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Re: SSD optimization on Debian (2014)

On Wed, 09 Jul 2014, Jochen Spieker wrote:
> >   * The "discard" options is not needed if your SSD has enough
> >     overprovisioning (spare space) or you leave (unpartitioned) free
> >     space on the SSD.
> >     See http://www.spinics.net/lists/raid/msg40866.html
> AFAIU, this discussion only relates to "online trim" (using the
> "discard" mount option). Running an occasional fstrim on your
> filesystems is almost free and, apart from RAID issues, I don't see any
> negative consequences. Am I missing something?

Yes.  Broken TRIM implementations, and with a recent kernel, broken QUEUED
TRIM implementations.

In both cases, you get filesystem corruption and data loss.

> What's "significant"? I remember seeing graphs on anandtech.com about

It depends on the load.  It needs overprovisioning for:

1. write performance  (large-block writes)
2. garbage collection (defrag erase blocks)
3. spare flash for sector remapping
4. error correction information

Both (1) and (2) are heavily dependent on what you're doing to the SSD, how
fast you're doing it (i.e. whether it has time to do some garbage collection
or not), and how good the SSD firmware is.

As you wrote, one can "extra overprovision" any non-crap SSD by leaving
untouched space (typically by never partitioning some of the SSD in the
first place).

Also, the less spare space a SSD has available, the more aggressively it
will have to garbage collect.

> If you use TRIM (either using the discard mount option or using fstrim
> regularly), your usable spare area is manufacturer spare area plus free
> space on your filesystem. If you don't use TRIM (and don't keep
> unpartitioned space), your spare area is only as large as what the
> manufacturer intended it to be.


  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh

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