Re: Desperate with 870 QVO and ZFS

Am 06.04.22 um 16:36 schrieb egoitz@ramattack.net:
> Hi Rainer!
> 
> Thank you so much for your help :) :)
> 
> Well I assume they are in a datacenter and should not be a power outage....
> 
> About dataset size... yes... our ones are big... they can be 3-4 TB easily each
> dataset.....
> 
> We bought them, because as they are for mailboxes and mailboxes grow and
> grow.... for having space for hosting them...

Which mailbox format (e.g. mbox, maildir, ...) do you use?

> We knew they had some speed issues, but those speed issues, we thought (as
> Samsung explains in the QVO site) they started after exceeding the speeding
> buffer this disks have. We though that meanwhile you didn't exceed it's
> capacity (the capacity of the speeding buffer) no speed problem arises. Perhaps
> we were wrong?.

These drives are meant for small loads in a typical PC use case,
i.e. some installations of software in the few GB range, else only
files of a few MB being written, perhaps an import of media files
that range from tens to a few hundred MB at a time, but less often
than once a day.

As the SSD fills, the space available for the single level write
cache gets smaller (on many SSDs, I have no numbers for this
particular device), and thus the amount of data that can be
written at single cell speed shrinks as the SSD gets full.

I have just looked up the size of the SLC cache, it is specified
to be 78 GB for the empty SSD, 6 GB when it is full (for the 2 TB
version, smaller models will have a smaller SLC cache).

But after writing those few GB at a speed of some 500 MB/s (i.e.
after 12 to 150 seconds), the drive will need several minutes to
transfer those writes to the quad-level cells, and will operate
at a fraction of the nominal performance during that time.
(QLC writes max out at 80 MB/s for the 1 TB model, 160 MB/s for the
2 TB model.)

And cheap SSDs often have no RAM cache (not checked, but I'd be
surprised if the QVO had one) and thus cannot keep bookkeeping date
in such a cache, further limiting the performance under load.

And the resilience (max. amount of data written over its lifetime)
is also quite low - I hope those drives are used in some kind of
RAID configuration. The 870 QVO is specified for 370 full capacity
writes, i.e. 370 TB for the 1 TB model. That's still a few hundred
GB a day - but only if the write amplification stays in a reasonable
range ...