MMP interval and fail_intervals in uberblock

When Multihost is enabled, and a pool is imported, uberblock writes
include ub_mmp_delay to allow an importing node to calculate the
duration of an activity test.  This value, is not enough information.

If zfs_multihost_fail_intervals > 0 on the node with the pool imported,
the safe minimum duration of the activity test is well defined, but does
not depend on ub_mmp_delay:

zfs_multihost_fail_intervals * zfs_multihost_interval

and if zfs_multihost_fail_intervals == 0 on that node, there is no such
well defined safe duration, but the importing host cannot tell whether
mmp_delay is high due to I/O delays, or due to a very large
zfs_multihost_interval setting on the host which last imported the pool.
As a result, it may use a far longer period for the activity test than
is necessary.

This patch renames ub_mmp_sequence to ub_mmp_config and uses it to
record the zfs_multihost_interval and zfs_multihost_fail_intervals
values, as well as the mmp sequence.  This allows a shorter activity
test duration to be calculated by the importing host in most situations.
These values are also added to the multihost_history kstat records.

It calculates the activity test duration differently depending on
whether the new fields are present or not; for importing pools with
only ub_mmp_delay, it uses

(zfs_multihost_interval + ub_mmp_delay) * zfs_multihost_import_intervals

Which results in an activity test duration less sensitive to the leaf
count.

In addition, it makes a few other improvements:
* It updates the "sequence" part of ub_mmp_config when MMP writes
  in between syncs occur.  This allows an importing host to detect MMP
  on the remote host sooner, when the pool is idle, as it is not limited
  to the granularity of ub_timestamp (1 second).
* It issues writes immediately when zfs_multihost_interval is changed
  so remote hosts see the updated value as soon as possible.
* It fixes a bug where setting zfs_multihost_fail_intervals = 1 results
  in immediate pool suspension.
* Update tests to verify activity check duration is based on recorded
  tunable values, not tunable values on importing host.
* Update tests to verify the expected number of uberblocks have valid
  MMP fields - fail_intervals, mmp_interval, mmp_seq (sequence number),
  that sequence number is incrementing, and that uberblock values match
  tunable settings.

Reviewed-by: Andreas Dilger <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Olaf Faaland <[email protected]>
Closes #7842 

1 files changed, 6 insertions, 0 deletions

diff --git a/include/sys/mmp.h b/include/sys/mmp.h
index 6f4672a38..527e3323b 100644
--- a/include/sys/mmp.h
+++ b/include/sys/mmp.h
@@ -31,6 +31,11 @@ extern "C" {
 #define	MMP_DEFAULT_INTERVAL		1000	/* ms */
 #define	MMP_DEFAULT_IMPORT_INTERVALS	20
 #define	MMP_DEFAULT_FAIL_INTERVALS	10
+#define	MMP_MIN_FAIL_INTERVALS		2	/* min if != 0 */
+#define	MMP_IMPORT_SAFETY_FACTOR	200	/* pct */
+#define	MMP_INTERVAL_OK(interval)	MAX(interval, MMP_MIN_INTERVAL)
+#define	MMP_FAIL_INTVS_OK(fails)	(fails == 0 ? 0 : MAX(fails, \
+					    MMP_MIN_FAIL_INTERVALS))
 
 typedef struct mmp_thread {
 	kmutex_t	mmp_thread_lock; /* protect thread mgmt fields */
@@ -46,6 +51,7 @@ typedef struct mmp_thread {
 	int		mmp_skip_error; /* reason for last skipped write */
 	vdev_t		*mmp_last_leaf;	/* last mmp write sent here */
 	uint64_t	mmp_leaf_last_gen;	/* last mmp write sent here */
+	uint32_t	mmp_seq;	/* intra-second update counter */
 } mmp_thread_t;