Re: Ext4: batched discard support

[Ric Wheeler <rwheeler@redhat.com>]

On 04/19/2010 01:58 PM, Greg Freemyer wrote:
> On Mon, Apr 19, 2010 at 12:30 PM, Eric Sandeen<sandeen@redhat.com>  wrote:
>> Greg Freemyer wrote:
>>> Adding Mark Lord in cc.
>>>
>>> He wrote a preliminary discard solution last summer.  I'm not sure how
>>> it has progressed.
>>
>> The difference here is that Mark's stuff wasn't as tightly integrated
>> with the kernel, IIRC.  What I saw was more at a user level - make a big
>> file, map it, discard all the blocks, unlink the file.
>>
>> It was a good first step, but I think we can do a lot better by using
>> fs-specific calls to be efficient&  targeted about the discards.
>>
>> Christoph has a similar approach for XFS, FWIW.
>>
>> -Eric
>
> I haven't looked closely at this patch, but I recall Mark consolidated
> numerous discontinuous trim/discard/unmap ranges into a single command
> to the SSD drive.
>
> That was why he felt he was getting superior performance.  ie. There
> was an overhead per command to the drive that was eliminated if a
> single more complex command with multiple ranges went to the SSD
> drive.
>
> But he's the one that did the work and the benchmarking, so I'll let
> him take it from here, especially if I mis-understood what he was
> doing.
>
> Greg

This work certainly builds on Mark's early results which clearly showed that 
several devices see a win from doing larger/batched discards instead of the fine 
grained ones.

Thanks!

Ric