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Date: Thu, 3 Aug 2006 20:57:17 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: cygwin copy problems usb 2.0
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On Aug  3 13:22, Igor Peshansky wrote:
> On Thu, 3 Aug 2006, Corinna Vinschen wrote:
> > On Aug  3 01:54, Eric Blake wrote:
> > > it would be accepted upstream.  Now if there were something more
> > > POSIX-y that we could do to speed things up, such as posix_fadvise,
> >
> > posix_fadvise can't be implemented nicely, AFAICS.  The POSIX semantics
> > require an already opened file and the advice is given for an offset and
> > a length.  The Windows semantics only allow to give the advice for the
> > whole file, and only switching between FILE_SEQUENTIAL_ONLY or "normal",
> > using ZwSetInformationFile.  By re-opening the file using ZwOpenFile it
> > would also be possible to toggle the FILE_RANDOM_ACCESS flag.  Still,
> > it's always for the whole file, not for an area giving offset and length.
> 
> Theoretically, it's possible to implement posix_fadvise only for offset=0
> and length=<length-of-file>, and have it fail with EINVAL otherwise...
> While technically not POSIX-compliant, it would still allow for better
> implementation of things like copy...

Right.  Now for the next problem.  Could anybody be bothered to actually
test the performance effect of setting FILE_SEQUENTIAL_ONLY when reading
and writing files sequentially as cp does?  It would be quite interesting
to get some real numbers on FAT and NTFS, before implementing posix_fadvice
just to find out that it has no practical effect.  If it's less than 10%
it's not worth the effort, imo.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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