Re: [RFC] delayed allocation for ext4

[David Chinner <dgc@sgi.com>]

On Sat, Dec 23, 2006 at 10:09:57PM +0300, Alex Tomas wrote:
> 
> Good day,
> 
> >>>>> David Chinner (DC) writes:
> 
>  DC> So that mean's we'll have 2 separate mechanisms for marking
>  DC> pages as delalloc. XFS uses the BH_delay flag to indicate
>  DC> that a buffer (block) attached to the page is using delalloc.
> 
> well, for blocksize=pagesize we can save 56 bytes on every page.

Sure, but it means that ext4 w/ delalloc won't work on lots of
machines....

>  DC> FWIW, how does this mechanism deal with block size < page size?
>  DC> Don't you have to track delalloc on a block basis rather than
>  DC> a page basis?
> 
> I'm still thinking how better to deal with that w/o much code duplication.

Code duplication in ext4, or across all filesystems?

>  DC> Ah, that's why you can get away with a page flag - you've ignored
>  DC> the partial page delay state problem. Any plans to use the
>  DC> existing method in the future so we will be able to use ext4 delalloc
>  DC> on machines with a page size larger than 4k?
> 
> what do you mean by "exsiting"? BH_delay?

Yes.

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group