Re: Desperate with 870 QVO and ZFS

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From: Stefan Esser <[email protected]>
To: [email protected]
Cc: [email protected], [email protected],
 [email protected], Rainer Duffner <[email protected]>
Message-ID: <[email protected]>
Subject: Re: Desperate with 870 QVO and ZFS
References: <[email protected]>
 <[email protected]>
 <[email protected]>
In-Reply-To: <[email protected]>

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Am 06.04.22 um 16:36 schrieb [email protected]:
> Hi Rainer!
>=20
> Thank you so much for your help :) :)
>=20
> Well I assume they are in a datacenter and should not be a power outage=
=2E...
>=20
> About dataset size... yes... our ones are big... they can be 3-4 TB eas=
ily each
> dataset.....
>=20
> We bought them, because as they are for mailboxes and mailboxes grow an=
d
> 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 spee=
ding
> 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 ...

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