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

Bryan Donlan
2009-08-13T20:55:00Z

On Thu, Aug 13, 2009 at 4:44 PM, <david@lang.hm> wrote:
> On Thu, 13 Aug 2009, Greg Freemyer wrote:
>
>> On Thu, Aug 13, 2009 at 12:33 PM, <david@lang.hm> wrote:
>>>
>>> On Thu, 13 Aug 2009, Markus Trippelsdorf wrote:
>>>
>>>> On Thu, Aug 13, 2009 at 08:13:12AM -0700, Matthew Wilcox wrote:
>>>>>
>>>>> I am planning a complete overhaul of the discard work.  Users can send
>>>>> down discard requests as frequently as they like.  The block layer will
>>>>> cache them, and invalidate them if writes come through.  Periodically,
>>>>> the block layer will send down a TRIM or an UNMAP (depending on the
>>>>> underlying device) and get rid of the blocks that have remained
>>>>> unwanted
>>>>> in the interim.
>>>>
>>>> That is a very good idea. I've tested your original TRIM implementation
>>>> on
>>>> my Vertex yesterday and it was awful ;-). The SSD needs hundreds of
>>>> milliseconds to digest a single TRIM command. And since your
>>>> implementation
>>>> sends a TRIM for each extent of each deleted file, the whole system is
>>>> unusable after a short while.
>>>> An optimal solution would be to consolidate the discard requests, bundle
>>>> them and send them to the drive as infrequent as possible.
>>>
>>> or queue them up and send them when the drive is idle (you would need to
>>> keep track to make sure the space isn't re-used)
>>>
>>> as an example, if you would consider spinning down a drive you don't hurt
>>> performance by sending accumulated trim commands.
>>>
>>> David Lang
>>
>> An alternate approach is the block layer maintain its own bitmap of
>> used unused sectors / blocks. Unmap commands from the filesystem just
>> cause the bitmap to be updated.  No other effect.
>
> how does the block layer know what blocks are unused by the filesystem?
>
> or would it be a case of the filesystem generating discard/trim requests to
> the block layer so that it can maintain it's bitmap, and then the block
> layer generating the requests to the drive below it?

Perhaps an interface (ioctl, etc) can be added to ask a filesystem to
discard all unused blocks in a certain range? (That is, have the
filesystem validate the request under any necessary locks before
passing it to the block IO layer)