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VDI: Swapping SSD, SAS, and NL-SAS Configurations
Hi, all,
Having a disucssion with a colleague about VDI sizing. We like the ESG whitepaper on VDI architecture. It recommends some VNX configurations with FAST Cache backed by 15K SAS drives.
We are pondering shifting a design to more capacity and replacing SAS with NL-SAS drives. We'll get much more capacity for the users' desktops with the same footprint in the datacenter. Of course we'll lose performance in the rotating disks that will need to be compensated for with a larger cache. Even with that larger cache, there is now a higher risk of performance problems during cache misses.
I am curious if there any guidelines or formulas that would help us in this shift. How much more EFD should we add to account for the loss of IOPS in the rotating disks? What are the risks of designing with a larger cache and slower, larger capacity rotating disks behind them? Maybe the vTexan himself, who provided such a great write-up on this whitepaper, can chime in.
Scott
Tommyt1
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January 24th, 2013 20:00
We usually we don't run into capacity issues when sizing for VDI just because of the number of spindles needed to support the workload. The change to NL-SAS could potentially be a lot of IOPS to have to compensate for - especially since my guess is you would want to run NL-SAS in a RAID 6 config which would add even more IOPS overhead. I guess the best numbers to use would be to size the array the old fashion way - http://www.vtexan.com/2011/09/26/quick-vdi-sizing-howto/ - then according to our whitepaper we can handle about 80% of the IOPS from VNX SP Cache and FAST Cache so we would only need to size the other 20%.of the IOPS in NL-SAS. But 80 IOPS for NL-SAS vs 150 IOPS for 15k is a big hurdle to have to overcome (at decent size scale).
The best thing to do is just work through the numbers - do you have some IOPS per Desktop and number of desktops we could work through ? or better yet - lets just forget about this and sell them a XtremIO box and call it good
Tommyt
@vTexan
orcsab
12 Posts
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January 24th, 2013 20:00
Hey! I first want to say that, as a proud member of EMC's midtier team, EMC a wonderful product shipping today that customers will love for VDI. It's called VNX. XtremIO will rock everyone's world, for sure. But VNX can rock their world today!
I think you touched on a magic number there that can be very helpful. If we assumed 80% of IOs are served by FAST Cache then I can size the NL-SAS drives behind that for the remaining IOPS. Yes, we have detailed measurements for this customer.
Of course, the problem with the 80% assumption is that the hit rates change as the relative size of cache with respect to spinning disks changes. So, I wonder if the hit rate goes down as we add more NL-SAS and the users star to use their space. As they do, IOPS on the rotating disks goes up and performance may falter. Hmm...
Thanks, Tommy!
Sanzone1
16 Posts
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February 11th, 2013 13:00
Scott...I would avoid FAST Cache and NL-SAS together. The timing to take the stuff on NL-SAS up to FAST Cache is going to be problematic. With a workload that is SO sensitive based on the user experience, I don't think you'll get the performance you need.
That 20% that's left over still needs some performance...and the process of promotion from Disk to FAST Cache is going to be way too slow with NL-SAS in the mix.
(Oh, yeah and XtremIO fixes this...and then some)
jim