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

[Ric Wheeler]

Chiming in here a bit late, but coalescing requests is also a good way 
to prevent read-modify-write cycles.

Specifically, if I remember the concern correctly, for the WRITE_SAME 
with unmap bit set, when the IO is not evenly aligned on the "erase 
chunk" (whatever they call it) boundary the device can be forced to do a 
read-modify-write (of zeroes) to the end or beginning of that region.

For a disk array, the WRITE_SAME with unmap bit when done cleanly on an 
aligned boundary can be done entirely in the array's cache. The 
read-modify-write can generate several reads to the back end disks which 
are significantly slower....

ric

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