This patch implements parallel mounting for zfs filesystems. This was first implemented on Illumos and ported to ZOL. After asking on the zfs-devel mailing list it was suggested that I start with the ZOL patch because it would be easier to port.
Delphix has seen huge decrease in latency of the mount phase with this patch (see results here: https://github.com/openzfs/openzfs/commit/a3f0e2b569). Performance improvements measured at Gandi are on par with those at Delphix.
Mailing thread here: https://lists.freebsd.org/pipermail/zfs-devel/2019-February/000516.html
- the ZOL patch pretty much applies directly to FreeBSD (unlike the Illumos patch)
- Had to take some code from the Illumos patch for #ifdef Illumos specific code that was removed when ported to ZOL
- After the ZOL port, a fix for a race condition was amended, this is included. (https://github.com/zfsonlinux/zfs/commit/e63ac16d25fbe991a356489c86d4077567dfea21)