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

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Date:	Sun, 13 Jun 2010 11:37:18 +0800
From:	"jeff.liu" <jeff.liu@...cle.com>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	Sunil Mushran <sunil.mushran@...cle.com>,
	Paul Eggert <eggert@...UCLA.EDU>, bug-coreutils@....org,
	Jim Meyering <jim@...ering.net>,
	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

Eric Sandeen wrote:
> 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.
Thanks for your confirming response.

Regards,
-Jeff
> 
> -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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