Re: about TRIM/DISCARD support and barriers

On Sun, 2008-11-23 at 13:39 +0000, David Woodhouse wrote:
We don't attempt to put non-contiguous ranges into a single TRIM yet.

We don't even merge contiguous ranges -- I still need to fix the
elevators to stop writes crossing writes,

I don't think we want to do that ... it's legal if the write isn't a
barrier and it will inhibit merging. That may be just fine for a SSD,
but it's not for spinning media since they get better performance out of
merged writes.

before we can stop discards
from also being barriers. (Discards are just writes, for the purpose of
that conversation).

Perhaps they shouldn't be ... they have different characteristics. For
instance, a discard may cross a read or write that has no sectors in
common with it; a discard may be merged as a non contiguous range
(assuming the drive supports multiple ranges), etc.

I think it might be better to give it its own type for the elevators.

James


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