linux-ext4 - Re: [PATCH 0/5] fallocate system call

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Date:	Mon, 30 Apr 2007 10:47:02 +1000
From:	David Chinner <dgc@....com>
To:	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
Cc:	torvalds@...l.org, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-ext4@...r.kernel.org, xfs@....sgi.com, suparna@...ibm.com,
	cmm@...ibm.com
Subject: Re: [PATCH 0/5] fallocate system call

On Thu, Apr 26, 2007 at 11:20:56PM +0530, Amit K. Arora wrote:
> Based on the discussion, this new patchset uses following as the
> interface for fallocate() system call:
> 
>  asmlinkage long sys_fallocate(int fd, int mode, loff_t offset, loff_t len)

Ok, so now for the hard questions - what are the semantics of
FA_ALLOCATE and FA_DEALLOCATE?

For FA_ALLOCATE, it's supposed to change the file size if we
allocate past EOF, right? What's the return value supposed to
be? Zero for success, error otherwise? Does this update a/m/ctime
at all? How persistent is this preallocation? Should it be
there "forever" or for the lifetime of the currently open fd
that it was preallocated on?

For FA_DEALLOCATE, does it change the filesize at all? Or does
it just punch a hole in the file? If it does change file size,
what happens when you punch out preallocation beyond EOF?
What's the return value supposed to be?

> Currently we have two modes FA_ALLOCATE and FA_DEALLOCATE, for
> preallocation and deallocation of preallocated blocks respectively. More
> modes can be added, when required.

FWIW, we definitely need a FA_PREALLOCATE mode (FA_ALLOCATE but does
not change file size) so we can preallocate beyond EOF for apps which
use O_APPEND (i.e. changing file size would cause problems for them).

> ToDos:
> =====
> 1>   Implementation on other architectures (other than i386, x86_64, 
> ppc64 and s390(x)) 

I'll have ia64 soon.

> 2>   A generic file system operation to handle fallocate
> (generic_fallocate), for filesystems that do _not_ have the fallocate
> inode operation implemented.
> 3>   Changes to glibc,
> 	a) to support fallocate() system call
> 	b) so that posix_fallocate() and posix_fallocate64() call
> 	   fallocate() system call
> 4>   Changes to XFS to implement the fallocate inode operation

And that's what I'm doing now, hence all the questions ;)

BTW, do you have a test program for this, or will I need to write
one myself?

Cheers,

Dave.
-- 
Dave Chinner
Principal Engineer
SGI Australian Software Group
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