ONTAP Discussions

Quantifying (spindle count:vm throughput) relationship + reallocate

fletch2007
3,351 Views

We added a 3rd DS4243 (24 disk 15K RPM) shelf to our existing aggr.

I ran some benchmarks before and after:

http://www.vmadmin.info/2011/08/more-spindlesmore-vm-throughput.html

Which led me to the whole reallocate volume question.

I've run the reallocate measure and the values returned are recommending reallocation be run on about 1/2 my volumes.

These volumes are 2-4Tb and run 200 VMs on a 3270 cluster

Is it safe to the reallocate live after hours? Or will this cause too much load?

What other things are there to be aware of?

I am looking to optimize the volumes for VM throughput and latency and plan to re-run the benchmark once reallocation is complete

thanks

4 REPLIES 4

chriskranz
3,351 Views

Thats a good question! I would definitely consider staggering the scans so that they don't overlap eachother. However it's difficult to say how long they'll take, on 2-4TB volumes it'll no doubt be hours, but it could stretch through a full night. I would probably be tempted to kick one off on Friday night to see how long it takes. It'll post something to syslog when it's finished.

If you are doing a single volume at a time, and your system is got a good average load of under 70% today, then I wouldn't expect the load to increase more than 5% during the scan. This is also the reason not to run multiple jobs at the same time on a busy system, I have seen it hammer a system when lots (10-20) volumes were processed at the same time.

Once you've completed the baseline, I'd consider creating a schedule to do these regularly, maybe just once a month or once a week if you see a big gain.

fletch2007
3,351 Views

I opened a case to make sure all the issues are covered and to formulate a plan for 1st reallocation then scheduled reallocation jobs.

For instance I read there are free space requirements for volume reallocate jobs.

I'll post our findings once we get some numbers for before and after reallocation.

thanks

ZANDER_MEARS
3,351 Views

I believe that if you have snapshots in the volume and use the -p switch there is no extra space requirements needed, though I've not personal experience of that

cheers

fletch2007
3,351 Views

The outcome of the Netapp case after several iterations with the engineer, was that reallocate was not recommended for deduped volumes!

this was somewhat shocking - we can't have optimal performance with the dedup feature enabled?  Effectively we are penalizing performance and accepting hotspots when we enable dedup?

On a technical level I do understand the tradeoffs, but wish there was a way to get the benefits of dedup without penalizing optimal performance.

I believe the answer will be some SSD or flash solution so the "hotspots" are on low latency SSD/flash...

thanks

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