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

[Chris Worley]

On Fri, Aug 14, 2009 at 6:30 PM, Greg Freemyer<greg.freemyer@xxxxxxxxx> wrote:
> On Fri, Aug 14, 2009 at 8:19 PM, Chris Worley<worleys@xxxxxxxxx> wrote:
>> On Fri, Aug 14, 2009 at 5:45 PM, Matthew Wilcox<matthew@xxxxxx> wrote:
>>> On Fri, Aug 14, 2009 at 05:21:32PM -0600, Chris Worley wrote:
>>>> Sooner is better than waiting to coalesce.  The longer an LBA is
>>>> inactive, the better for any management scheme.  If you wait until
>>>> it's reused, you might as well forgo the advantages of TRIM/UNMAP.  If
>>>> a the controller wants to coalesce, let it coalesce.
>>>
>>> I'm sorry, you're wrong.  There is a tradeoff point, and it's different
>>> for each drive model.  Sending down a steady stream of tiny TRIMs is
>>> going to give terrible performance.
>>
>> Sounds like you might be using junk for a device?
>>
>> For junk, a little coalescing may be warranted... like in the I/O
>> schedular, but no more than 100usecs wait before posting, or then you
>> effect high performing devices too.
>>
>> Chris
>
> Why?
>
> AIUI, on every write a high performing device allocates a new erase
> block from its free lists, writes to it, and puts the now unused erase
> block on the free list.

So erase blocks are 512 bytes (if I write 512 bytes, an erase block is
now freed)?  Not true.

>  That erase block becomes available for reuse
> some milliseconds later.
>
> As long as the SSD has enough free erase blocks to work with I see no
> disadvantage in delaying a discard by minutes, hours or days in most
> cases.  The exception is when the filesystem is almost full and the
> SSD is short of erase blocks to work with.

That "exception..." is another good reason why.

>
> In that case it will want to get as many free erase blocks as it can
> as fast as it can get them.

Exactly.

Chris
>
> Greg
>
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