seeking advice on sparse files on xfs

[Joe Landman]

On 12/19/2012 10:00 PM, Dave Chinner wrote:

[...]

>> I git cloned the coreutils to look at current state of the code, and
>> saw exactly what is represented on slide 14.
>>
>> "Brute force – read each sector in full, before
>> skipping while writing the copy"
>
> http://code.metager.de/source/xref/coreutils/src/extent-scan.c
>
> It uses FIEMAP.
>
> Indeed, take a 1TB sparse file (empty) and copy it:
>
> $ cp --version
> cp (GNU coreutils) 8.13
> $ ls -lh blah
> -rw-r--r-- 1 root root 1.0T Nov 30 06:42 blah
> $ xfs_bmap -v blah
> blah: no extents
> $ strace cp --sparse=always blah fred
> .....
> stat("fred", 0x7fff98d7ded0)            = -1 ENOENT (No such file or directory)
> stat("blah", {st_mode=S_IFREG|0600, st_size=1099511627776, ...}) = 0
> stat("fred", 0x7fff98d7dc50)            = -1 ENOENT (No such file or directory)
> open("blah", O_RDONLY)                  = 3
> fstat(3, {st_mode=S_IFREG|0600, st_size=1099511627776, ...}) = 0
> open("fred", O_WRONLY|O_CREAT|O_EXCL, 0600) = 4
> fstat(4, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
> ioctl(3, FS_IOC_FIEMAP, 0x7fff98d7c9d0) = 0
> ftruncate(4, 1099511627776)             = 0
> close(4)                                = 0
> close(3)                                = 0
> ...
> $ xfs_bmap -v fred
> fred: no extents
> $
>
> Looks like cp already does what you want - it didn't copy a TB Of
> zeros..... ;)

I have much older coreutils by default on CentOS 6.x.  Will recheck this 
with newer version, and pull down the extent-scan.  My strace had lots 
of zeros ...

>
>>> And what you really want is a version of cp that supports these:
>>>

[...]

>> Was thinking of hacking something up at a much higher level
>> (cheating by parsing bmap data and stuff like that).
>
> Don't. You'll only get it wrong, just like the initial attempts to
> use FIEMAP in cp did. There's a reason coreutils is moving to
> SEEK_HOLE/SEEK_DATA instead of FIEMAP for efficient sparse file
> handling....

Thanks.  This isn't what I wanted to do if I could avoid it.

>
> Cheers,
>
> Dave.
>


--
Joe