On Thu, Apr 26, 2007 at 11:20:56PM +0530, Amit K. Arora wrote:

quoted text
> 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

quoted text
> 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:

quoted text
> =====

> 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

quoted text
> (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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