On Sat, Jan 16, 2021 at 10:39:51AM +0300, Andrei Borzenkov wrote:

15.01.2021 06:54, Zygo Blaxell пишет:

quoted

On the other hand, I'm in favor of deprecating the whole discard option
and going with fstrim instead.  discard in its current form tends to
increase write wear rather than decrease it, especially on metadata-heavy
workloads.  discard is roughly equivalent to running fstrim thousands
of times a day, which is clearly bad for many (most?  all?) SSDs.

My (probably naive) understanding so far was that trim on SSD marks
areas as "unused" which means SSD need to copy less residual data from
erase block when reusing it. Assuming TRIM unit is (significantly)
smaller than erase block.

I would appreciate if you elaborate how trim results in more write on SSD?

The areas are not only marked as unused, but also zeroed.  To keep the
zeroing semantic, every discard must be persisted, thus requiring a write
to the SSD's metadata (not btrfs metadata) area.


Meow!
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