Bob Friesenhahn <bfrie...@simple.dallas.tx.us> wrote:
> I have heard quite a few times that TRIM is "good" for SSD drives but
> I don't see much actual use for it. Every responsible SSD drive
> maintains a reserve of unused space (20-50%) since it is needed for
> wear leveling and to repair failing spots. This means that even when
> a SSD is 100% full it still has considerable space remaining. A very
> simple SSD design solution is that when a SSD block is "overwritten"
> it is replaced with an already-erased block from the free pool and the
> old block is submitted to the free pool for eventual erasure and
> re-use. This approach avoids adding erase times to the write latency
> as long as the device can erase as fast as the average date write
> rate.
The question in case if SSDs is:
ZFS is COW, but does the SSD know which block is "in use" and which is not?
If the SSD did know whether a block is in use, it could erase unused blocks
in advance. But what is an "unused block" on a filesystem that supports
snapshots?
>From the perspective of the SSD I see only the following difference between
a COW filesystem an a conventional filesystem. A conventional filesystem
may write more often to the same block number than a COW filesystem does.
But even for the non-COW case, I would expect that the SSD frequently remaps
overwritten blocks to previously erased spares.
My conclusion is that ZFS on a SSD works fine in case that the the primary used
blocks plus all active snapshots use less space than the official size - the
spare reserve from the SSD. If you however fill up the medium, I expect a
performance degradation.
Jörg
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