Something that I have seen popping up multiple times now is the discussion around VSAN and spindles for performance. Someone mentioned on the community forums they were going to buy 20 x 600GB SAS drives for their VSAN environment for each of their 3 hosts. These were 10K SAS disks, which obviously outperform the 7200 RPM SATA drives. I figured I would do some math first:
- Server with 20 x 600GB 10K SAS = $9,369.99 per host
- Server with 3 x 4TB Nearline SAS = $4,026.91 per host
So that is about a 4300 dollar difference. Note that I did not spec out the full server, so it was a base model without any additional memory etc, just to illustrate the Perf vs Capacity point. Now as mentioned, of course the 20 spindles would deliver additional performance. Because after all you have additional spindles and better performing spindles. So lets do the math on that one taking some average numbers in to account:
- 20 x 10K RPM SAS with 140 IOps each = 2800 IOps
- 3 x 7200 RPM NL-SAS with 80 IOps each = 240 IOps
That is a whopping 2560 IOps difference in total. That does sound like an awe full lot doesn’t it? To a certain extent it is a lot, but will it really matter in the end? Well the only correct answer here is: it depends.
I mean, if we were talking about a regular RAID based storage system it would be clear straight away… the 20 disks would win for sure. However we are talking VSAN here and VSAN heavily leans on SSD for performance. Meaning that each diskgroup is fronted by an SSD and that SSD is used for both Read Caching (70% of capacity) and write buffering (30%) of capacity. Illustrated in the diagram below.
The real question is what is your expected IO pattern? Will most IO come from read cache? Do you expect a high data change rate and as such could de-staging be problematic when backed by just 3 spindles? Then on top of that, how and when will data be de-staged? I mean, if data sits in write buffer for a while it could be the data changes 3 or 4 times before being destaged, preventing the need to hit the slow spindles. It all depends on your workload, your IO pattern, your particular use case. Looking at the difference in price, I guess it makes sense to ask yourself the question what $ 4300 could buy you?
Well for instance 3 x 400GB Intel S3700 capable of delivering 75k read IOps and 35k write IOps (~800 dollars per SSD). That is extra, as with the server with 20 disks you would also still need to buy SSD and as the rule of thumb is roughly 10% of your disk capacity you can see what either the savings are or the performance benefits could be. In other words, you can double up on the cache without any additional costs compared to the 20-disk server. I guess personally I would try to balance it a bit, I would go for higher capacity drives but probably not all the way up to 4TB. I guess it also depends on the server type you are buying, will they have 2.5″ drive slots or 3.5″? How many drive slots will you have and how many disks will you need to hit the capacity requirements? Are there any other requirements? As this particular user mentioned for instance he expected extremely high sustained IOs and potentially full backups daily, as you can imagine that could impact the number of spindles desired/required to meet performance expectations.
The question remains, what should you do? To be fair, I cannot answer that question for you… I just wanted to show that these are all things one should think about before buying hardware.
Just a nice little fact, today a VSAN host can have 5 Disk Groups with 7 disks, so 35 disks in total. With 32 hosts in a cluster that is 1120 disks… That is some nice capacity right with 4TB disks that are available today.
I also want to point out that a tool is being developed as we speak which will help you making certain decisions around hardware, cache sizing etc. Hopefully more news on that soon,
** Update, as of 26/11/2013 the VSAN Beta Refresh allows for 7 disks in a disk group… **