Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks?

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 2 Jun 2008 13:43:20 +0200 Jens Axboe <jens.axboe@oracle.com> wrote:

> On Sun, Jun 01 2008, Andrew Morton wrote:
> > > > I expect major users of this system call will be applications which do
> > > > small-sized overwrites into large files, mainly databases.  That is,
> > > > once the application developers discover its existence.  I'm still
> > > > getting expressions of wonder from people who I tell about the
> > > > five-year-old fadvise().
> > > 
> > > Hey, you have one user now, its called s2disk. But for this call to be
> > > useful, we'd need asynchronous variant... is there such thing?
> > 
> > Well if you're asking the syscall to shove more data into the block
> > layer than it can concurrently handle, sure, the block layer will
> > block.  It's tunable...
> 
> Ehm, lets get the history right, please :-)
> 
> The block layer pretty much doesn't care about how large the queue
> size is, it's largely at 128 to prevent the vm from shitting itself
> like it has done in the past (and continues to do I guess, though
> your reply leaves me wondering).
> 
> So you think the vm will be fine with a huge number of requests?
> It wont go nuts scanning and reclaiming, wasting oodles of CPU
> cycles?

The VFS did screw up a couple of times with unbounded queues.  It did
get fixed and it is a design objective for the writeback code to _not_
depend upon request exhaustion for proper behaviour.

But it hasn't had a large amount of testing with unbounded queues and
there may still be problems in there.