This makes it possible to manually force updating capacity data
after the disk got resized. Without it it might be neccessary to
reboot before FreeBSD notices updated disk size under eg VMWare.
Discussed with: imp@
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6108
as before. The common scheduling bits have moved from inline code in
each of the CAM periph drivers into a library that implements the
default scheduling.
In addition, a number of rate-limiting and I/O preference options can
be enabled by adding CAM_IOSCHED_NETFLIX to your config file. A number
of extra stats are also maintained. CAM_IOSCHED_NETFLIX isn't on by
default because it uses a separate BIO_READ and BIO_WRITE queue, so
doesn't honor BIO_ORDERED between these two types of operations. We
already didn't honor it for BIO_DELETE, and we don't depend on
BIO_ORDERED between reads and writes anywhere in the system (it is
currently used with BIO_FLUSH in ZFS to make sure some writes are
complete before others start and as a poor-man's soft dependency in
one place in UFS where we won't be issuing READs until after the
operation completes). However, out of an abundance of caution, it
isn't enabled by default.
Plus, this also brings in NCQ TRIM support for those SSDs that support
it. A black list is also provided for known rogues that use NCQ trim
as an excuse to corrupt the drive. It was difficult to separate out
into a separate commit.
This code has run in production at Netflix for over a year now.
Sponsored by: Netflix, Inc
Differential Revision: https://reviews.freebsd.org/D4609
This adds Samsung PM851 to the list. It can be found in Lenovo Thinkpad
T440 for instance.
Reviewed by: Kevin Bowling <kevin.bowling@kev009.com>,
Jason Wolfe <j@nitrology.com>
Approved by: Kevin Bowling <kevin.bowling@kev009.com>,
Jason Wolfe <j@nitrology.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D5753
the if statement it pairs with). While not an error today, a careless
edit in the future could cause problems (though given the nature of
this specific code, the problems quite likely would be some variation
of "most direct access SCSI storage devices won't attach," which is
unlikely to go unnoticed).
PVS-Studio: V705
camdd(8) utility.
CCBs may be queued to the driver via the new CAMIOQUEUE ioctl, and
completed CCBs may be retrieved via the CAMIOGET ioctl. User
processes can use poll(2) or kevent(2) to get notification when
I/O has completed.
While the existing CAMIOCOMMAND blocking ioctl interface only
supports user virtual data pointers in a CCB (generally only
one per CCB), the new CAMIOQUEUE ioctl supports user virtual and
physical address pointers, as well as user virtual and physical
scatter/gather lists. This allows user applications to have more
flexibility in their data handling operations.
Kernel memory for data transferred via the queued interface is
allocated from the zone allocator in MAXPHYS sized chunks, and user
data is copied in and out. This is likely faster than the
vmapbuf()/vunmapbuf() method used by the CAMIOCOMMAND ioctl in
configurations with many processors (there are more TLB shootdowns
caused by the mapping/unmapping operation) but may not be as fast
as running with unmapped I/O.
The new memory handling model for user requests also allows
applications to send CCBs with request sizes that are larger than
MAXPHYS. The pass(4) driver now limits queued requests to the I/O
size listed by the SIM driver in the maxio field in the Path
Inquiry (XPT_PATH_INQ) CCB.
There are some things things would be good to add:
1. Come up with a way to do unmapped I/O on multiple buffers.
Currently the unmapped I/O interface operates on a struct bio,
which includes only one address and length. It would be nice
to be able to send an unmapped scatter/gather list down to
busdma. This would allow eliminating the copy we currently do
for data.
2. Add an ioctl to list currently outstanding CCBs in the various
queues.
3. Add an ioctl to cancel a request, or use the XPT_ABORT CCB to do
that.
4. Test physical address support. Virtual pointers and scatter
gather lists have been tested, but I have not yet tested
physical addresses or scatter/gather lists.
5. Investigate multiple queue support. At the moment there is one
queue of commands per pass(4) device. If multiple processes
open the device, they will submit I/O into the same queue and
get events for the same completions. This is probably the right
model for most applications, but it is something that could be
changed later on.
Also, add a new utility, camdd(8) that uses the asynchronous pass(4)
driver interface.
This utility is intended to be a basic data transfer/copy utility,
a simple benchmark utility, and an example of how to use the
asynchronous pass(4) interface.
It can copy data to and from pass(4) devices using any target queue
depth, starting offset and blocksize for the input and ouptut devices.
It currently only supports SCSI devices, but could be easily extended
to support ATA devices.
It can also copy data to and from regular files, block devices, tape
devices, pipes, stdin, and stdout. It does not support queueing
multiple commands to any of those targets, since it uses the standard
read(2)/write(2)/writev(2)/readv(2) system calls.
The I/O is done by two threads, one for the reader and one for the
writer. The reader thread sends completed read requests to the
writer thread in strictly sequential order, even if they complete
out of order. That could be modified later on for random I/O patterns
or slightly out of order I/O.
camdd(8) uses kqueue(2)/kevent(2) to get I/O completion events from
the pass(4) driver and also to send request notifications internally.
For pass(4) devcies, camdd(8) uses a single buffer (CAM_DATA_VADDR)
per CAM CCB on the reading side, and a scatter/gather list
(CAM_DATA_SG) on the writing side. In addition to testing both
interfaces, this makes any potential reblocking of I/O easier. No
data is copied between the reader and the writer, but rather the
reader's buffers are split into multiple I/O requests or combined
into a single I/O request depending on the input and output blocksize.
For the file I/O path, camdd(8) also uses a single buffer (read(2),
write(2), pread(2) or pwrite(2)) on reads, and a scatter/gather list
(readv(2), writev(2), preadv(2), pwritev(2)) on writes.
Things that would be nice to do for camdd(8) eventually:
1. Add support for I/O pattern generation. Patterns like all
zeros, all ones, LBA-based patterns, random patterns, etc. Right
Now you can always use /dev/zero, /dev/random, etc.
2. Add support for a "sink" mode, so we do only reads with no
writes. Right now, you can use /dev/null.
3. Add support for automatic queue depth probing, so that we can
figure out the right queue depth on the input and output side
for maximum throughput. At the moment it defaults to 6.
4. Add support for SATA device passthrough I/O.
5. Add support for random LBAs and/or lengths on the input and
output sides.
6. Track average per-I/O latency and busy time. The busy time
and latency could also feed in to the automatic queue depth
determination.
sys/cam/scsi/scsi_pass.h:
Define two new ioctls, CAMIOQUEUE and CAMIOGET, that queue
and fetch asynchronous CAM CCBs respectively.
Although these ioctls do not have a declared argument, they
both take a union ccb pointer. If we declare a size here,
the ioctl code in sys/kern/sys_generic.c will malloc and free
a buffer for either the CCB or the CCB pointer (depending on
how it is declared). Since we have to keep a copy of the
CCB (which is fairly large) anyway, having the ioctl malloc
and free a CCB for each call is wasteful.
sys/cam/scsi/scsi_pass.c:
Add asynchronous CCB support.
Add two new ioctls, CAMIOQUEUE and CAMIOGET.
CAMIOQUEUE adds a CCB to the incoming queue. The CCB is
executed immediately (and moved to the active queue) if it
is an immediate CCB, but otherwise it will be executed
in passstart() when a CCB is available from the transport layer.
When CCBs are completed (because they are immediate or
passdone() if they are queued), they are put on the done
queue.
If we get the final close on the device before all pending
I/O is complete, all active I/O is moved to the abandoned
queue and we increment the peripheral reference count so
that the peripheral driver instance doesn't go away before
all pending I/O is done.
The new passcreatezone() function is called on the first
call to the CAMIOQUEUE ioctl on a given device to allocate
the UMA zones for I/O requests and S/G list buffers. This
may be good to move off to a taskqueue at some point.
The new passmemsetup() function allocates memory and
scatter/gather lists to hold the user's data, and copies
in any data that needs to be written. For virtual pointers
(CAM_DATA_VADDR), the kernel buffer is malloced from the
new pass(4) driver malloc bucket. For virtual
scatter/gather lists (CAM_DATA_SG), buffers are allocated
from a new per-pass(9) UMA zone in MAXPHYS-sized chunks.
Physical pointers are passed in unchanged. We have support
for up to 16 scatter/gather segments (for the user and
kernel S/G lists) in the default struct pass_io_req, so
requests with longer S/G lists require an extra kernel malloc.
The new passcopysglist() function copies a user scatter/gather
list to a kernel scatter/gather list. The number of elements
in each list may be different, but (obviously) the amount of data
stored has to be identical.
The new passmemdone() function copies data out for the
CAM_DATA_VADDR and CAM_DATA_SG cases.
The new passiocleanup() function restores data pointers in
user CCBs and frees memory.
Add new functions to support kqueue(2)/kevent(2):
passreadfilt() tells kevent whether or not the done
queue is empty.
passkqfilter() adds a knote to our list.
passreadfiltdetach() removes a knote from our list.
Add a new function, passpoll(), for poll(2)/select(2)
to use.
Add devstat(9) support for the queued CCB path.
sys/cam/ata/ata_da.c:
Add support for the BIO_VLIST bio type.
sys/cam/cam_ccb.h:
Add a new enumeration for the xflags field in the CCB header.
(This doesn't change the CCB header, just adds an enumeration to
use.)
sys/cam/cam_xpt.c:
Add a new function, xpt_setup_ccb_flags(), that allows specifying
CCB flags.
sys/cam/cam_xpt.h:
Add a prototype for xpt_setup_ccb_flags().
sys/cam/scsi/scsi_da.c:
Add support for BIO_VLIST.
sys/dev/md/md.c:
Add BIO_VLIST support to md(4).
sys/geom/geom_disk.c:
Add BIO_VLIST support to the GEOM disk class. Re-factor the I/O size
limiting code in g_disk_start() a bit.
sys/kern/subr_bus_dma.c:
Change _bus_dmamap_load_vlist() to take a starting offset and
length.
Add a new function, _bus_dmamap_load_pages(), that will load a list
of physical pages starting at an offset.
Update _bus_dmamap_load_bio() to allow loading BIO_VLIST bios.
Allow unmapped I/O to start at an offset.
sys/kern/subr_uio.c:
Add two new functions, physcopyin_vlist() and physcopyout_vlist().
sys/pc98/include/bus.h:
Guard kernel-only parts of the pc98 machine/bus.h header with
#ifdef _KERNEL.
This allows userland programs to include <machine/bus.h> to get the
definition of bus_addr_t and bus_size_t.
sys/sys/bio.h:
Add a new bio flag, BIO_VLIST.
sys/sys/uio.h:
Add prototypes for physcopyin_vlist() and physcopyout_vlist().
share/man/man4/pass.4:
Document the CAMIOQUEUE and CAMIOGET ioctls.
usr.sbin/Makefile:
Add camdd.
usr.sbin/camdd/Makefile:
Add a makefile for camdd(8).
usr.sbin/camdd/camdd.8:
Man page for camdd(8).
usr.sbin/camdd/camdd.c:
The new camdd(8) utility.
Sponsored by: Spectra Logic
MFC after: 1 week
This allows to set delete method via tunable, before device capabilities
are known. Also allow ZERO method for devices not reporting LBP, if user
explicitly requests it -- it may be useful if storage supports compression
and WRITE SAME, but does not support UNMAP.
MFC after: 2 weeks
Previously such LUNs were silently ignored. But while they indeed unable
to process most of SCSI commands, some, like RTPG, they still can.
MFC after: 1 month
properly.
If there is garbage in the flags field, it can sometimes include a
set CDAI_FLAG_STORE flag, which may cause either an error or
perhaps result in overwriting the field that was intended to be
read.
sys/cam/cam_ccb.h:
Add a new flag to the XPT_DEV_ADVINFO CCB, CDAI_FLAG_NONE,
that callers can use to set the flags field when no store
is desired.
sys/cam/scsi/scsi_enc_ses.c:
In ses_setphyspath_callback(), explicitly set the
XPT_DEV_ADVINFO flags to CDAI_FLAG_NONE when fetching the
physical path information. Instead of ORing in the
CDAI_FLAG_STORE flag when storing the physical path, set
the flags field to CDAI_FLAG_STORE.
sys/cam/scsi/scsi_sa.c:
Set the XPT_DEV_ADVINFO flags field to CDAI_FLAG_NONE when
fetching extended inquiry information.
sys/cam/scsi/scsi_da.c:
When storing extended READ CAPACITY information, set the
XPT_DEV_ADVINFO flags field to CDAI_FLAG_STORE instead of
ORing it into a field that isn't initialized.
sys/dev/mpr/mpr_sas.c,
sys/dev/mps/mps_sas.c:
When fetching extended READ CAPACITY information, set the
XPT_DEV_ADVINFO flags field to CDAI_FLAG_NONE instead of
setting it to 0.
sbin/camcontrol/camcontrol.c:
When fetching a device ID, set the XPT_DEV_ADVINFO flags
field to CDAI_FLAG_NONE instead of 0.
sys/sys/param.h:
Bump __FreeBSD_version to 1100061 for the new XPT_DEV_ADVINFO
CCB flag, CDAI_FLAG_NONE.
Sponsored by: Spectra Logic
MFC after: 1 week
VMware returns BUSY status when storage has transient connectivity issues.
It is often better to wait and let VM admin fix the problem then crash.
Discussed with: ken
MFC after: 1 week
This includes a new summary mode (-s) for camcontrol defects that
quickly tells the user the most important thing: how many defects
are in the requested list. The actual location of the defects is
less important.
Modern drives frequently have more than the 8191 defects that can
be reported by the READ DEFECT DATA (10) command. If they don't
have that many grown defects, they certainly have more than 8191
defects in the primary (i.e. factory) defect list.
The READ DEFECT DATA (12) command allows for longer parameter
lists, as well as indexing into the list of defects, and so allows
reporting many more defects.
This has been tested with HGST drives and Seagate drives, but
does not fully work with Seagate drives. Once I have a Seagate
spec I may be able to determine whether it is possible to make it
work with Seagate drives.
scsi_da.h: Add a definition for the new long block defect
format.
Add bit and mask definitions for the new extended
physical sector and bytes from index defect
formats.
Add a prototype for the new scsi_read_defects() CDB
building function.
scsi_da.c: Add a new scsi_read_defects() CDB building function.
camcontrol(8) was previously composing CDBs manually.
This is long overdue.
camcontrol.c: Revamp the camcontrol defects subcommand. We now
go through multiple stages in trying to get defect
data off the drive while avoiding various drive
firmware quirks.
We start off by requesting the defect header with
the 10 byte command. If we're in summary mode (-s)
and the drive reports fewer defects than can be
represented in the 10 byte header, we're done.
Otherwise, we know that we need to issue the
12 byte command if the drive reports the maximum
number of defects.
If we're in summary mode, we're done if we get a
good response back when asking for the 12 byte header.
If the user has asked for the full list, then we
use the address descriptor index field in the 12
byte CDB to step through the list in 64K chunks.
64K is small enough to work with most any ancient
or modern SCSI controller.
Add support for printing the new long block defect
format, as well as the extended physical sector and
bytes from index formats. I don't have any drives
that support the new formats.
Add a hexadecimal output format that can be turned
on with -X.
Add a quiet mode (-q) that can be turned on with
the summary mode (-s) to just print out a number.
Revamp the error detection and recovery code for
the defects command to work with HGST drives.
Call the new scsi_read_defects() CDB building
function instead of rolling the CDB ourselves.
Pay attention to the residual from the defect list
request when printing it out, so we don't run off
the end of the list.
Use the new scsi_nv library routines to convert
from strings to numbers and back.
camcontrol.8: Document the new defect formats (longblock, extbfi,
extphys) and command line options (-q, -s, -S and
-X) for the defects subcommand.
Explain a little more about what drives generally
do and don't support.
Sponsored by: Spectra Logic
MFC after: 1 week
None of existing STEC devices need UNMAP or even support it well, having
many limitations and even hanging sometimes executing those commands.
New devices that may use UNMAP going to be released under HGST name.
MFC after: 3 days
These changes prevent sysctl(8) from returning proper output,
such as:
1) no output from sysctl(8)
2) erroneously returning ENOMEM with tools like truss(1)
or uname(1)
truss: can not get etype: Cannot allocate memory
there is an environment variable which shall initialize the SYSCTL
during early boot. This works for all SYSCTL types both statically and
dynamically created ones, except for the SYSCTL NODE type and SYSCTLs
which belong to VNETs. A new flag, CTLFLAG_NOFETCH, has been added to
be used in the case a tunable sysctl has a custom initialisation
function allowing the sysctl to still be marked as a tunable. The
kernel SYSCTL API is mostly the same, with a few exceptions for some
special operations like iterating childrens of a static/extern SYSCTL
node. This operation should probably be made into a factored out
common macro, hence some device drivers use this. The reason for
changing the SYSCTL API was the need for a SYSCTL parent OID pointer
and not only the SYSCTL parent OID list pointer in order to quickly
generate the sysctl path. The motivation behind this patch is to avoid
parameter loading cludges inside the OFED driver subsystem. Instead of
adding special code to the OFED driver subsystem to post-load tunables
into dynamically created sysctls, we generalize this in the kernel.
Other changes:
- Corrected a possibly incorrect sysctl name from "hw.cbb.intr_mask"
to "hw.pcic.intr_mask".
- Removed redundant TUNABLE statements throughout the kernel.
- Some minor code rewrites in connection to removing not needed
TUNABLE statements.
- Added a missing SYSCTL_DECL().
- Wrapped two very long lines.
- Avoid malloc()/free() inside sysctl string handling, in case it is
called to initialize a sysctl from a tunable, hence malloc()/free() is
not ready when sysctls from the sysctl dataset are registered.
- Bumped FreeBSD version to indicate SYSCTL API change.
MFC after: 2 weeks
Sponsored by: Mellanox Technologies
trims to the device assumes the list is sorted. Don't apply the
optimization of not sorting the queue when we have SSDs to the
delete_queue, since it causes more discard traffic to the drive. While
one could argue that the higher levels should coalesce the trims,
that's not done today, so some optimization at this level is needed.
CR: https://phabric.freebsd.org/D142
Nobody yet reported disk supporting I/Os less then our MAXPHYS value, but
since we any way have code to read Block Limits VPD page, that is easy.
MFC after: 2 weeks
Instead of rereading VPD pages on every device open, do it only on initial
device probe, and in cases when device reported via UNIT ATTENTIONs that
something has changed. Capacity is still rereaded on every open because
it is more critical for operation and more probable to change in run time.
On my tests with Intel 530 SSDs on mps(4) HBA this change reduces time
GEOM needs to retaste the device (that includes few open/close cycles)
from ~150ms to ~30ms.
MFC after: 2 weeks
Sponsored by: iXsystems, Inc.
The latest draft of SBC-3 tells: "A MAXIMUM UNMAP LBA COUNT field set to
a non-zero value indicates the maximum number of LBAs that may be unmapped
by an UNMAP command." To me it does not sound like that limit is set per
single descriptor, but rather per all command. And I have at least one
device that behaves exactly that way. This patch fixes the problem there.
MFC after: 1 week
- Replace ordered_tag_count counter with single flag;
- From da remove outstanding_cmds counter, duplicating pending_ccbs list;
- From da_softc remove unused links field.
information.
The existing algorithm selects a preferred leaf vdev based on offset of the zio
request modulo the number of members in the mirror. It assumes the devices are
of equal performance and that spreading the requests randomly over both drives
will be sufficient to saturate them. In practice this results in the leaf vdevs
being under utilized.
The new algorithm takes into the following additional factors:
* Load of the vdevs (number outstanding I/O requests)
* The locality of last queued I/O vs the new I/O request.
Within the locality calculation additional knowledge about the underlying vdev
is considered such as; is the device backing the vdev a rotating media device.
This results in performance increases across the board as well as significant
increases for predominantly streaming loads and for configurations which don't
have evenly performing devices.
The following are results from a setup with 3 Way Mirror with 2 x HD's and
1 x SSD from a basic test running multiple parrallel dd's.
With pre-fetch disabled (vfs.zfs.prefetch_disable=1):
== Stripe Balanced (default) ==
Read 15360MB using bs: 1048576, readers: 3, took 161 seconds @ 95 MB/s
== Load Balanced (zfslinux) ==
Read 15360MB using bs: 1048576, readers: 3, took 297 seconds @ 51 MB/s
== Load Balanced (locality freebsd) ==
Read 15360MB using bs: 1048576, readers: 3, took 54 seconds @ 284 MB/s
With pre-fetch enabled (vfs.zfs.prefetch_disable=0):
== Stripe Balanced (default) ==
Read 15360MB using bs: 1048576, readers: 3, took 91 seconds @ 168 MB/s
== Load Balanced (zfslinux) ==
Read 15360MB using bs: 1048576, readers: 3, took 108 seconds @ 142 MB/s
== Load Balanced (locality freebsd) ==
Read 15360MB using bs: 1048576, readers: 3, took 48 seconds @ 320 MB/s
In addition to the performance changes the code was also restructured, with
the help of Justin Gibbs, to provide a more logical flow which also ensures
vdevs loads are only calculated from the set of valid candidates.
The following additional sysctls where added to allow the administrator
to tune the behaviour of the load algorithm:
* vfs.zfs.vdev.mirror.rotating_inc
* vfs.zfs.vdev.mirror.rotating_seek_inc
* vfs.zfs.vdev.mirror.rotating_seek_offset
* vfs.zfs.vdev.mirror.non_rotating_inc
* vfs.zfs.vdev.mirror.non_rotating_seek_inc
These changes where based on work started by the zfsonlinux developers:
https://github.com/zfsonlinux/zfs/pull/1487
Reviewed by: gibbs, mav, will
MFC after: 2 weeks
Sponsored by: Multiplay
When safety requirements are met, it allows to avoid passing I/O requests
to GEOM g_up/g_down thread, executing them directly in the caller context.
That allows to avoid CPU bottlenecks in g_up/g_down threads, plus avoid
several context switches per I/O.
The defined now safety requirements are:
- caller should not hold any locks and should be reenterable;
- callee should not depend on GEOM dual-threaded concurency semantics;
- on the way down, if request is unmapped while callee doesn't support it,
the context should be sleepable;
- kernel thread stack usage should be below 50%.
To keep compatibility with GEOM classes not meeting above requirements
new provider and consumer flags added:
- G_CF_DIRECT_SEND -- consumer code meets caller requirements (request);
- G_CF_DIRECT_RECEIVE -- consumer code meets callee requirements (done);
- G_PF_DIRECT_SEND -- provider code meets caller requirements (done);
- G_PF_DIRECT_RECEIVE -- provider code meets callee requirements (request).
Capable GEOM class can set them, allowing direct dispatch in cases where
it is safe. If any of requirements are not met, request is queued to
g_up or g_down thread same as before.
Such GEOM classes were reviewed and updated to support direct dispatch:
CONCAT, DEV, DISK, GATE, MD, MIRROR, MULTIPATH, NOP, PART, RAID, STRIPE,
VFS, ZERO, ZFS::VDEV, ZFS::ZVOL, all classes based on g_slice KPI (LABEL,
MAP, FLASHMAP, etc).
To declare direct completion capability disk(9) KPI got new flag equivalent
to G_PF_DIRECT_SEND -- DISKFLAG_DIRECT_COMPLETION. da(4) and ada(4) disk
drivers got it set now thanks to earlier CAM locking work.
This change more then twice increases peak block storage performance on
systems with manu CPUs, together with earlier CAM locking changes reaching
more then 1 million IOPS (512 byte raw reads from 16 SATA SSDs on 4 HBAs to
256 user-level threads).
Sponsored by: iXsystems, Inc.
MFC after: 2 months
reduce lock congestion and improve SMP scalability of the SCSI/ATA stack,
preparing the ground for the coming next GEOM direct dispatch support.
Replace big per-SIM locks with bunch of smaller ones:
- per-LUN locks to protect device and peripheral drivers state;
- per-target locks to protect list of LUNs on target;
- per-bus locks to protect reference counting;
- per-send queue locks to protect queue of CCBs to be sent;
- per-done queue locks to protect queue of completed CCBs;
- remaining per-SIM locks now protect only HBA driver internals.
While holding LUN lock it is allowed (while not recommended for performance
reasons) to take SIM lock. The opposite acquisition order is forbidden.
All the other locks are leaf locks, that can be taken anywhere, but should
not be cascaded. Many functions, such as: xpt_action(), xpt_done(),
xpt_async(), xpt_create_path(), etc. are no longer require (but allow) SIM
lock to be held.
To keep compatibility and solve cases where SIM lock can't be dropped, all
xpt_async() calls in addition to xpt_done() calls are queued to completion
threads for async processing in clean environment without SIM lock held.
Instead of single CAM SWI thread, used for commands completion processing
before, use multiple (depending on number of CPUs) threads. Load balanced
between them using "hash" of the device B:T:L address.
HBA drivers that can drop SIM lock during completion processing and have
sufficient number of completion threads to efficiently scale to multiple
CPUs can use new function xpt_done_direct() to avoid extra context switch.
Make ahci(4) driver to use this mechanism depending on hardware setup.
Sponsored by: iXsystems, Inc.
MFC after: 2 months
