Multi-modifier protection (MMP)

Add multihost=on|off pool property to control MMP.  When enabled
a new thread writes uberblocks to the last slot in each label, at a
set frequency, to indicate to other hosts the pool is actively imported.
These uberblocks are the last synced uberblock with an updated
timestamp.  Property defaults to off.

During tryimport, find the "best" uberblock (newest txg and timestamp)
repeatedly, checking for change in the found uberblock.  Include the
results of the activity test in the config returned by tryimport.
These results are reported to user in "zpool import".

Allow the user to control the period between MMP writes, and the
duration of the activity test on import, via a new module parameter
zfs_multihost_interval.  The period is specified in milliseconds.  The
activity test duration is calculated from this value, and from the
mmp_delay in the "best" uberblock found initially.

Add a kstat interface to export statistics about Multiple Modifier
Protection (MMP) updates. Include the last synced txg number, the
timestamp, the delay since the last MMP update, the VDEV GUID, the VDEV
label that received the last MMP update, and the VDEV path.  Abbreviated
output below.

$ cat /proc/spl/kstat/zfs/mypool/multihost
31 0 0x01 10 880 105092382393521 105144180101111
txg   timestamp  mmp_delay   vdev_guid   vdev_label vdev_path
20468    261337  250274925   68396651780       3    /dev/sda
20468    261339  252023374   6267402363293     1    /dev/sdc
20468    261340  252000858   6698080955233     1    /dev/sdx
20468    261341  251980635   783892869810      2    /dev/sdy
20468    261342  253385953   8923255792467     3    /dev/sdd
20468    261344  253336622   042125143176      0    /dev/sdab
20468    261345  253310522   1200778101278     2    /dev/sde
20468    261346  253286429   0950576198362     2    /dev/sdt
20468    261347  253261545   96209817917       3    /dev/sds
20468    261349  253238188   8555725937673     3    /dev/sdb

Add a new tunable zfs_multihost_history to specify the number of MMP
updates to store history for. By default it is set to zero meaning that
no MMP statistics are stored.

When using ztest to generate activity, for automated tests of the MMP
function, some test functions interfere with the test.  For example, the
pool is exported to run zdb and then imported again.  Add a new ztest
function, "-M", to alter ztest behavior to prevent this.

Add new tests to verify the new functionality.  Tests provided by
Giuseppe Di Natale.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #745
Closes #6279

man/man5/zfs-module-parameters.5

@@ -1410,6 +1410,79 @@ otherwise would under the old \fBzfs_mg_alloc_failures\fR facility.
 Default value: \fB0\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_multihost_history\fR (int)
+.ad
+.RS 12n
+Historical statistics for the last N multihost updates will be available in
+\fB/proc/spl/kstat/zfs/<pool>/multihost\fR
+.sp
+Default value: \fB0\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_interval\fR (ulong)
+.ad
+.RS 12n
+Used to control the frequency of multihost writes which are performed when the
+\fBmultihost\fR pool property is on.  This is one factor used to determine
+the length of the activity check during import.
+.sp
+The multihost write period is \fBzfs_multihost_interval / leaf-vdevs\fR milliseconds.
+This means that on average a multihost write will be issued for each leaf vdev every
+\fBzfs_multihost_interval\fR milliseconds.  In practice, the observed period can
+vary with the I/O load and this observed value is the delay which is stored in
+the uberblock.
+.sp
+On import the activity check waits a minimum amount of time determined by
+\fBzfs_multihost_interval * zfs_multihost_import_intervals\fR.  The activity
+check time may be further extended if the value of mmp delay found in the best
+uberblock indicates actual multihost updates happened at longer intervals than
+\fBzfs_multihost_interval\fR.  A minimum value of \fB100ms\fR is enforced.
+.sp
+Default value: \fB1000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_import_intervals\fR (uint)
+.ad
+.RS 12n
+Used to control the duration of the activity test on import.  Smaller values of
+\fBzfs_multihost_import_intervals\fR will reduce the import time but increase
+the risk of failing to detect an active pool.  The total activity check time is
+never allowed to drop below one second.  A value of 0 is ignored and treated as
+if it was set to 1
+.sp
+Default value: \fB10\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_multihost_fail_intervals\fR (uint)
+.ad
+.RS 12n
+Controls the behavior of the pool when multihost write failures are detected.
+.sp
+When \fBzfs_multihost_fail_intervals = 0\fR then multihost write failures are ignored.
+The failures will still be reported to the ZED which depending on its
+configuration may take action such as suspending the pool or offlining a device.
+.sp
+When \fBzfs_multihost_fail_intervals > 0\fR then sequential multihost write failures
+will cause the pool to be suspended.  This occurs when
+\fBzfs_multihost_fail_intervals * zfs_multihost_interval\fR milliseconds have
+passed since the last successful multihost write.  This guarantees the activity test
+will see multihost writes if the pool is imported.
+.sp
+Default value: \fB5\fR.
+.RE
+
 .sp
 .ne 2
 .na
@@ -1529,8 +1602,8 @@ Default value: \fB1,048,576\fR.
 \fBzfs_read_history\fR (int)
 .ad
 .RS 12n
-Historic statistics for the last N reads will be available in
-\fR/proc/spl/kstat/zfs/POOLNAME/reads\fB
+Historical statistics for the last N reads will be available in
+\fB/proc/spl/kstat/zfs/<pool>/reads\fR
 .sp
 Default value: \fB0\fR (no data is kept).
 .RE
@@ -1684,8 +1757,8 @@ Default value: \fB32\fR.
 \fBzfs_txg_history\fR (int)
 .ad
 .RS 12n
-Historic statistics for the last N txgs will be available in
-\fR/proc/spl/kstat/zfs/POOLNAME/txgs\fB
+Historical statistics for the last N txgs will be available in
+\fB/proc/spl/kstat/zfs/<pool>/txgs\fR
 .sp
 Default value: \fB0\fR.
 .RE