Re: brtfs on top of dmcrypt with SSD. No corruption iff write cache off?

Hi,

"-o discard" is supported, but can have some negative consequences on
performance on some SSDs or at least whether it adds worthwhile performance
is up for debate ....

The problem is, that TRIM can't be benchmarked in a traditional way -
by running one run with TRIM and another one without and later
comparing numbers.
Sure, enabling TRIM causes your benchmarks to run a bit slower in the
first place (as the TRIM commands need to be generated and interpreted
by the SSD, which is not free) - however without TRIM the SSD won't
have any clue what data is "dead" and what needs to be rescued until
you overwrite it.

On my SandForce SSD I had disabled TRIM (because those drives are
known for the good recovery without TRIM), and for now the SSD is in
the following state:

dd oflag=sync if=/some_large_file_in_page_cache of=testfile bs=4M
567558236 bytes (568 MB) copied, 23.7798 s, 23.9 MB/s

233 SandForce_Internal      0x0000   000   000   000    Old_age
Offline      -       3968
241 Lifetime_Writes_GiB     0x0032   000   000   000    Old_age
Always       -       1280


So that drive is writing at 24mb/s and has a write amplification of
3.1 - usually you see values between one and two.
Completly "normal" desktop useage.

- Clemens