Author Topic: [Warning] Beware of QLC SSDs  (Read 7808 times)

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Offline David Hess

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Re: [Warning] Beware of QLC SSDs
« Reply #25 on: September 05, 2021, 08:29:02 am »
The problem is that if the scrub operation results in a write, and the power fails during the write, then data may be corrupted.  So either the drive needs to have power loss protection or scrubbing has to be disabled before power is removed.

Or the drive must be able to recover from a failed scrub operation.

The drive needs to be able to recover from a failed write which could be triggered by a scrub operation, which short of a RAID type of organization will be impossible, and maybe even that will not be enough.

An interrupted write to a multilevel storage cell destroys any previous data but there is another failure mode which also applies to SLC.   The state machine which controls the write operation can run amok if the supply voltage drops out of tolerance, and attempt to write to completely unrelated pages.  This exact behavior was observed in drive tests.  If those unrelated pages happen to be metadata like the Flash translation tables, then the drive will likely become inoperable.

This is completely separate from protecting data in transit which is stored in buffers.  If only data in transit was at risk, then interrupted write operations would be no different than those on hard drives and modern filesystems handle this just fine by updating metadata as a separate operation.

I do not know why the state machine which controls destructive operations cannot be made proof against power loss however so far all drives with power loss protection rely on sufficient stored energy to complete any write, and presumably erase, operation.  For a while SandForce advertised controllers that did not require a capacitor bank for power loss protection but testing showed that they still destructively failed.
 

Offline mrflibble

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Re: [Warning] Beware of QLC SSDs
« Reply #26 on: September 05, 2021, 08:39:40 am »
... The state machine which controls the write operation can run amok if the supply voltage drops out of tolerance, and attempt to write to completely unrelated pages.  This exact behavior was observed in drive tests.  If those unrelated pages happen to be metadata like the Flash translation tables, then the drive will likely become inoperable.
Okay, that would indeed be bad. Do you have a link to those drive tests?
 

Offline David Hess

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Re: [Warning] Beware of QLC SSDs
« Reply #27 on: September 05, 2021, 05:42:21 pm »
... The state machine which controls the write operation can run amok if the supply voltage drops out of tolerance, and attempt to write to completely unrelated pages.  This exact behavior was observed in drive tests.  If those unrelated pages happen to be metadata like the Flash translation tables, then the drive will likely become inoperable.

Okay, that would indeed be bad. Do you have a link to those drive tests?

Not anymore - they were years ago and nobody seems to like testing drives in this way anymore so there are no newer reports.  They used an external circuit to cut power to the drive under control of the Raspberry Pi which was running the test.

A search turns up this report but I was thinking of a different one:

https://hardware.slashdot.org/story/13/12/27/208249/power-loss-protected-ssds-tested-only-intel-s3500-passes
 

Offline mrflibble

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Re: [Warning] Beware of QLC SSDs
« Reply #28 on: September 08, 2021, 08:50:26 pm »
Do you have a link to those drive tests?
Not anymore - they were years ago and nobody seems to like testing drives in this way anymore so there are no newer reports.  They used an external circuit to cut power to the drive under control of the Raspberry Pi which was running the test.

A search turns up this report but I was thinking of a different one:

https://hardware.slashdot.org/story/13/12/27/208249/power-loss-protected-ssds-tested-only-intel-s3500-passes
I suspect there is a correlation between the amount of bricked SSDs in the wild, and the urge to do power failure tests. That's usually how these things come to be. Large amount of consumer reports of problems with drive XYZ, which motivates someone to dig a little deeper.

And the reverse of that could very well be what we have now. No epidemic of bricked SSDs, so no power fail tests of the sort you mention.

 

Offline amyk

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Re: [Warning] Beware of QLC SSDs
« Reply #29 on: September 13, 2021, 12:53:15 am »
I suspect "industrial" SSDs that use real SLC flash and are basically designed for embedded systems which don't have any real controlled power-off sequence (e.g. a DOS-based PC where you can just wait for the prompt to appear and turn off the power switch right away) will still pass those tests.

Not going to be the fastest nor biggest, and definitely not cheap, but I don't think hundreds of MB/s are really necessary in such applications either, and the ultra-low "seek" time compared to a HDD will already give a good speedup.
 


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