On 2/15/16 11:26 AM, Jim Wilcoxson wrote: > I'm developing a backup program, HashBackup, and recently added sparse > "hole skipping" using fiemap. That is probably not safe. Mapping can change immediately after fiemap returns, and you could miss data. cp went down that path a few years ago, and it caused pain. SEEK_HOLE/SEEK_DATA would be a better choice. > Today I ran into a weird issue with xfs > on Linux 3.10. > > A test program creates a sparse file with 10000*(4K data, 4M hole). > HB calls fiemap with start=0, length=0xFFFF...FFFF, mapped_extents=0 > to get the number of extents. Fiemap is coming back with > mapped_extents=1364 instead of 10000. Take a look at xfs_bmap -v output to get a clear picture of what is actually on disk. It's possible that some combination of speculative preallocation or other things has merged extents. In general, the kernel has plenty of leeway in terms of how it chooses to lay out a file. "seek/write" doesn't necessarily guarantee a hole. > The fiemap.txt file says: > > "If fm_extent_count is zero, then the fm_extents[] array is ignored > (no extents will be returned), and the fm_mapped_extents count will > hold the number of extents needed in fm_extents[] to hold the file's > current mapping." > > It doesn't say the filesystem can choose to return fewer extents if it > wants, but maybe xfs interprets it this way and fiemap has to be > called in a loop until the extent_last flag is set. If that's the > case, fiemap.txt should be updated. It's not clear that it is returning fewer extents. See what xfs_bmap says, and compare. But also, don't use fiemap results for anything that requires data integrity... -Eric > Thanks, > Jim > (not on the xfs mailing list) > _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
