Is it safe to use consumer MLC SSDs in a server?

A few thoughts;

  • SSDs have ‘overcommit’ memory. This is the memory used in place of cells ‘damaged’ by writing. Low end SSDs may only have 7% of overcommit space; mid-range around 28%; and enterprise disks as much as 400%. Consider this factor.
  • How much will you be writing to them per day? Even middle-of-the-range SSDs such as those based on Sandforce’s 1200 chips rarely appreciate more than around 35GB of writes per day before seriously cutting into the overcommitted memory.
  • Usually, day 1 of a new SSD is full of writing, whether that’s OS or data. If you have significantly more than >35GB of writes on day one, consider copying it across in batches to give the SSD some ‘tidy up time’ between batches.
  • Without TRIM support, random write performance can drop by up to 75% within weeks if there’s a lot of writing during that period – if you can, use an OS that supports TRIM
  • The internal garbage collection processes that modern SSDs perform is very specifically done during quiet periods, and it stops on activity. This isn’t a problem for a desktop PC where the disk could be quiet for 60% of its usual 8 hour duty cycle, but you run a 24hr service… when will this process get a chance to run?
  • It’s usually buried deep in specs but like cheapo ‘regular’ disks, inexpensive SSDs are also only expected to have a duty cycle of around 30%. You’ll be using them for almost 100% of the time – this will affect your MTBF rate.
  • While SSDs don’t suffer the same mechanical problems regular disks do, they do have single and multiple-bit errors – so strongly consider RAIDing them even though the instinct is not to. Obviously it’ll impact on all that lovely random write speed you just bought but consider it anyway.
  • It’s still SATA not SAS, so your queue management won’t be as good in a server environment, but then again the extra performance boost will be quite dramatic.

Good luck – just don’t ‘fry’ them with writes 🙂

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