Re: sync & asyck i/o

[Daniel Phillips]

"Stephen C. Tweedie" wrote:
> 
> Hi,
> 
> On Tue, Feb 06, 2001 at 02:52:40PM +0000, Alan Cox wrote:
> > > According to the man page for fsync it copies in-core data to disk
> > > prior to its return. Does that take async i/o to the media in account?
> > > I.e. does it wait for completion of the async i/o to the disk?
> >
> > Undefined.
> 
> > In practice some IDE disks do write merging and small amounts of write
> > caching in the drive firmware so you cannot trust it 100%.
> 
> It's worth noting that it *is* defined unambiguously in the standards:
> fsync waits until all the data is hard on disk.  Linux will obey that
> if it possibly can: only in cases where the hardware is actively lying
> about when the data has hit disk will the guarantee break down.

Sometimes I want to know that the write is safely on disk and sometimes
I only need to know that the io has gone over the bus and is on its way
to disk.  In the latter case the buffer/page can be unlocked a lot
sooner.  Please correct me if I'm wrong, but I don't think the current
API can make that distinction for IDE, much less provide a uniform way
of controlling this behaviour across all types of block devices.  We
need that, or else we have to choose between the following: 1) slow 2)
risky.

I'd like to be able to set a bit in the buffer_head that says 'get back
to me when it's on disk' vs 'get back to me when it's hit the bus'.
	
--
Daniel