linux-ext4 - Re: bug#6131: [PATCH]: fiemap support for efficient sparse file copy

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Date:	Fri, 11 Jun 2010 09:03:29 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	"jeff.liu" <jeff.liu@...cle.com>
CC:	Sunil Mushran <sunil.mushran@...cle.com>,
	Paul Eggert <eggert@...UCLA.EDU>,
	Jim Meyering <jim@...ering.net>, bug-coreutils@....org,
	Joel Becker <Joel.Becker@...cle.com>,
	Chris Mason <chris.mason@...cle.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Tao Ma <tao.ma@...cle.com>
Subject: Re: bug#6131: [PATCH]: fiemap support for efficient sparse file copy

jeff.liu wrote:
> Sunil Mushran wrote:
>> On 06/10/2010 04:47 PM, Paul Eggert wrote:
>>> On 06/09/2010 11:56 PM, jeff.liu wrote:
>>>   
>>>> Yeah, I just realized that the behaviour I observed is caused by the
>>>> delay allocation mechanism of
>>>> the particular FS.
>>>>      
>>> If the file system is using delayed allocation, then can
>>> the fiemap ioctl tell us that a file contains a hole (because nothing
>>> has been
>>> allocated there), but read() would tell us that the file contains
>>> nonzero data at the same location
>>> (because it's sitting in a buffer somewhere)?  If so, we'd need to do
>>> something like invoke
>>> fdatasync() on the file before issuing the fiemap ioctl, to force
>>> allocation; or perhaps
>>> there's another ioctl that will do the allocation without having to
>>> actually do a sync.
>>>    
>> I guess we'll have to use FIEMAP_FLAG_SYNC.
> Hi Sunil,
> 
> Thanks for the comments.
> So we can ensure the source file synced before mapping in this way.
> 
> Hi Jim and Paul,
> 
> How about the tiny patch below?

I agree that this is needed, thanks.

-Eric

> From d6d619a169ff68a9a310a69d8089b9fbf83b5f91 Mon Sep 17 00:00:00 2001
> From: Jie Liu <jeff.liu@...cle.com>
> Date: Fri, 11 Jun 2010 16:29:02 +0800
> Subject: [PATCH 1/1] copy.c: add FIEMAP_FLAG_SYNC to fiemap ioctl
> 
> * src/copy.c (fiemap_copy): Force kernel to sync the source
> file before mapping.
> 
> Signed-off-by: Jie Liu <jeff.liu@...cle.com>
> ---
>  src/copy.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/src/copy.c b/src/copy.c
> index f149be4..f48c74d 100644
> --- a/src/copy.c
> +++ b/src/copy.c
> @@ -191,6 +191,7 @@ fiemap_copy (int src_fd, int dest_fd, size_t buf_size,
>    do
>      {
>        fiemap->fm_length = FIEMAP_MAX_OFFSET;
> +      fiemap->fm_flags = FIEMAP_FLAG_SYNC;
>        fiemap->fm_extent_count = count;
> 
>        /* When ioctl(2) fails, fall back to the normal copy only if it

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