Both of these features are not needed by many consumers and result in avoidable
reads which in turn puts them on profiles due to cache-line ping ponging.
On top of that the current lockgmr entry point is slower than necessary
single-threaded. As an attempted clean up preparing for other changes,
provide new routines which don't support any of the aforementioned features.
With these patches in place vop_stdlock and vop_stdunlock disappear from
flamegraphs during -j 104 buildkernel.
Reviewed by: jeff (previous version)
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D22665
The current vnode layout is not smp-friendly by having frequently read data
avoidably sharing cachelines with very frequently modified fields. In
particular v_iflag inspected for VI_DOOMED can be found in the same line with
v_usecount. Instead make it available in the same cacheline as the v_op, v_data
and v_type which all get read all the time.
v_type is avoidably 4 bytes while the necessary data will easily fit in 1.
Shrinking it frees up 3 bytes, 2 of which get used here to introduce a new
flag field with a new value: VIRF_DOOMED.
Reviewed by: kib, jeff
Differential Revision: https://reviews.freebsd.org/D22715
vn_open_cred() assumes that it is called from the top-level of a VFS
syscall. Writers must call bwillwrite() before locking any VFS
resource to wait for cleanup of dirty buffers.
ZFS getextattr() and setextattr() VOPs do call vn_open_cred(), which
results in wait for unrelated buffers while owning ZFS vnode lock (and
ZFS does not use buffer cache). VN_OPEN_INVFS allows caller to skip
bwillwrite.
Note that ZFS is still incorrect there, because it starts write on an
mp and locks a vnode while holding another vnode lock.
Reported by: Willem Jan Withagen <wjw@digiware.nl>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
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.
MFC after: 2 weeks
Sponsored by: iXsystems, Inc.
illumos/illumos-gate@555d674d5d555d674d5dhttps://www.illumos.org/issues/10592
This is a collection of recent fixes from ZoL:
8eef997679b Error path in metaslab_load_impl() forgets to drop ms_sync_lock
928e8ad47d3 Introduce auxiliary metaslab histograms
425d3237ee8 Get rid of space_map_update() for ms_synced_length
6c926f426a2 Simplify log vdev removal code
21e7cf5da89 zdb -L should skip leak detection altogether
df72b8bebe0 Rename range_tree_verify to range_tree_verify_not_present
75058f33034 Remove unused vdev_t fields
Portions contributed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Author: Serapheim Dimitropoulos <serapheim@delphix.com>
MFC after: 4 weeks
illumos/illumos-gate@663207adb1663207adb1
10601 Pool allocation classes
https://www.illumos.org/issues/10601
illumos port of ZoL Pool allocation classes. Includes at least these two
commits:
441709695 Pool allocation classes misplacing small file blocks
cc99f275a Pool allocation classes
10757 Add -gLp to zpool subcommands for alt vdev names
https://www.illumos.org/issues/10757
Port from ZoL of
d2f3e292d Add -gLp to zpool subcommands for alt vdev names
Note that a subsequent ZoL commit changed -p to -P
a77f29f93 Change full path subcommand flag from -p to -P
Portions contributed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Portions contributed by: Håkan Johansson <f96hajo@chalmers.se>
Portions contributed by: Richard Yao <ryao@gentoo.org>
Portions contributed by: Chunwei Chen <david.chen@nutanix.com>
Portions contributed by: loli10K <ezomori.nozomu@gmail.com>
Author: Don Brady <don.brady@delphix.com>
11541 allocation_classes feature must be enabled to add log device
illumos/illumos-gate@c1064fd7cec1064fd7cehttps://www.illumos.org/issues/11541
After the allocation_classes feature was integrated, one can no longer add a
log device to a pool unless that feature is enabled. There is an explicit check
for this, but it is unnecessary in the case of log devices, so we should handle
this better instead of forcing the feature to be enabled.
Author: Jerry Jelinek <jerry.jelinek@joyent.com>
FreeBSD notes.
I faithfully added the new -g, -L, -P flags, but only -g does something:
vdev GUIDs are displayed instead of device names. -L, resolve symlinks,
and -P, display full disk paths, do nothing at the moment.
The use of special vdevs is backward compatible for read-only access, so
root pools should be bootable, but exercise caution.
MFC after: 4 weeks
10499 Multi-modifier protection (MMP)
illumos/illumos-gate@e0f1c0afa4e0f1c0afa4https://www.illumos.org/issues/10499
Port the following ZFS commits from ZoL to illumos.
379ca9cf2 Multi-modifier protection (MMP)
bbffb59ef Fix multihost stale cache file import
0d398b256 Do not initiate MMP writes while pool is suspended
10701 Correct lock ASSERTs in vdev_label_read/write
illumos/illumos-gate@58447f688d58447f688dhttps://www.illumos.org/issues/10701
Port of ZoL commit:
0091d66f4e Correct lock ASSERTs in vdev_label_read/write
At a minimum, this fixes a blown assert during an MMP test run when running on
a DEBUG build.
11770 additional mmp fixes
illumos/illumos-gate@4348eb90124348eb9012https://www.illumos.org/issues/11770
Port a few additional MMP fixes from ZoL that came in after our
initial MMP port.
4ca457b065 ZTS: Fix mmp_interval failure
ca95f70dff zpool import progress kstat
(only minimal changes from above can be pulled in right now)
060f0226e6 MMP interval and fail_intervals in uberblock
Note from the committer (me).
I do not have any use for this feature and I have not tested it. I only
did smoke testing with multihost=off.
Please be aware.
I merged the code only to make future merges easier.
Portions contributed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Portions contributed by: Tim Chase <tim@chase2k.com>
Portions contributed by: sanjeevbagewadi <sanjeev.bagewadi@gmail.com>
Portions contributed by: John L. Hammond <john.hammond@intel.com>
Portions contributed by: Giuseppe Di Natale <dinatale2@llnl.gov>
Portions contributed by: Prakash Surya <surya1@llnl.gov>
Portions contributed by: Brian Behlendorf <behlendorf1@llnl.gov>
Author: Olaf Faaland <faaland1@llnl.gov>
MFC after: 4 weeks
illumos/illumos-gate@9c2acf00e29c2acf00e2https://www.illumos.org/issues/10554
During the port of MMP (illumos bug 10499) from ZoL, I found this
earlier ZoL project is a prerequisite. Here is the original
description. This addition will enable us to sync an open TXG to the
main pool on demand. The functionality is similar to 'sync(2)' but
'zpool sync' will return when data has hit the main storage instead of
potentially just the ZIL as is the case with the 'sync(2)' cmd.
Portions contributed by: Jerry Jelinek <jerry.jelinek@joyent.com>
Author: Alek Pinchuk <apinchuk@datto.com>
MFC after: 3 weeks
Relnotes: possibly
I overlooked the fact that zfsproc is required by dtrace modules that
use illumos compatible taskq KPI. So, move the symbol definition to
the opensolaris module that provides compatibility support for both ZFS
and DTrace. Also, rename zfsproc to system_proc to reflect that it is
not specific to ZFS.
Reported by: ae
MFC after: 5 weeks
X-MFC with: ae
The purpose of this change is to group kernelthreads specific to a
particular ZFS pool under a kernel process. There can be many dozens of
threads per pool. This change improves observability of those threads.
This change consists of several subchanges:
1. illumos taskq_create_proc can now pass its process parameter to
taskqueue. Also, use zfsproc instead of NULL for taskq_create. Caveat:
zfsproc might not be initialized yet. But in that case it is still NULL,
so not worse than before.
2. illumos sys/proc.h: kthread id is stored in t_did field, not t_tid.
3. zfs: enable SPA_PROCESS on the kernel side. The change is a bit hairy
as newproc() is implemented privately to spa.c. I couldn't think of a
better way to populate process name than to poke inside the argument for
the process routine.
4. illumos thread_create: allow assigning thread to process other than
zfsproc.
5. zfs: expose spa_proc to other users, assign sync and quiesce threads
to it.
Pool-specific threads created using (relatively new) zthr mechanism are
still assigned to the zfskern process rather than to a respective
zpool-xxx process. I am going to address this a bit later.
Reviewed by: no one
MFC after: 5 weeks
Relnotes: perhaps
Differential Revision: https://reviews.freebsd.org/D9720
Since physical device asize is calculated from psize and the asize is stored
in pool label, we can use asize to set the value of psize, which is used to
calculate the location of the pool labels.
MFC after: 1 week
Port illumos change: https://www.illumos.org/issues/11667
Move lz4.c out of zfs tree to opensolaris/common/lz4, adjust it to be
usable from kernel/stand/userland builds, so we can use just one single
source. Add lz4.h to declare lz4_compress() and lz4_decompress().
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D22037
The previous code came from OpenSolaris, which in my understanding require
allocation size to be known to free memory. To store that size previous
code allocated additional 8 byte header. But I have noticed that zlib
with present settings allocates 64KB context buffers for each call, that
could be efficiently cached by UMA, but addition of those 8 bytes makes
them fall back to physical RAM allocations, that cause huge overhead and
lock congestion on small blocks. Since FreeBSD's free() does not have
the size argument, switching to it solves the problem, increasing write
speed to ZVOLs with 4KB block size and GZIP compression on my 40-threads
test system from ~60MB/s to ~600MB/s.
MFC after: 1 week
Sponsored by: iXsystems, Inc.
This patch modifies the zfs_ioc_snapshot_list_next() ioctl to enable it
to take input parameters that alter the way looping through the list of
snapshots is performed. The idea here is to restrict functions that
throw away some of the snapshots returned by the ioctl to a range of
snapshots that these functions actually use. This improves efficiency
and execution speed for some rollback and send operations.
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes#8077zfsonlinux/zfs@4c0883fb4a
MFC after: 2 weeks
except for filesystems that set the MNTK_VMSETSIZE_BUG, Set the flag for ZFS.
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D21883
illumos/illumos-gate@c4ab0d3f46c4ab0d3f46https://www.illumos.org/issues/10809
Port ZoL ee36c709c3d Performance optimization of AVL tree comparator functions
This is a followup to r337567 that imported the ZoL commit directly into
FreeBSD. It seems that at the time we did not have some of the earlier
changes, so some pieces of the ZoL change were not applicable. Also,
the illumos version got a few style cleanups. Some changes were missed
or incorrectly merged (e.g., vdev_cache_lastused_compare and
metaslab_rangesize_compare).
Obtained from: ZoL, illumos
MFC after: 25 days
X-MFC after: r353634
illumos/illumos-gate@7931524763
FreeBSD note: some tweaking was needed to avoid a conflict with
sys/rangelock.h.
Author: Matthew Ahrens <mahrens@delphix.com>
Obtained from: illumos
MFC after: 3 weeks
illumos/illumos-gate@52abb70e0752abb70e07https://www.illumos.org/issues/9691
When iterating over a ZAP object, we're almost always certain to
iterate over the entire object. If there are multiple leaf blocks, we
can realize a performance win by issuing reads for all the leaf blocks
in parallel when the iteration begins.
For example, if we have 10,000 snapshots, "zfs destroy -nv
pool/fs@1%9999" can take 30 minutes when the cache is cold. This
change provides a >3x performance improvement, by issuing the reads
for all ~64 blocks of each ZAP object in parallel.
Author: Matthew Ahrens <mahrens@delphix.com>
Obtained from: illumos
MFC after: 2 weeks
illumos/illumos-gate@d0cb1fb926d0cb1fb926https://www.illumos.org/issues/9425
Problem Statement
ZFS Channel program scripts currently require a timeout, so that hung
or long-running scripts return a timeout error instead of causing ZFS
to get wedged. This limit can currently be set up to 100 million Lua
instructions. Even with a limit in place, it would be desirable to
have a sys admin (support engineer) be able to cancel a script that is
taking a long time.
Proposed Solution
Make it possible to abort a channel program by sending an interrupt
signal.In the underlying txg_wait_sync function, switch the cv_wait to
a cv_wait_sig to catch the signal. Once a signal is encountered, the
dsl_sync_task function can install a Lua hook that will get called
before the Lua interpreter executes a new line of code. The
dsl_sync_task can resume with a standard txg_wait_sync call and wait
for the txg to complete. Meanwhile, the hook will abort the script and
indicate that the channel program was canceled. The kernel returns a
EINTR to indicate that the channel program run was canceled.
FreeBSD note: the return value of cv_wait_sig() has inverted meaning
between us and illumos.
Author: Don Brady <don.brady@delphix.com>
Obtained from: illumos
MFC after: 4 weeks
illumos/illumos-gate@a0b03b161ca0b03b161chttps://www.illumos.org/issues/10330
3 recent ZoL changes in the vdev and metaslab code which we can pull over:
PR 8324 c853f382db 8324 Change target size of metaslabs from 256GB to 16GB
PR 8290 b194fab0fb 8290 Factor metaslab_load_wait() in metaslab_load()
PR 8286 419ba59145 8286 Update vdev_is_spacemap_addressable() for new spacemap
encoding
Author: Serapheim Dimitropoulos <serapheimd@gmail.com>
Obtained from: illumos, ZoL
MFC after: 2 weeks
illumos/illumos-gate@e914ace2e9e914ace2e9https://www.illumos.org/issues/10343
On the openzfs feature/porting matrix, this is listed as:
prefix to refcount funcs/types
Having these changes will make it easier to share other work across the
different ZFS operating systems.
PR 7963 424fd7c3e Prefix all refcount functions with zfs_
PR 7885 & 7932 c13060e47 Linux 4.19-rc3+ compat: Remove refcount_t compat
PR 5823 & 5842 4859fe796 Linux 4.11 compat: avoid refcount_t name conflict
Author: Tim Schumacher <timschumi@gmx.de>
Obtained from: illumos, ZoL
MFC after: 3 weeks
illumos/illumos-gate@aa02ea0194aa02ea0194
10572 Fix race in dnode_check_slots_free()
https://www.illumos.org/issues/10572
The Fix from ZoL:
Currently, dnode_check_slots_free() works by checking dn->dn_type
in the dnode to determine if the dnode is reclaimable. However,
there is a small window of time between dnode_free_sync() in the
first call to dsl_dataset_sync() and when the useraccounting code
is run when the type is set DMU_OT_NONE, but the dnode is not yet
evictable, leading to crashes. This patch adds the ability for
dnodes to track which txg they were last dirtied in and adds a
check for this before performing the reclaim.
This patch also corrects several instances when dn_dirty_link was
treated as a list_node_t when it is technically a multilist_node_t.
10579 Don't allow dnode allocation if dn_holds != 0
https://www.illumos.org/issues/10579
The fix from ZoL:
This patch simply fixes a small bug where dnode_hold_impl() could
attempt to allocate a dnode that was in the process of being freed,
but which still had active references. This patch simply adds the
required check.
Author: Tom Caputi <tcaputi@datto.com>
Reported by: delphij
MFC after: 2 weeks
X-MFC with: r353176
illumos/illumos-gate@946342a260946342a260https://www.illumos.org/issues/10452
illumos is missing a few small follow up ZoL bug fixes for the large dnode
feature. We should pull those in.
Those commits are in the ZoL tree as (newest to oldest):
PR 8435 - 75d6b7ddca269542279975f716a343bb40a79baf - Add missing copyright
notice to large_dnode tests
PR 7433 - e14a32b1c844d924b9f093375c0badcf10f61741 - Fix object reclaim when
using large dnodes
PR 6616 - 48fbb9ddbf2281911560dfbc2821aa8b74127315 - Free objects when
receiving full stream as clone
PR 6695 - 39f56627ae988d09b4e3803c01c22b2026b2310e - receive_freeobjects()
skips freeing some object
Portions contributed by: Ned Bass <bass6@llnl.gov>
Portions contributed by: Tom Caputi <tcaputi@datto.com>
Author: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Obtained from: illumos, ZoL
MFC after: 2 weeks
X-MFC with: r353176
Atomics are used for page busy and valid state when the shared busy is
held. The details of the locking protocol and valid and dirty
synchronization are in the updated vm_page.h comments.
Reviewed by: kib, markj
Tested by: pho
Sponsored by: Netflix, Intel
Differential Revision: https://reviews.freebsd.org/D21594
fcmpset can have two kinds of semantics, weak and strong.
For practical purposes, strong semantics means that if fcmpset fails
then the reported current value is always different from the expected
value. Weak semantics means that the reported current value may be the
same as the expected value even though fcmpset failed. That's a so
called "sporadic" failure.
I originally implemented atomic_cas expecting strong semantics, but many
platforms actually have weak one.
Reported by: pkubaj (not confirmed if same issue)
Discussed with: kib, mjg
MFC after: 19 days
X-MFC with: r353340
This patch fixes 2 issues with the DMU free throttle implemented
in dmu_free_long_range(). The first issue is that get_next_chunk()
was calculating the number of L1 blocks the free would dirty
incorrectly. In some cases involving extremely large files, this
code would greatly overestimate the number of affected L1 blocks,
causing excessive calls to txg_wait_open(). This patch corrects
the calculation.
The second issue is that the free throttle uses the total number
of free'd blocks in all (open, quiescing, and syncing) txgs to
determine whether to throttle. This causes large frees (such as
those created by the first issue) to cause 4 txg syncs before
any further frees were allowed to proceed. This patch ensures
that the accounting is done entirely in a per-txg fashion, so
that frees from a given txg don't affect those that immediately
follow it.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
zfsonlinux/zfs@f4c594da94
Freeing throttle should account for holes
Deletion throttle currently does not account for holes in a file.
This means that it can activate when it shouldn't.
To fix it we switch the throttle to be based on the number of
L1 blocks we will have to dirty when freeing
Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
zfsonlinux/zfs@65282ee9e0
Submitted by: Alek Pinchuk <pinchuk.alek@gmail.com>
Reviewed by: allanjude
MFC after: 2 weeks
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D21895
membar_producer is supposed to be a store-store barrier.
Also, in the code that FreeBSD has ported from illumos membar_producer
is used only with regular stores to regular memory (with respect to
caching).
We do not have an MI primitive for the store-store barrier, so
atomic_thread_fence_rel is the closest we have as it provides
(load | store) -> store barrier.
Previously, membar_producer was an empty function call on all 32-bit
arm-s, 32-bit powerpc, riscv and all mips variants. I think that it was
inadequate.
On other platforms, such as amd64, arm64, i386, powerpc64, sparc64,
membar_producer was implemented using stronger primitives than required
for a store-store barrier with respect to regular memory access.
For example, it used sfence on amd64 and lock-ed nop in i386 (despite TSO).
On powerpc64 we now use recommended lwsync instead of eieio.
On sparc64 FreeBSD uses TSO mode.
On arm64/aarch64 we now use dmb sy instead of dmb ish. Not sure if this
is an improvement, actually.
After this change we can drop opensolaris_atomic.S for aarch64, amd64,
powerpc64 and sparc64 as all required atomic operations have either
direct or light-weight mapping to FreeBSD native atomic operations.
Discussed with: kib
MFC after: 4 weeks
atomic_cas_32 is implemented using atomic_fcmpset_32 on all platforms.
Ditto for atomic_cas_64 and atomic_fcmpset_64 on platforms that have it.
The only exception is sparc64 that provides MD atomic_cas_32 and
atomic_cas_64.
This is slightly inefficient as fcmpset reports whether the operation
updated the target and that information is not needed for cas.
Nevertheless, there is less code to maintain and to add for new platforms.
Also, the operations are done inline now as opposed to function calls before.
atomic_add_64_nv is implemented using atomic_fetchadd_64 on platforms
that provide it.
casptr, cas32, atomic_or_8, atomic_or_8_nv are completely removed as they
have no users.
atomic_mtx that is used to emulate 64-bit atomics on platforms that lack
them is defined only on those platforms.
As a result, platform specific opensolaris_atomic.S files have lost most of
their code. The only exception is i386 where the compat+contrib code
provides 64-bit atomics for userland use. That code assumes availability of
cmpxchg8b instruction. FreeBSD does not have that assumption for i386
userland and does not provide 64-bit atomics. Hopefully, this can and will
be fixed.
MFC after: 3 weeks
As long as we support ZFS on 32-bit platforms we should do this for all
64-bit variables that are modified in a lockless fashion using atomic
operations. Otherwise, there is a risk of a reading a torn value.
Here is a rationale for why I am doing this in dmu_object_alloc_impl:
- it's very recent code
- the code deals with object IDs and a number of objects in a file
system can overflow 32 bits
- incorrect allocation of an object ID may result in hard to debug
problems
- fixing all plain reads of 64-bit atomic variables is not a trivial
undertaking to do in one shot, so I chose to do it incrementally
MFC after: 3 weeks
X-MFC after: r353301, r353176
The compatibility code for the atomic operations in ZFS code is a bit
messy. In some cases the native definitions are directly made
available, in some cases there are emulated operations in
opensolaris_atomic.c and in yet other cases there are atomic operations
implemented in assembly that were obtained from OpenSolaris / illumos.
This commit adds atomic_swap_64 for use with i386 userland.
The code is copied from illumos.
I am not sure why FreeBSD does not provide that operation natively.
Maybe because we try (or pretend) to support processors that did not
have the necessary instructions.
While here I also added atomic_load_64 for the same reasons.
This is original code based on iilumos atomic_swap_64 and FreeBSD
atomic_load_acq_64_i586.
Pointyhat to: avg
MFC after: 1 week
8423 8199 7432 Implement large_dnode pool feature
7432 Large dnode pool feature
8199 multi-threaded dmu_object_alloc()
8423 Implement large_dnode pool feature
10406 large_dnode changes broke zfs recv of legacy stream
llumos/illumos-gate@54811da5ac54811da5achttps://www.illumos.org/issues/8423https://www.illumos.org/issues/8199https://www.illumos.org/issues/7432illumos/illumos-gate@811964cd9f811964cd9fhttps://www.illumos.org/issues/10406
ZoL issues:
Improved dnode allocation #6564
Clean up large dnode code #6262
Fix dnode_hold() freeing dnode behavior #8172
Fix dnode allocation race #6414, #6439
Partial: Raw sends must be able to decrease nlevels #6821, #6864
Remove unnecessary txg syncs from receive_object() Closes#7197
This updates FreeBSD large_dnode code (that was imported from ZoL) to a
version that was committed to illumos. It has some cleanups,
improvements and fixes comparing to what we have in FreeBSD now.
I think that the most significant update is 8199 multi-threaded
dmu_object_alloc().
This commit reverts r351077 that was a revert of r351074 and r351076 and
restores those changes. Required atomic operations should be available
now on all platforms where we build ZFS.
Obtained from: illumos
MFC after: 3 weeks
Previously, the code used a plain store on platforms that lacked
atomic_swap_64 and possibly some other platforms as the condition worked
only if atomic_swap_64 was a macro.
MFC after: 1 week
X-MFC after: r353166, r353167
Some 32-bit platforms do not provide 64-bit atomic operations that ZFS
requires, either in userland or at all. We emulate those operations for
those platforms using a mutex. That is not entirely correct and it's
very efficient. Besides, the loads are plain loads, so torn values are
possible.
Nevertheless, the emulation seems to work for some definition of work.
This change adds atomic_swap_64, which is already used in ZFS code, and
atomic_load_64 that can be used to prevent torn reads.
MFC after: 1 week
