illumos gate

Implementation is now feature-complete, new webrev at:
http://cr.illumos.org/~webrev/skiselkov/3525/

Porting the OpenZFS change (which was based off the original work here that wasn't integrated), as well as some subsequent bug fixes (as well as two small prerequisites), the following OpenZFS commits were used:

commit 5ff2249fa579c20c12bce627e284415720fd4412
Author: Alexander Motin <mav@FreeBSD.org>
Date:   Tue Dec 3 12:59:30 2019 -0500

    Fix use-after-free in case of L2ARC prefetch failure

    In case L2ARC read failed, l2arc_read_done() creates _different_ ZIO
    to read data from the original storage device.  Unfortunately pointer
    to the failed ZIO remains in hdr->b_l1hdr.b_acb->acb_zio_head, and if
    some other read try to bump the ZIO priority, it will crash.

    The problem is reproducible by corrupting L2ARC content and reading
    some data with prefetch if l2arc_noprefetch tunable is changed to 0.
    With the default setting the issue is probably not reproducible now.

    Reviewed-by: Tom Caputi <tcaputi@datto.com>
    Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
    Signed-off-by: Alexander Motin <mav@FreeBSD.org>
    Sponsored-By: iXsystems, Inc.
    Closes #9648

From 37c22948e5b1ce9f9f11c66676ecca15caf1f264 Mon Sep 17 00:00:00 2001
From: George Amanakis <gamanakis@gmail.com>
Date: Tue, 31 Mar 2020 13:46:48 -0400
Subject: [PATCH] Reset l2ad_hand and l2ad_first in l2arc_evict

Increasing l2arc_write_size or l2arc_write_boost can result in
l2arc_write_buffers() not having enough space to perform its writes and
panic zio_write_phys().

Instead of resetting l2ad_hand to l2ad_start at the end of
l2arc_write_buffers() and not taking into account a possible
user-mediated increase of l2arc_write_max, we do this in l2arc_evict(),
right after l2arc_write_size() has run. If there is not enough space to
evict (ie we will exceed l2ad_end) we evict to the end of the device,
reset l2ad_hand to l2ad_start, set l2ad_first to 0 and iterate
l2arc_evict(). We avoid infinite iteration of l2arc_evict() by making
sure in l2arc_write_size() that l2ad_start + size does not exceed
l2ad_end.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #10154

From ca6c7a94c994b9979824f8fccf1c6ddb00ee5598 Mon Sep 17 00:00:00 2001
From: Brian Behlendorf <behlendorf1@llnl.gov>
Date: Fri, 15 Mar 2019 14:17:38 -0700
Subject: [PATCH] Fix l2arc_evict() destroy race

When destroying an arc_buf_hdr_t its identity cannot be discarded
until it is entirely undiscoverable.  This not only includes being
unhashed, but also being removed from the l2arc header list.
Discarding the header's identify prematurely renders the hash
lock useless because it will always hash to bucket zero.

This change resolves a race with l2arc_evict() by discarding the
identity after it has been removed from the l2arc header list.
This ensures either the header is not on the list or contains
the correct identify.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7688
Closes #8144

From 77f6826b83b7e27f0996f6d192202c36f65e41fd Mon Sep 17 00:00:00 2001
From: George Amanakis <gamanakis@gmail.com>
Date: Fri, 10 Apr 2020 13:33:35 -0400
Subject: [PATCH] Persistent L2ARC

This commit makes the L2ARC persistent across reboots. We implement
a light-weight persistent L2ARC metadata structure that allows L2ARC
contents to be recovered after a reboot. This significantly eases the
impact a reboot has on read performance on systems with large caches.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Saso Kiselkov <skiselkov@gmail.com>
Co-authored-by: Jorgen Lundman <lundman@lundman.net>
Co-authored-by: George Amanakis <gamanakis@gmail.com>
Ported-by: Yuxuan Shui <yshuiv7@gmail.com>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #925
Closes #1823
Closes #2672
Closes #3744
Closes #9582

From 9249f1272ec08316f4482cba145b42ba935d3b02 Mon Sep 17 00:00:00 2001
From: George Amanakis <gamanakis@gmail.com>
Date: Fri, 17 Apr 2020 12:27:40 -0400
Subject: [PATCH] Persistent L2ARC minor fixes

Minor fixes on persistent L2ARC improving code readability and fixing
a typo in zdb.c when byte-swapping a log block. It also improves the
pesist_l2arc_007_pos.ksh test by giving it more time to retrieve log
blocks on the cache device.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Adam D. Moss <c@yotes.com>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #10210

From 657fd33bcff17e44ad55dffdf294d7c107b4bf5d Mon Sep 17 00:00:00 2001
From: George Amanakis <gamanakis@gmail.com>
Date: Thu, 7 May 2020 19:34:03 -0400
Subject: [PATCH] Improvements on persistent L2ARC

Functional changes:

We implement refcounts of log blocks and their aligned size on the
cache device along with two corresponding arcstats. The refcounts are
reflected in the header of the device and provide valuable information
as to whether log blocks are accounted for correctly. These are
dynamically adjusted as log blocks are committed/evicted. zdb also uses
this information in the device header and compares it to the
corresponding values as reported by dump_l2arc_log_blocks() which
emulates l2arc_rebuild(). If the refcounts saved in the device header
report higher values, zdb exits with an error. For this feature to work
correctly there should be no active writes on the device. This is also
employed in the tests of persistent L2ARC. We extend the structure of
the cache device header by adding the two new variables mirroring the
refcounts after the existing variables to preserve backward
compatibility in terms of persistent L2ARC.

1) a new arcstat "l2_log_blk_asize" and refcount "l2ad_lb_asize" which
   reflect the total aligned size of log blocks on the device. This is
   also reflected in the header of the cache device as "dh_lb_asize".
2) a new arcstat "l2arc_log_blk_count" and refcount "l2ad_lb_count" 
   which reflect the total number of L2ARC log blocks present on cache
   devices.  It is also reflected in the header of the cache device as
   "dh_lb_count".

In l2arc_rebuild_vdev() if the amount of committed log entries in a log
block is 0 and the device header is valid we update the device header.
This will facilitate trimming of the whole device in this case when
TRIM for L2ARC is implemented.

Improve loop protection in l2arc_rebuild() by using the starting offset
of the payload of each log block instead of the starting offset of the
log block.

If the zio in l2arc_write_buffers() fails, restore the lbps array in the
header of the device to its previous state in l2arc_write_done().

If l2arc_rebuild() ends the rebuild process without restoring any L2ARC
log blocks in ARC and without any other error, this means that the lbps
array in the header is pointing to non-existent or invalid log blocks.
Reset the device header in this case.

In l2arc_rebuild() change the zfs_dbgmsg messages to
spa_history_log_internal() making them user visible with zpool history
command.

Non-functional changes:

Make the first test in persistent L2ARC use `zdb -lll` to increase
coverage in `zdb.c`.

Rename psize with asize when referring to log blocks, since
L2ARC_SET_PSIZE stores the vdev aligned size for log blocks. Also
rename dh_log_blk_entries to dh_log_entries to make it clear that
it is a mirror of l2ad_log_entries. Added comments for both changes.

Fix inaccurate comments for example in l2arc_log_blk_restore().

Add asserts at the end in l2arc_evict() and l2arc_write_buffers().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #10228

From 2054f35e564b59e23950ee573e91dbae3fd2ccd9 Mon Sep 17 00:00:00 2001
From: George Amanakis <gamanakis@gmail.com>
Date: Fri, 10 Jul 2020 17:10:03 -0400
Subject: [PATCH] Fix a persistent L2ARC bug in l2arc_write_done()

In case l2arc_write_done() handles a zio that was not successful check
that the list of log block pointers is not empty when restoring them
in the device header. Otherwise zero them out. In any case perform the
actual write updating the device header after the zio of
l2arc_write_buffers() completes as l2arc_write_done() may have touched
the memory holding the log block pointers in the device header.

Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #10540
Closes #10543

git commit f0a052391861a2b96cf28973c3b7f2854591aa79

commit  f0a052391861a2b96cf28973c3b7f2854591aa79
Author: George Amanakis <gamanakis@gmail.com>
Date:   2020-10-16T16:10:02.000Z

    3525 Persistent L2ARC
    Portions contributed by: Saso Kiselkov <skiselkov@gmail.com>
    Portions contributed by: Jorgen Lundman <lundman@lundman.net>
    Portions contributed by: Brian Behlendorf <behlendorf1@llnl.gov>
    Portions contributed by: Alexander Motin <mav@FreeBSD.org>
    Portions contributed by: Jason King <jason.king@joyent.com>
    Reviewed by: C Fraire <cfraire@me.com>
    Reviewed by: Toomas Soome <tsoome@me.com>
    Approved by: Dan McDonald <danmcd@joyent.com>