Re: thin provisioned LUN support

James Bottomley
2008-11-07T16:06:49Z

On Fri, 2008-11-07 at 11:00 -0500, Martin K. Petersen wrote:
> >>>>> "Chris" == Chris Mason <chris.mason@oracle.com> writes:
>
> Chris> Hmmm, it's surprising to me that arrays who tell us please use
> Chris> the noop elevator suddenly want us to merge discard requests.
> Chris> The array really needs to be able to deal with this internally.
>
> Let's also not forget that we're talking about merging discard
> requests for the purpose making internal array housekeeping efficient.
> That involves merging discards up to the internal array block sizes
> which may be on the order of 512/768/1024 KB.
>
> If we were talking about merging discards up to a 4/8/16 KB boundary
> that might be something we'd have a chance to do within a reasonable
> amount of time (bigger than normal read/write I/O but not hours).
>
> But keeping discard state around for long enough to attempt to
> aggregate 768KB (and 768KB-aligned) chunks is icky.

Icky but possible. It's the same rb tree affair we use to keep vma
lists (with the same characteristics). The point is that technically we
can do this pretty easily ... all the way down to not losing any
potential discards that the array would ignore. However, procedurally
it would certainly be sending the wrong message to the array vendors
(the message being "sure the OS will sanitise any crap you care to
dump").

On the other hand, if we have to do it for flash and MMC anyway ...

James