Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)

[Robert Hancock]

On 08/15/2009 11:39 AM, jim owens wrote:
> ***begin rant***
>
> I have not seen any analysis of the benefit and cost to the
> end user of the TRIM or array UNMAP. We now see that TRIM
> as implemented by some (all?) SSDs will come at high cost.
> The cost is all born by the host. Do we get any benefit, or
> is it all for the device vendor. And when we subtract the cost
> from the benefit, does the user actually benefit and how?
>
> I'm tired of working around shit storage products and broken
> device protocols from the "T" committees. I suggest we just
> add a "white list" of devices that handle the discard fast
> and without us needing NCQ queue drain. Then only send TRIM
> to devices that are on the white list and throw the others
> away in the block device layer.

They all will require NCQ queue drain. It's an inherent requirement of 
the protocol that you can't overlap NCQ and non-NCQ commands, and the 
trim command is not NCQ.
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