Why you should avoid placing SSDs in traditional Arrays!
By Anatol Studler on Aug 25, 2008
This is quite surprising to me, as it is comparable to place a 8-cylinder bi-turbo engine with 450HP into an entry level car (try to avoid to use any brands ;-)).
You might ask for an explanation? Here it is:
Traditional midrange arrays are developed to handle hundreds of traditional (15k RPM) harddisk drives. A traditional harddisk is capable of running about 250 IO/s. Now if we compare this with the actual enterprise class Solid State Disks available on the market, a single solid state disk can do about 50k IO/s read or 12k IO/s write. So in fact it is about 100x faster than a 15k RPM harddisk.
The controller of a midrange array system can probably do about 500k IO/s against it's internal cache. So in fact if we place about 10x Solid State Disks into such a storage system, it would simple consume the complete power of the controller. I didn't even start talking about RAID functionality!!!!
There is another major reason that makes such solutions ridiculous! It is the lantency you are adding by using FC networks. While a traditional harddisk works with a latency of about 3,125us (3.1ms), an enterprise class Solid State Disk works with a latency of less than 100us (0.1ms). By using FC, you might loose 1 IO/s with a traditional disk drive by adding the overhead of switches, cable length and array controllers! With a SSD and a latency of less than 100us, the overhead can end up at loosing 10'000 IO/s in the read performance.
So, where do I place the SSD technology?
The answer is simple!
AS CLOSE AS YOU CAN TO THE SERVER!
The best protocol and technology today is SAS (Serial Attached SCSI). The only limitation of SAS is the cable length as it is limited to about 8m, but there is no additional protocol overhead as on FC!
There are two ways to implement SAS attached SSDs.
- Directly in a Server, as most of the servers anyway uses SAS attached internal harddisks.
- Attached via SAS JBOD (Just a Bunch Of Disks) if you need more disks than a server could cover.
Thats where most vendors have to stop as they have no solution or good answer.
Sun's ZFS is exactely the product that is capable of using all the benefits of SSDs in combination with the benefits of traditional storage (DENSITY). Combining the two technologies within one file system provides performance AND density under one umbrella. The magic word is Hybrid Storage Pool.
While the slow part of ZFS (density) remains on traditional fibre channel storage arrays, the important parts (performance) like ZIL (ZFS Intend Log) and L2ARC (Level 2 Adaptive Replacement Cache) remains on SSD technology.