Re: [libvirt] [PATCH] Fix sparse volume allocation reporting

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