Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks?

[Pavel Machek <pavel@suse.cz>]

On Tue 2008-06-03 10:01:22, Michael Kerrisk wrote:
> On Sun, Jun 1, 2008 at 1:40 PM, Pavel Machek <pavel@suse.cz> wrote:
> > Hi!
> >
> >> > > > All I can say so far is that I find the same as you do:
> >> > > > SYNC_FILE_RANGE_WRITE (after writing) takes a significant amount of time,
> >> > > > more than half as long as when you add in SYNC_FILE_RANGE_WAIT_AFTER too.
> >> > > >
> >> > > > Which make the sync_file_range call pretty pointless: your usage seems
> >> > > > perfectly reasonable to me, but somehow we've broken its behaviour.
> >> > > > I'll be investigating ...
> >> > >
> >> > > It will block on disk queue fullness - sysrq-W will tell.
> >> >
> >> > Ah, thank you.  What a disappointment, though it's understandable.
> >> > Doesn't that very severely limit the usefulness of the system call?
> >>
> >> A bit.  The request queue size is runtime tunable though.
> >
> > Which /sys is that? What happens if I set the queue size to pretty
> > much infinity, will memory management die horribly?
> >
> >> 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?
> >
> > Okay, I can fork and do the call from another process, but...
> >
> >> > I admit the flag isn't called SYNC_FILE_RANGE_WRITE_WITHOUT_WAITING,
> >> > but I don't suppose Pavel and I are the only ones misled by it.
> >>
> >> Yup, this caveat/restriction should be in the manpage.
> >
> > Michael, this is something for you I guess?
> 
> Pavel,
> 
> Just to confirm: you are meaning that the sentence
> 
>     Notice that even this this may and will block if you attempt
>     to write more than request queue size.
> 
> should be added to the man page under the description of
> SYNC_FILE_RANGE_WRITE, right?

Yes.
							Pavel

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