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

[Greg Freemyer <greg.freemyer@gmail.com>]

On Fri, Aug 14, 2009 at 8:19 PM, Chris Worley<worleys@gmail.com> wrote:
> On Fri, Aug 14, 2009 at 5:45 PM, Matthew Wilcox<matthew@wil.cx> 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.  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.

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

Greg

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