freebsd-nq/cmd
Serapheim Dimitropoulos 4d044c4c1d OpenZFS 9238 - ZFS Spacemap Encoding V2
Motivation
==========

The current space map encoding has the following disadvantages:
[1] Assuming 512 sector size each entry can represent at most 16MB for a segment.
    This makes the encoding very inefficient for large regions of space.
[2] As vdev-wide space maps have started to be used by new features (i.e.
    device removal, zpool checkpoint) we've started imposing limits in the
    vdevs that can be used with them based on the maximum addressable offset
    (currently 64PB for a top-level vdev).

New encoding
============

The layout can be found at space_map.h and it remains backwards compatible with
the old one. The introduced two-word entry format, besides extending the limits
imposed by the single-entry layout, also includes a vdev field and some extra
padding after its prefix.

The extra padding after the prefix should is reserved for future usage (e.g.
new prefixes for future encodings or new fields for flags). The new vdev field
not only makes the space maps more self-descriptive, but also opens the doors
for pool-wide space maps (expected to be used in the log spacemap project).

One final important note is that the number of bits used for vdevs is reduced
to 24 bits for blkptrs. That was decided as we don't know of any setups that
use more than 16M vdevs for the time being and we wanted to fit the vdev field
in the space map. In addition that gives us some extra bits in dva_t.

Other references:
=================

The new encoding is also discussed towards the end of the Log Space Map
presentation from 2017's OpenZFS summit.
Link: https://www.youtube.com/watch?v=jj2IxRkl5bQ

Authored by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <gwilson@zfsmail.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Approved by: Gordon Ross <gwr@nexenta.com>
Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Tim Chase <tim@chase2k.com>

OpenZFS-commit: https://github.com/openzfs/openzfs/commit/90a56e6d
OpenZFS-issue: https://www.illumos.org/issues/9238
Closes #7665
2018-07-05 12:02:34 -07:00
..
arc_summary Add Python 3 rewrite of arc_summary.py 2018-02-28 08:52:34 -08:00
arcstat Fix some typos 2018-02-28 08:57:10 -08:00
dbufstat Fix some typos 2018-02-28 08:57:10 -08:00
fsck_zfs Add /sbin/fsck.zfs helper 2013-01-09 16:54:58 -08:00
mount_zfs Update build system and packaging 2018-05-29 16:00:33 -07:00
raidz_test Support -fsanitize=address with --enable-asan 2018-01-10 10:49:27 -08:00
vdev_id vdev_id: new slot type ses 2017-12-20 09:42:07 -08:00
zdb OpenZFS 9238 - ZFS Spacemap Encoding V2 2018-07-05 12:02:34 -07:00
zed Fix typo in comment, handeling->handling 2018-06-18 21:51:06 -07:00
zfs Add tunables for channel programs 2018-06-15 15:10:42 -07:00
zgenhostid Add zgenhostid utility script 2017-07-25 13:22:03 -04:00
zhack Project Quota on ZFS 2018-02-13 14:54:54 -08:00
zinject Add support for decryption faults in zinject 2018-05-02 15:36:20 -07:00
zpool Fix missing option '-e' in zpool online usage 2018-06-26 10:17:55 -07:00
zstreamdump Fix issues with raw sends of spill blocks 2018-04-17 11:19:03 -07:00
ztest OpenZFS 9238 - ZFS Spacemap Encoding V2 2018-07-05 12:02:34 -07:00
zvol_id Fedora 28: Fix misc bounds check compiler warnings 2018-04-04 10:16:47 -07:00
Makefile.am Retire legacy test infrastructure 2017-08-15 17:26:38 -07:00