[smartmontools-support] TBW for SATA disks

[Claudio Kuenzler]

> I had a bit of a search online and I found these scripts which use
> smartctl to calculate a TBW percentage for each disk:
>
>
> https://askubuntu.com/questions/865792/how-can-i-monitor-the-tbw-on-my-samsung-ssd
>
> It would seem that if you get a TBW under 90% your disk probably needs
> to be replaced, at least thats turned out to be the case with my
> Samsung SSDs in a RAIDZ2 pool. Instead of having to use one of these
> scripts or commands it would be better to have this feature integrated
> into smartctl.
>

I agree with you. When you are about to reach the "TBW warranty" limit, it
seems to be a wise idea to replace the drive in question.
There's no hard requirement or suggestion from any vendor though. But
seeing this in real life (
https://www.claudiokuenzler.com/blog/1163/western-digital-green-ssd-dead-without-pre-fail-indication),
a replacement before actually reaching the TBW limit could help run into
serious problems later.
Not sure though if smartctl should automatically (by default) alert when
soon reaching the drive's TBW limit.


> Is TBW calculation/output already present but I've not found the right
> option or is there a better way to check the health of SATA SSDs than
> using one of these TBW scripts?
>

smartctl shows the attribute 241 on most SSDs (so far all I know) but
sometimes the values differ.

Sometimes you get a Total_LBAs_Written. Here on a Samsung SSD:

241 Total_LBAs_Written      0x0032   099   099   000    Old_age   Always
    -       41440294917

Sometimes this is translated into GB, seen on a WD Blue SDD:

241 Host_Writes_GiB         0x0030   253   253   ---    Old_age   Offline
   -       139897

Obviously the TBW calculation needs to be adjusted, depending on what value
is shown.

I am working on implementing a TBW check in check_smart.pl (the monitoring
plugin using smartctl in the background). For this purpose I created an
overview of TBW limits (
https://github.com/Napsty/check_smart/blob/master/SSD-TBW-Warranty.md).
This list can be used as base for the TBW check. I hope I can get to it
rather sooner than later.

cheers,
ck
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