Dispelling myths about VSAN and flash.
I’ve been having the same conversation with several customers lately that is concerning.
Myth #1 “VSAN must use flash devices from a small certified list”
Reality: The reality is that that there are over 600 different flash devices that have been certified (and this list is growing).
Myth #2 “The VSAN certified flash devices are expensive!”
Reality: ” Capacity tier flash devices can be found in the 50-60 cents per GB range from multiple manufacturers. Caching tier devices can be found for under $1 per GB. These prices have fallen from $2.5 a GB when VSAN was released in 2014. I expect this downward price trend to continue.
Myth #3 “I could save money with another vendor who will support using cheaper consumer grade flash. They said it would be safe”.
Reality: Consumer grade drives lack capacitors to protect both upper and lower pages. In order to protect lower cost NAND these drives use volatile DRAM buffers to hold and coalesce writes. Low end consumer grade drives will ignore flush after write commands coming from the operating system, and on power loss can simply loose the data in this buffer. Other things that can happen is meta data corruption (loss of the lookup table resulting in large portions of the drive becoming unavailable) shorn writes (where writes do not align properly with their boundary and loose data as well as improperly return it on read) and non-serialized writes that could potentially file system or application level recovery journals. Ohio State and HP Labs put together a great paper on all the things that can (and will) go wrong here. SSD’s have improved since this paper, and others have done similar tests of drives with and without proper power loss protection. The findings point to enterprise class drives with power loss protection being valuable.
Myth #4 “Those consumer grade drives are just as fast!”
Reality: IO latency consistency is less reliable on writes and garbage collection takes significantly more time as there is less spare capacity to manage it. Flash is great when its fast, but when its not consistent applications can miss SLA’s. If using consumer grade flash in a VSAN home lab, make sure you disable the high latency drive detection. In our labs under heavy sustained load we’ve seen some fairly terrible performance out of consumer flash devices.
In conclusion, there are times and places for cheap low end consumer grade flash (like in my notebook or home lab) but for production use where persistent data matters it should be avoided.