linux-kernel - Re: sync_file_range(SYNC_FILE_RANGE

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Date:	Sun, 1 Jun 2008 08:23:43 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Pavel Machek <pavel@....cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks?

On Sat, 31 May 2008, Andrew Morton wrote:
> On Sat, 31 May 2008 19:44:49 +0100 (BST) Hugh Dickins <hugh@...itas.com> wrote:
> > 
> > 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?

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.

Hugh
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