bmap-tools VS httm

> But I recall reading elsewhere a discussion about some userspace program which did depend on holes being present in the filesystem as actual holes (visible to SEEK_HOLE and so on) and not as runs of zeros.

> treatment of on-disk segments as "what was written by programs" can cause areas of 0 to not be written by bmaptool copy

https://github.com/intel/bmap-tools/issues/75

IMO, the issue here isn't filesystem or zfs behavior, it's that bmap-tool wants an extra "don't care bit" per block, which filesystems (traditionally) don't track, and programs interacting with filesystem don't expect to exist.

Some of the comments I've made in this issue describe options to make things better.

(>_<) Oh man, I knew about [0] when I posted (which is why I said it just reduces the chance of hitting the bug (by a lot)). But after spending all Saturday JST on it, I went to bed before [1] was posted.

Skimming through #6958 though, it seems like it's the lesser of evils, compared to #15526... I think? It's less obvious (to me) what the impact of #6958 is. Is it silent undetectable corruption of your precious data potentially over years, or more likely to cause a crash or runtime error?

Reports like https://github.com/intel/bmap-tools/issues/65 make it seem more like the latter.

But I have to read more. But since the zfs_dmu_offset_next_sync setting was disabled by default until recently, I still suspect (but yeah, don't know for sure) that disabling is the safest thing we can currently do on unmodified ZFS systems.

My projects and many projects include a THIRD-PARTY-LICENSES.html file when I distribute binaries. See: https://github.com/kimono-koans/httm/blob/master/third_party/LICENSES_THIRD_PARTY.html

The only real advantage I can think of with nested ZFS is that the files in the KVM would obviously be individual inodes in the nested ZFS filesystem and datasets, in addition to the one inode per virtual volume on the hypervisor (or zvol). This would allow for granular file management on the kvm and the use of tools like https://github.com/kimono-koans/httm which is like a command line time machine for ZFS.

> It's worth noting that copy_file_range is used by a lot of things.

Yes, but the trigger feature, block cloning, only landed in the latest 2.2 release. If you immediately hopped on 2.2, and used a system with lots copy_file_range and FICLONE use, yes, you may have a problem (like, as you note, on Gentoo, where this problem surfaced).

Most people were just hopping on the bandwagon. My distro ships 2.1.5, so I have a 6 month wait until this feature lands, so I was just building copy_file_range support into my ZFS apps, right before news of this bug hit.[0]

> There are other things required to trigger the bug that are a lot less common though.

Exactly. My guess is the incidence of this will exceedingly rare for the common user/small NAS user/etc. I've run a corruption detector[0], and what I've found mostly indicates false positives. Some are build artifact fingerprints, which I don't care about, and which were deleted with the next build. The ones with an extant file on another system, I confirmed were a diff match with the origin using `rsync -rincv` and whats on snapshots with `httm --map-aliases`. So far no positive matches.

[0]: https://github.com/kimono-koans/httm

cargo install --git https://github.com/kimono-koans/httm --branch clones strace -f -o stderr.txt -e ioctl -- httm -r -R ~/.zshenv

httm prints the size, date and corresponding locations of available unique versions (deduplicated by modify time and size) of files residing on snapshots, but can also be used interactively to select and restore files, even snapshot mounts by file! httm might change the way you use snapshots (because ZFS/BTRFS/NILFS2 aren't designed for finding for unique file versions) or the Time Machine concept (because httm is very fast!).

All btrfs snapshot tools can have different layouts. It's mostly a nightmare for any one tool to support. Although its not the tool you're looking for, FYI AFAIK httm supports all/most btrfs layouts, but it took more work than necessary to get there.

ZFS only option which requires super user privileges.

[0]: https://github.com/kimono-koans/httm/blob/master/httm.1

httm - Interactive, file-level Time Machine-like tool for ZFS/btrfs/nilfs2