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I'm looking at getting a Crucial M225 SSD. Now the question that's on my mind, is is it worth RAIDing these things? In fact are there any RAID downsides particular to SSDs that you don't see with typical magnetic HDDs.

For 10% more, I can get 2x 128GB drives over the single 256GB drive. Now bare in mind that the 256GB drive has got about 5-10% better performance than the 128GB one.

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up vote 8 down vote accepted

I would definitely get the single, larger drive. It's unlikely that RAID-ing two smaller drives will provide a useful performance increase, and the TRIM command (which helps preserve an SSD's performance over time) probably won't work.

The SSD's controller should also use any free space to help maintain performance and maximise the life of the drive (by minimising the write amplification ratio). With two smaller drives you start out with a lower potential to take advantage of this.

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I second this. Every SSD manufacturer's firmware I've seen to date still has bugs in it when it comes to any kind of RAID setup. –  churnd Nov 1 '09 at 23:20
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With 2 drives, RAID 0 would be your best and pretty much only option, considering your aim of keeping disk space.

However RAID 0 reliability is roughly estimated as Mean_Time_To_Failure / Number_Disks, which means on your case, half the MTTF of your 128Gb SSD. Not good.

Furthermore, while you may get double the performance on a RAID 0 scenario under optimal conditions, this will only happen rarely on reads and writes smaller than the stripe size and with evenly distributed blocks. Depending on what you do with your machine you would have to play around with the stripe size between (32Kb, 64Kb, 128Kb, 256Kb or even 512Kb) to get the best of your RAID. Too much may seem good, but will mean less evenly distributed blocks. Too little and you will only rarely benefit from increased performance.

Get the one big disk. SSDs are already a very good improvement over platter hard drives:

  • You get a guaranteed 5-10% performance boost with no questions asked
  • One disk is much simpler to administer than a RAID
  • Your disk reliability is increased over a RAID 0
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Normally RAID-0 is risky. With spinning disks you essentially cut your expected time to failure (ie, lifetime) in half.

However, with SSDs, I believe that two disks in RAID-0 should last as about as long as a single SSD of equivalent capacity. Ex: two 64GB SSDs in RAID-0 versus a single 128GB SSD. The lifespan of a SSD is essentially a factor of the number of write cycles. With RAID-0 each drive in the array should see half the number of write cycles. This is in contrast to two spinning drives in RAID-0. While each spinning drive would see only half the amount of writes, each drive would spend the same out of time spun up and the read/write head would see about the same amount of mechanical wear.

As far as I know no studies have been done on this. So this is informed speculation and not fact.

However it's likely that you won't be able to use TRIM support on your RAID'ed SSDs. This may or may not be an issue. Intel's second-gen drives show no performance decrease even when TRIM support is absent and OCZ has a newer firmware that enables garbage collection that doesn't rely on TRIM support. So you may not miss TRIM.

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