[libvirt] [PATCH] Fix sparse volume allocation reporting

Cole Robinson crobinso at redhat.com
Thu Apr 2 18:39:58 UTC 2009

Daniel P. Berrange wrote:
> On Thu, Mar 26, 2009 at 05:55:00PM +0100, Daniel Veillard wrote:
>> On Thu, Mar 26, 2009 at 01:19:12PM +0000, Daniel P. Berrange wrote:
>>> On Tue, Mar 24, 2009 at 03:29:21PM -0400, Cole Robinson wrote:
>> [...]
>>> Having read the man page again, I'm inclined to say using st_blksize
>>> is always wrong no matter what, because it is quite clear that 'st_blocks'
>>> is always in 512 byte units. So perhaps we might be better of doing
>>>    #ifndef DEV_BSIZE
>>>    #define DEV_BSIZE 512
>>>    #endif
>>> And then always using DEV_BSIZE. 
>>   In those kind of cases I go down to the spec and it states (in the
>> informative section though):
>> -----------------------------------------
>> http://www.opengroup.org/onlinepubs/000095399/basedefs/sys/stat.h.html
>> "The unit for the st_blocks member of the stat structure is not defined
>> within IEEE Std 1003.1-2001. In some implementations it is 512 bytes. It
>> may differ on a file system basis. There is no correlation between
>> values of the st_blocks and st_blksize, and the f_bsize (from
>> <sys/statvfs.h>) structure members.
>> Traditionally, some implementations defined the multiplier for st_blocks
>> in <sys/param.h> as the symbol DEV_BSIZE."
>> -----------------------------------------
>>   So I agree with Dan, we need to drop st_blksize in any volume size
>> computation, and fallback to 512 if not defined, apparently only
>> src/storage_backend.c referenced it :-)
>>   I still find the "It may differ on a file system basis" to be a bit
>> frightening considering the sandard doesn't seems to indicate how to
>> extract that information from the filesystem :-( , oh well ...
> I guess if we come across a filesystem where it is not 512, then someone
> will have created an ioctl() or equivalent to let us find out the true
> value. Failing that, relying on 512 seems like a good plan.
> Daniel

Okay, updated patch attached.


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