ZFS "partially filled holes lose birth time"

[Richard Laager]

Note: This is different from Launchpad bug #1557151. This is another, similar, bug.

Bug description from Matt Ahrens at OpenZFS:
"If a ZFS object contains a hole at level one, and then a data block is
 created at level 0 underneath that l1 block, l0 holes will be created.
 However, these l0 holes do not have the birth time property set; as a
 result, incremental sends will not send those holes.

 Fix is to modify the dbuf_read code to fill in birth time data."
-- https://www.illumos.org/issues/6513

From pcd on IRC in #zfsonlinux:
"basically, what happens is this: if you zero out an entire l1 indirect
 block's worth of data (several megabytes), we save space by storing that
 entire indirect block as a single hole in an l2 indirect block with
 birth time N. If you then modify some of the data under that, but not
 all of it, when the l1 indirect block is filled back in with mostly
 holes and some data blocks, the wholes will not have any"

Fixed in ZoL here:
https://github.com/zfsonlinux/zfs/commit/bc77ba73fec82d37c0b57949ec29edd9aa207677

This has *not* been merged into a ZoL release yet, nor the release branch.

This is a very unfortunate bug because the fix only helps you moving forward. A separate bug has been opened to propose a fix for that:
https://www.illumos.org/issues/7175

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Richard Laager (rlaager)]

And also:
https://github.com/zfsonlinux/zfs/issues/4996
fix pending here:
https://github.com/zfsonlinux/zfs/pull/5061

Debian is shipping a patch to add an ignore_hole_birth tunable (and defaulting to on). We should get that in Ubuntu, except that we also want to rename it as per here:
https://github.com/zfsonlinux/zfs/pull/5239/commits/9af8a46cc80a1b14c12c95edac4b10b8b3b9a7f2

[fmyhr (fmyhr-u)]

17.04 has ZFS 0.6.5.7 that works around this bug using /sys/module/zfs/parameters/ignore_hole_birth tunable. Will a backport of this bugfix to 16.04 LTS be forthcoming?