Re: [PATCH] add -o flush for fat

[Chris Mason <mason@suse.com>]

On Fri, Aug 04, 2006 at 06:31:49PM -0700, Andrew Morton wrote:
> 
> Could we have more detail on what you're trying to do here?
> 
> You've obviously made some decisions about how to handle under-writeback
> data, dirty data, integrity versus cleaning, etc.  What were those
> decisions and what led you to them?

The basic idea is that someone watching the blinking light on the usb
stick knows that when the light goes out, he can pull the stick out.
It's an alternative to mount -o sync that it closer to what users are
asking for.

There's no attempt at integrity, it only tries to keep a fairly constant
flow of data to the device.

> 
> We should document the precise semantics of these two functions, especially
> wrt data integrity, handling of dirty-but-under-writeback pages, etc. 
> (Right now I'm not sure what those semantics are).

They are loosely defined.  One way to handle
dirty-but-under-writeback pages is to add a timer that
filemap_fdatawrites the block device.  Since fdatawrite is used in the
file release call, regular files don't need this, only the block device.

This timer isn't added in the patch, but it's an easy addition later.

> 
> I'm also wondering what sync_inode(nr_to_write=0) does ;) I think it writes
> one page...

It starts a write on the inode and nothing else ;)  There are lots of
ways to write the inode and wait, but this was the only one I found to
write the inode without waiting and then write the data pages while
waiting for in flight io.

-chris