[ilovecaching]

I’m confused… you’re talking about avoid a local copy of sparse regions… Linux already does that at the level of the inode. There’s also a seek operation to move past the next hole. Not sure why you would carry around metadata the filesystem is already tracking for you.

[masklinn]

> Not sure why you would carry around metadata the filesystem is already tracking for you.

Because bmap files are independent of the filesystem and OS, and thus would probably like to work even with filesystems which don't support sparse files, and OS which don't expose holes?

For instance until NFS 4.2 in 2016 you could write sparse files to an NFS volume, but there was no way to detect holes when reading. exfat doesn't support sparse files at all. And according to their man pages, OpenBSD and NetBSD have yet to support SEEK_HOLE/SEEK_DATA (which are non-standard extensions of POSIX lseek(2)).

Plus according to its history the bmaptools project was created about a year after the release of kernel 3.1, which introduced support for SEEK_HOLE and SEEK_DATA. Doesn't take much of a leap to assume that the project's creator didn't consider that widespread enough to be reliable (Debian wouldn't release a 3.x-based version until the following year).

[andrewshadura]

Seeking through holes also doesn't work very well for compressed images, since usually there is no way to tell apart an insignificant hole from a long sequence of zeroes or other filler data.

[agildehaus]

An example from my work:

We have a Yocto build that results in about 120MB worth of files that make up our app and Yocto. Originally we had a script that would write a bootloader, partition and format ext4 our target's eMMC, and decompress a 120MB tarball to that filesystem.

That worked well, but we wanted our script to become OS-independent, as our field team ran Windows laptops. It's quite difficult to get Windows to do an ext4 format, and I wanted our tool to have a minimal number of dependencies (e2fsprogs requirement? some proprietary thing from Paragon? no thanks)

So instead, have Yocto produce an image containing the bootloader and all four pre-formatted ext4 filesystems. No operating system needs to do the format if the filesystem already exits within, it's just a raw block write. But now the image is 4GB, the size of our eMMC, and writing all of it would be painfully slow.

Thankfully Yocto also outputs a bmap file which maps the parts of that 4GB which are empty space -- blocks we don't need to write when commissioning our target device. So our commissioning tool was rewritten in Go, and I wrote a bmap implementation in Go to do the write. Flashing our target is as fast as it used to be, but now that tool can be easily made to work on multiple operating systems.