2011-08-22 11:04 PM
We added a 3rd DS4243 (24 disk 15K RPM) shelf to our existing aggr.
I ran some benchmarks before and after:
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
2011-08-23 01:56 AM
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.
2011-08-23 03:37 PM
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.
2011-10-07 07:02 AM
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
2011-10-07 07:37 AM
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...