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An SSD is internally a RAID0 that can be up to 16 wide (that's how they get their performance). Erm. Actually they get their performance because they write to

by moe 16y ago

An SSD is internally a RAID0 that can be up to 16 wide (that's how they get their performance). Erm. Actually they get their performance because they write to NAND instead of spinning platters. SSD controllers are also quite a bit smarter than plain RAID0. RAID0 works very well with good quality drives. A RAID0 over three disks has about 1/3 the MTBF of a single disk. That can still be a worthwhile trade-off if you need the extra capacity, but if you're mostly after performance and reliability then a pair of SSDs, or even a single SSD, is the better choice.

macemoneta 16y ago
You'll probably find this interesting reading on the internal architecture of SSDs: http://www.denali.com/wordpress/index.php/dmr/2010/02/02/ssd-interfaces-and-performance-effects Also, while RAID0 reduces the MTBF, it's not linear. Drive life is not magically shortened as a result of the drive being in a RAID array (if you take care to isolate synchronous vibration). The life of the array is equal to the shortest drive life. In other words, if a drive would have failed after 25,000 hours in standalone operation, it will still fail in 25,000 hours in an array. The other drives may run to 100,000 hours, but it's a "weakest link" failure mode.
- moe 16y ago
  Also, while RAID0 reduces the MTBF, it's not linear. Well, it is inverse proportional. The life of the array is equal to the shortest drive life. Erm. To be clear: Your risk of having a RAID0-set (over 3 disks) fail during a given timespan is 3 times higher than having a single-disk-"set" fail in the same timespan. In other words, if a drive would have failed after 25,000 hours in standalone operation, it will still fail in 25,000 hours in an array. That calculation makes no sense. If you have a single drive then that will fail, on average, after 25k hours. If you stripe over three of these drives then your array will, on average, fail after 8333 hours.
  - macemoneta 16y ago
    While the probability of failure is nearly a function of the number of drives, the MTBF/MTTF calculations do not work that way. For example, if there were a probability of 5% that the disk would fail within three years, in a three disk RAID0 array, that probability of failure would be: P=(1-(1-.05)^3)=.14263 In other words, 14.3% probability of failure within three years. That doesn't mean it will fail in that time frame. It means if you have a large population of that configuration, that is the rate you would be dealing with for drive replacement planning. The MTBF and MTTF calculations apply to populations of drives (e.g. a given model) not to a given drive. The values provide no predictability for the failure of any specific drive. Using the values for that purpose is a common misapplication. A drive with a MTTF of 1,000,000 power-on hours can fail in 15 minutes or never during its useful life. As a result, a three drive array will have a higher probability of failure over a given interval, but the MTTF/MTBF of the drives is essentially unchanged. Think of it this way... The probability of winning the lottery is one in 20,000,000. The probability that someone (anyone) will win the lottery in a given week may be one out of ten - 10%. In other words, some person wins the lottery, on average, one time in ten weeks. That doesn't mean that your probability of winning the lottery is 10%. It also doesn't mean that the average probability of winning the lottery is 10%. It also doesn't change the probability of winning the lottery; it's still one in 20,000,000, even if three people win in a 10 week interval.
    - moe 16y ago
      Hm. Thanks for repeating what I just said, I guess. But what was your point again?
      - macemoneta 16y ago
        tl;dr: For RAID0 arrays there is a non-linear increase in the probability of failure, but the MTTF/MTBF doesn't change much.
        moe 16y ago
        Could it be you're just arguing for arguments sake? My original point was: A RAID0 over 3 disks is about 3 times more likely to fail than a single disk running standalone. Fail means "total data loss". You confirm that point with your own math, yet still seem to be trying to argue that there was no difference. Sorry, that makes no sense to me.

by moe 16y ago

macemoneta 16y ago

You'll probably find this interesting reading on the internal architecture of SSDs: http://www.denali.com/wordpress/index.php/dmr/2010/02/02/ssd-interfaces-and-performance-effects Also, while RAID0 reduces the MTBF, it's not linear. Drive life is not magically shortened as a result of the drive being in a RAID array (if you take care to isolate synchronous vibration). The life of the array is equal to the shortest drive life. In other words, if a drive would have failed after 25,000 hours in standalone operation, it will still fail in 25,000 hours in an array. The other drives may run to 100,000 hours, but it's a "weakest link" failure mode.

moe 16y ago
Also, while RAID0 reduces the MTBF, it's not linear. Well, it is inverse proportional. The life of the array is equal to the shortest drive life. Erm. To be clear: Your risk of having a RAID0-set (over 3 disks) fail during a given timespan is 3 times higher than having a single-disk-"set" fail in the same timespan. In other words, if a drive would have failed after 25,000 hours in standalone operation, it will still fail in 25,000 hours in an array. That calculation makes no sense. If you have a single drive then that will fail, on average, after 25k hours. If you stripe over three of these drives then your array will, on average, fail after 8333 hours.
- macemoneta 16y ago
  While the probability of failure is nearly a function of the number of drives, the MTBF/MTTF calculations do not work that way. For example, if there were a probability of 5% that the disk would fail within three years, in a three disk RAID0 array, that probability of failure would be: P=(1-(1-.05)^3)=.14263 In other words, 14.3% probability of failure within three years. That doesn't mean it will fail in that time frame. It means if you have a large population of that configuration, that is the rate you would be dealing with for drive replacement planning. The MTBF and MTTF calculations apply to populations of drives (e.g. a given model) not to a given drive. The values provide no predictability for the failure of any specific drive. Using the values for that purpose is a common misapplication. A drive with a MTTF of 1,000,000 power-on hours can fail in 15 minutes or never during its useful life. As a result, a three drive array will have a higher probability of failure over a given interval, but the MTTF/MTBF of the drives is essentially unchanged. Think of it this way... The probability of winning the lottery is one in 20,000,000. The probability that someone (anyone) will win the lottery in a given week may be one out of ten - 10%. In other words, some person wins the lottery, on average, one time in ten weeks. That doesn't mean that your probability of winning the lottery is 10%. It also doesn't mean that the average probability of winning the lottery is 10%. It also doesn't change the probability of winning the lottery; it's still one in 20,000,000, even if three people win in a 10 week interval.
  - moe 16y ago
    Hm. Thanks for repeating what I just said, I guess. But what was your point again?
    - macemoneta 16y ago
      tl;dr: For RAID0 arrays there is a non-linear increase in the probability of failure, but the MTTF/MTBF doesn't change much.
      - moe 16y ago
        Could it be you're just arguing for arguments sake? My original point was: A RAID0 over 3 disks is about 3 times more likely to fail than a single disk running standalone. Fail means "total data loss". You confirm that point with your own math, yet still seem to be trying to argue that there was no difference. Sorry, that makes no sense to me.