On 2011-Jul-26 17:24:05 +0800, "Fajar A. Nugraha" <w...@fajar.net> wrote: >Shouldn't modern SSD controllers be smart enough already that they know: >- if there's a request to overwrite a sector, then the old data on >that sector is no longer needed
ZFS never does update-in-place and UFS only does update-in-place for
metadata and where the application forces update-in-place. This means
there will generally (always for ZFS) be a delay between when a
filesystem "frees" (is no longer interested in the contents of) a
sector and when it overwrites that sector. Without TRIM support, a
SSD can only use "overwrite" to indicate that the contents of a sector
are not needed. Which, in turn, means there is a pool of sectors that
the FS knows are unused but the SSD doesn't - and is therefore forced
to preserve.
Since an overwrite almost never matches the erase page, this increases
wear on the SSD because it is forced to rewrite unwanted data in order
to free up pages for erasure to support external write requests. It
also reduces performance for several reasons:
- The SSD has to unnecessarily copy data - which takes time.
- The space recovered by each erasure is effectively reduced by the
amount of rewritten data so more time-consuming erasures are needed
for a given external write load.
- The pools of "unused but not erased" and "erased" ("available")
sectors are smaller, increasing the probability that an external
write will require a synchronous erase cycle to complete.
>- allocate a "clean" sector from pool of available sectors (part of
>wear-leveling mechanism)
As above, in the absence of TRIM, the pool will be smaller (and more
likely to be empty).
>- clear the old sector, and add it to the pool (possibly done in
>background operation)
Otherwise a sector could never be rewritten.
>It seems to be the case with sandforce-based SSDs. That would pretty
>much let the SSD work just fine even without TRIM (like when used
>under HW raid).
Better SSDs mitigate the problem by having more "hidden" space
(keeping the "available" pool larger to reduce the probability of a
synchronous erase being needed) and higher performance (masking the
impact of the additional internal writes and erasures).
If TRIM support was available then the performance would still
improve. This means you either get better system performance from
the same SSD, or you can get the same system performance from a
lower-performance (cheaper) SSD.
--
Peter Jeremy
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