Hi, I have a rather extensive FIEMAP test which works fine on ext4 but fails on btrfs. I've took a look at one of the failures, and simplified it to the following: 1. create a 4KiB non-sparse file 2. truncate it to 8KiB 3. truncate it to 4KiB + 1 byte IOW: $ dd if=/dev/urandom of=file bs=4096 count=1 $ truncate -s 8192 file $ truncate -s 4097 file Let's assume that the FS block size is 4KiB, as it is returned the FIGETBSZ ioctl. These actions result in: 1. the file will have only 1 block mapped on ext4 2. the file will have 2 blocks mapped on btrfs IOW, on ext4: $ stat file File: ‘file’ Size: 4097 Blocks: 8 IO Block: 4096 regular file and on btrfs $ stat file File: ‘file’ Size: 4097 Blocks: 16 IO Block: 4096 regular file Notice 8 vs 16 blocks. Interesting enough that just creating a 4KiB file and then truncating it to 4097 bytes works as I expect in btrfs - results in a file with only the first block mapped. It looks like ext4 is "perfect" in detecting sparse 4KiB blocks while btrfs sometimes maps seemingly sparse 4KiB blocks. Is this considered to be a defect or this is fine since the FS does not probably give any guarantees WRT mapped and unmapped blocks? Thanks! -- Best Regards, Artem Bityutskiy -- 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
