a solid base, thoughts or comments?

Thanks!

Ric