Re: [RFC] new ->perform_write fop

[Nick Piggin <npiggin@suse.de>]

On Fri, May 21, 2010 at 11:15:18AM -0400, Christoph Hellwig wrote:
> Nick, what exactly is the problem with the reserve + allocate design?
> 
> In a delalloc filesystem (which is all those that will care about high
> performance large writes) the write path fundamentally consists of those
> two operations.  Getting rid of the get_blocks mess and replacing it
> with a dedicated operations vector will simplify things a lot.

Nothing wrong with it, I think it's a fine idea (although you may still
need a per-bh call to connect the fs metadata to each page).

I just much prefer to have operations after the copy not able to fail,
otherwise you get into all those pagecache corner cases.

BTW. when you say reserve + allocate, this is in the page-dirty path,
right? I thought delalloc filesystems tend to do the actual allocation
in the page-cleaning path? Or am I confused?

 
> Punching holes is a rather problematic operation, and as mentioned not
> actually implemented for most filesystems - just decrementing counters
> on errors increases the chances that our error handling will actually
> work massively.

It's just harder for the pagecache. Invalidating and throwing out old
pagecache and splicing in new pages seems a bit of a hack.