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

[Greg Freemyer]

On Fri, Aug 14, 2009 at 8:38 PM, Chris Worley<worleys@xxxxxxxxx> wrote:
> 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.

Seriously, how do you  know?  Are you under NDA?

The write paper I read about typical SSD design described a partial
erase block write as:

Internal logic/micro-controller performs:

Read erase block, modify erase block, allocate new erase block, write
new erase block, free now unused old erase block, old erase block
added to a hardware erase queue the performs the actual erase in the
background at the relatively slow speed of multiple milliseconds..

The purpose of the trim/discard command being to allow the ssd to have
enough free erase blocks ready to go that the writes don't have to
stall while they wait for a erase block to pop out of the erase queue.

Greg
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