Storage – Is It Safe to Use Consumer MLC SSDs in a Server

storage

We (and by we I mean Jeff) are looking into the possibility of using Consumer MLC SSD disks in our backup data center.

We want to try to keep costs down and usable space up – so the Intel X25-E's are pretty much out at about 700$ each and 64GB of capacity.

What we are thinking of doing is to buy some of the lower end SSD's that offer more capacity at a lower price point. My boss doesn't think spending about 5k for disks in servers running out of the backup data center is worth the investment.

These drives would be used in a 6 drive RAID array on a Lenovo RD120. The RAID controller is an Adaptec 8k (rebranded Lenovo).

Just how dangerous of an approach is this and what can be done to mitigate these dangers?

Best Answer

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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