NetApp told how important Flash Storage has become and that classical disks will not die the next few years. They say, classical SAS disks 3.5″ 15k will disappear from market, but get replaced by 2.5″ 10k disks. Because they’re cheaper and faster. If I compare the IOPS, they’re not faster, but may be cheaper.
145IOPS @ 2.5″ 10k
177 IOPS @ 3.5″ 15k
If you’d like to calculate this by yourself like me, you can use this calculator from wmarrow: http://www.wmarow.com/strcalc/
IOPS per GB
Another interesting thing was, why bigger disks aren’t faster than smaller ones. Most people think, high density disks have less read/write head movements and smaller ways to read data from the platter. Yes, it is, but.
A 3.5″ seagate barracuda XT disk with 7200 rpm has an average IOPS of ~75. If you now calculate IOPS/GB, you get 0.036 IOPS per GB. If you compare to a disk that can store only 400GB, you have 0.18 IOPS per GB. Now compared to a faster 2.5″ 10k rpm disk with 400GB (~145 IOPS) , it’s 0.36 IOPS per GB. The smaller 3.5″ is 5x faster and the same size faster rotating, smaller disk 10x faster.
Cache Pool vs. Flash Cache
In a classical NetApp Storage, Flash Cache is used for Controller caching. They use Cache sizes like 12/24/40GB for FAS 2200/3200 Series and up to 3/6/16TB for FAS 6200 Series. Using this huge amount of read cache, the underlying disks are able to write down a lot more write IO’s directly. That’s why NetApp also doesn’t talk about write cache when they’re selling a FAS, they only tell you about read cache in the systems.
By the way, NetApp doesn’t support Storage Tiering. But that’s because they say, the Flash Pools that support the Array is like their top tier in tiering, but works even better than classical tiering.
Flash Pools are disk pools composed of SSDs to support and/or extend the existing Flash Cache. The difference is that you’re allowed to control for which storage pool the cache can be used. This is an advantage as soon as you calculate reqired IOPS for a server LUN by yourself. Here’s an example; Target is 40’000 IOPS and 2TB of disk space is needed.
Example calculation of IOPS with SAS Disks and Flash Cache
However, you’ve also the possibility to create standard disk pools using SSD disk drives. Pay attention and don’t confuse with “Flash Pools” (see above). SSD disk pools can be used to directly assign LUNs to servers with high load, or systems where you cannot accept high cache rewarming time e.g. after a host / os failure. SSD Pools give you low capacity but high read/write performance.
Didn’t heard about that before? Me too. It’s a technology to use SSDs installed at ESX hosts for read caching the FAS arrays. Using a vCenter Plugin, the FAS controls the local SSD disks and puts requested data on it for caching. NetApp sais, that could be even faster than stored on the array itself. And you’ve got more additional, cheap and lots of read cache.
– Flash Accel is the light green part in the picture.
Don’t know for what time this link stays available, I’ve listened to this recorded Webcast [german] using this link:
The Presentation was also available to download as PDF here (written in german):