How to check if the block is present in a sparse file (for simple copy-on-write)?

Question

How to get sparse block size and check if data is present at the given offset in sparse file in reiserfs/ext3 in Linux?

I want to use it to implement simple copy-on-write block device using FUSE.

Or I should better keep a bitmap in a separate file?

Answer 1

Well, http://lxr.linux.no/#linux+v2.6.33/arch/um/drivers/cow_user.c indicates that User Mode Linux uses an explicit bitmap for this, FWIW.

Answer 2

/usr/src/linux/Documentation/filesystems/fiemap.txt

The fiemap ioctl is an efficient method for userspace to get file extent mappings. Instead of block-by-block mapping (such as bmap), fiemap returns a list of extents.

There's a quick example of usage in git://kernel.ubuntu.com/cking/debug-code/fiemap/. A sparse file will lack extents for the "missing" portions.