Re: thin provisioned LUN support

Ric Wheeler
2008-11-07T19:50:30Z

jim owens wrote:
> Theodore Tso wrote:
>> On Fri, Nov 07, 2008 at 01:09:48PM -0500, Ric Wheeler wrote:
>>> I don't think that trim bugs should be that common - we just have to
>>> be very careful never to send down a trim for any uncommitted block.
>>>
>>
>> The trim code probably deserves a very aggressive unit test to make
>> sure it works correctly, but yeah, we should be able to control any
>> trim bugs.
>>
>>> Simple is always good, but I still think that the coalescing (even
>>> basic coalescing) will be a critical performance feature.
>>
>> Will we be able to query the device and find out its TRIM/UNMAP
>> alignment requirements? There is also a balanace between performance
>> (at least if the concern is sending too many separate TRIM commands)
>> and giving the SSD more flexibility in its wear-leveling allocation
>> decisions by sending TRIM commands sooner rather than later.
>
> This is all good if the design is bounded by the requirements
> of trim for flash devices. Because AFAIK the use of trim for
> flash ssd is a performance optimization. The ssd won't loose
> functionality if the trim is less than the chunk size. It may
> run slower and wear out faster, but that is all.
>
> If I understand correctly, with thin provisioning, unmapping
> less than the chunk will not release that chunk for other use.
> So you have lost the thin provision feature of the array.
>
> The concern (Chris I think) and I have is that doing a design
> to handle thin provision arrays *when chunk > fs_block_size*
> that guarantees you will *always* release on chunk boundaries
> is a lot more complicated.
>
> To do that you kind of have to build a filesystem into the
> block layer to persistently store "mapped/unmapped blocks
> in chunk" and then do the "unmap-this-chunk" when a region
> is all unmapped.
>
> 250 MB per 1TiB 512b sector disk for a simple 1-bit-per-sector
> state. And that assumes you don't replicate it for safety.
> That is what the array vendors are trying to avoid by pushing
> it off to the OS.
>
> Whoever supports thin provisioning better get their unmapping
> correct because those big customers will be looking for who
> to blame if they don't get all the features.
>
> jim

I do think that what we have today is a reasonable start, especially if
we can do some coalescing of the unmap commands just like we do for
normal IO.

It does not have to be perfect, but it will work well for devices with a
reasonable chunk size.

A vendor could always supply a clean up script to run when we get too
out of sync between what the fs & storage device think is really
allocated. (Bringing back that wonderful model of windows defrag your
disk :-))

ric