Questions concerning fstrim and online discard.

[Andreas Hartmann <andihartmann@01019freenet.de>]

Hello!

Please, may I ask you some questions because I'm a little bit confused
about the behaviour of fstrim and online discard (ext4)?


I'm using the following configuration / partitioning with a SSD
(Controller: SF-2281; Corsair Force GT 240GB):

- dm_crypt: cryptsetup luksOpen - version 1.4.1, device-mapper
  version 1.02.75
- lvm version 2.02.96
- ext4, option discard; kernel version 3.4.11

All seems to work fine, I can't see any errors in messages or in dmesg
according trim / discard.

Now, I tested fstrim from util-linux version 2.19, and surprisingly got
this unexpected error:

fstrim: /: FITRIM ioctl failed: Operation not supported

Why do I get this error? Online discard always seemed to work fine (= I
didn't get any error).

I detected, that there is another necessary option to get a
working fstrim:
cryptsetup luksOpen must be run with option --allow-discards.


Now my questions:
- Did online discard work without --allow-discards? If not, why didn't I
  get any error message?
- Is there a difference between online discard and fstrim, means,
  fstrim doesn't work without --allow-discards, but online discard does?
- fstrim / always says after each reboot:
  /: 1701429248 bytes was trimmed
  This can't be true as the root partition contains mainly root and
  nothing else (no /var or /usr or even /opt). There has been no
  changes in fs since last power on / off cycle and fstrim during the
  last cycle.
  Doesn't fstrim remember the already trimmed regions? Does it trim
  always the complete unused regions?
- Which trim should be used with a SF-2281 controller concerning
  endurance? fstrim or online discard?


Thank you very much for your advice!
Kind regards,
Andreas