On 5/18/11 3:28 PM, Jiaying Zhang wrote: > > > On Tue, May 17, 2011 at 8:19 PM, Eric Sandeen <sandeen@xxxxxxxxxx <mailto:sandeen@xxxxxxxxxx>> wrote: > > On 5/17/11 5:59 PM, Jiaying Zhang wrote: > > There is a bug in commit c8d46e41 "ext4: Add flag to files with blocks > > intentionally past EOF" that if we fallocate a file with FALLOC_FL_KEEP_SIZE > > flag and then ftruncate the file to a size larger than the file's i_size, > > any allocated but unwritten blocks will be freed but the file size is set > > to the size that ftruncate specifies. > > > > Here is a simple test to reproduce the problem: > > 1. fallocate a 12k size file with KEEP_SIZE flag > > 2. write the first 4k > > 3. ftruncate the file to 8k > > Then 'ls -l' shows that the i_size of the file becomes 8k but debugfs > > shows the file has only the first written block left. > > To be honest I'm not 100% certain what the fiesystem -should- do in this case. > > If I go through that same sequence on xfs, I get 4k written / 8k unwritten: > > # xfs_bmap -vp testfile > testfile: > EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS > 0: [0..7]: 2648750760..2648750767 3 (356066400..356066407) 8 00000 > 1: [8..23]: 2648750768..2648750783 3 (356066408..356066423) 16 10000 > > size 8k: > # ls -l testfile > -rw-r--r-- 1 root root 8192 May 17 22:33 testfile > > and diskspace used 12k: > # du -hc testfile > 12K testfile > 12K total > > I think this is a different result from ext4, either with or without your patch. > > On ext4 I get size 8k, but only the first 4k mapped, as you say. > > I agree that truncating to a size larger than i_size is un-specified by > POSIX. However, I think the problem with the current behavior is that > we have an inconsistency between file's i_size and its extent tree. > Now we have 8k i_size but the file has only 4k space allocated. That > can confuse applications. That's called "a sparse file" right? Apps should not be confused by that ... -Eric -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
