Re: Latest advice on SSD?

On Thu, Apr 12, 2018 at 8:11 AM, Mark Kirkwood
<mark(dot)kirkwood(at)catalyst(dot)net(dot)nz> wrote:
> The 512 Gb model is big enough that the SLC cache and performance is gonna
> be ok. What would worry me is the lifetime: individual 512 Gb 850 EVOs are
> rated at 150 Tb over 5 years. Compare that to the Intel S3710 - 400 Gb is
> rated at 8 Pb over 5 years. These drives are fast enough so that you *might*
> write more than 4x 150 = 600 Tb over 5 years...
>
>
> In addition - Samsung are real cagey about the power loss reliability of
> these drives - I suspect that if you do lose power unexpectedly then data
> corruption will result (no capacitors to keep RAM cache in sync).

I have done a lot of pull-the-plug testing on Samsung 850 M2 drives as
a side effect of a HA demo setup. I haven't kept any numbers, but on a
tiny database with a smallish 100tps workload I am seeing data
corruption in about 1% of cases. Things like empty pg_control files,
sections of WAL replaced with zeroes and/or old data. OS level write
cache tuning is not enough to get rid of it.

Based on that and the fact that interrupting SSD garbage collection
might also cause data loss, my recommendation is to either avoid
consumer drives for important databases. Or if you are adventurous
have multiple replicas in different power domains and have operational
procedures in place to reimage hosts on power loss.

--
Ants Aasma
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26, A-2700 Wiener Neustadt
Web: https://www.cybertec-postgresql.com