Why Compellent proposes fewer disks
Sorry to see blog posts like this one.
I like your blog and often I read it but, this time I think you have done wrong calculations and the result isn’t very good.
Probably, as per admission, you don’t know very well Compellent’s stuff and you need some help to better understand how it is possible to achieve similar results.
I would like to say in advance that I don’t know this particular deal/customer environment but I’m a Compellent reseller and I work every day to configure systems.
First of all I use 99th and not 95th percentile (Compellent suggests it to me), second I use 177 IOPS per second and not 220 IOPS per second on 15K rpm disks. We are using internal Compellent documents to do the right sizing to be sure to do the right configurations. Compellent is very, very, very – sometimes too picky- in customers data collections (iops, space, latencies, # of servers involved, type of workload and so on) so it seems very strange to me what you are writing.
Compellent, opposed to NetApp, has Fast Track, Data Progression and dynamic block cache and those help to achieve more performance with less resources than competitors, these features works all together to deliver, some time, awesome numbers!
Follow me with my calculations: 11 active disks (I suppose that 12 disks tray means 1 spare) deliver near 2000 raw IOPS but:
it is the ability to write data on the fastest portion of the disk (external tracks, about 20% of the disk space) and obtain more IOPS and less latency (NetApp hasn’t this feature).
If you can read/write data on external tracks you will gain, in practice, something like 15/30% of better performance and very low latencies from your disks (the advantage depends by number of servers and dispersion of writes).
the new total is 2000+20% = 2400 IOPS.
It is automated tiered storage (its not important what you or your company think about it but, if it is well implemented, it gives very useful advantages). Data Progression works on a single tier of disks too also thanks to Fast Track!
So you can write data in RAID10 on Fastest tracks and then they will be migrated according to policies/profiles to RAID5/6 in other portions of disks.
With NetApp you need to choose in advance the type of RAID (RAID 4 or DP to save space or RAID10 to have better performance, but not all on the same volume!).
We don’t add more IOPS here but there aren’t constraints and less performance due to the use of a certain raid!
Dynamic Block Cache
Compellent’s cache is small (512 MB for mirrored writes and 3GB per controller for reads) but it is very flexible and auto tuned! (BTW, I don’t know the NetApp proposition for the deal you speak about, but if it is a FAS 2040 we are speaking of similar amounts of cache).
You can choose for each LUN the cache behavior and the block is dynamic from 2KB to 256KB, it means that the controller allocates the right space needed and not prefixed blocks. In real world it means that you have more space and flexibility if compared to fixed blocks to manage IO peaks.
I hope I cleared at best why Compellent is proposing less disks than competitors in most of cases.
Doubts, questions or comments? Let me know.