Martin Gregorie  wrote:
> Its somewhat unclear to me what exactly TRIM is meant to improve when
> used on an SMR drive: I can see that it would be a big help if it could
> defragment files etc, but surely that can't be done at the drive level
> since it implies some knowledge of the filing system's logical structure?

On SMR, writing is expensive because you have to rewrite the whole track
(and overlapping ones I think).  TRIM means the drive has a view of what
data is valid and what isn't.  That means the drive won't have to rewrite
data that isn't valid.  It can aggregate writes into empty tracks so that it
minimises the amount of rewriting it has to do - if the drive is 'empty' it
doesn't need to bother at all, just fill up the empty space.

Like SSDs, it also means a more dynamic mapping of block addresses to
physical location - rather than classical cylinder/head/sector addressing
where there was a quasi-static mapping from block to location (unless you
had sectors with errors), here that mapping gets permuted on a much more
regular basis.

That's for Drive-Managed SMR.  For Host-Managed SMR the filesystem does
this, being better placed to optimise the process.

Theo

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