Re: [RFC] new ->perform_write fop

[Chris Mason <chris.mason@oracle.com>]

On Fri, May 14, 2010 at 06:38:21PM +1000, Dave Chinner wrote:
> On Fri, May 14, 2010 at 05:22:19PM +1000, Nick Piggin wrote:
> > On Fri, May 14, 2010 at 04:41:45PM +1000, Dave Chinner wrote:
> > > On Thu, May 13, 2010 at 11:30:57PM -0400, Josef Bacik wrote:
> > > > So this is what I had envisioned, we make write_begin take a nr_pages pointer
> > > > and tell it how much data we have to write, then in the filesystem we allocate
> > > > as many pages as we feel like, idealy something like
> > > > 
> > > > min(number of pages we need for the write, some arbitrary limit for security)
> > > 
> > > Actually, i was thinking that the RESERVE call determines the size
> > > of the chunk (in the order of 1-4MB maximum). IOWs, we pass in the
> > > start offset of the write, the entire length remaining, and the
> > > RESERVE call determines how much it will allow in one loop.
> > > 
> > > 	written = 0;
> > > 	while (bytes_remaining > 0) {
> > > 		chunklen = ->allocate(off, bytes_remaining, RESERVE);
> > > 		write_begin(&pages, off, chunklen);
> > > 		copied = copy_pages(&pages, iov_iter, chunklen);
> > > 		.....
> > > 		bytes_remaining -= copied;
> > > 		off += copied;
> > > 		written += copied;
> > > 	}
> > 
> > How much benefit are you expecting to get?
> 
> If the max chunk size is 4MB, then three orders of magnitudes fewer
> allocation calls for x86_64 (i.e. one instead of 1024).  For
> filesystems with significant allocation overhead (like gaining
> cluster locks in gfs2), this will be a *massive* win.

It's a pretty big deal in btrfs too.  A 4K write write is much less
expensive than it used to be, but the part where we mark a range of
bytes as delayed allocation goes faster if that range is bigger.

-chris