illumos gate

From the original change log:

This restriction appears to have been put in place to avoid errors with
    updating the configuration cache file.  However:

• The majority of asynchronous event handling does not involve configuration
  cache file updates.
• The configuration cache file need not be on the root file system, so the
  check was not complete.
• Other classes of errors (e.g. file system full) can also prevent a
  successful update yet do not prevent asynchronous event processing.
• Some system configurations (e.g. NanoBSD) are designed to never have a R/W
  root, so ZFS event processing is permanently disabled in these systems.
• Failure to handle asynchronous events promptly can extend the window of
  time that a pool is in a critical state.

At worst, a missed configuration cache update will force the operator to
    perform a manual "zfs import" (note -f is not required) to inform the system
    about a newly created pool.  To minimize the likelihood of this rare
    occurrence, configuration cache write failures now emit FMA events so the
    operator can take corrective action, and the write is retried every 5 minutes.
    This is implemented as a tunable, spa.c:zfs_ccw_retry_interval.

uts/common/fs/zfs/spa.c:

• Modify spa_async_dispatch to honor configuration cache write limiting. If
  other events are pending, a configuration cache write will be attempted at
  the same time, so the rate limiting only applies when the asynchronous
  dispatch system is otherwise idle. Async events should be rare (e.g.
  device arrival/departure) and configuration cache rarer, so a more
  complicated system to strictly honor the retry limit seems unwarranted.
• Remove check in spa_async_dispatch() for the root file system being R/W.

uts/common/fs/zfs/spa_config.c:

• Instead of silently ignoring configuration cache write failures, report
  them via a new FMA event. The current zfs_ereport_post() doesn't allow
  arbitrary name=value pairs to be appended to the report, so the
  configuration cache file name is not available to it. This limitation
  should be addressed in a future update.

uts/common/fs/zfs/sys/spa_impl.h:

• Add a field to the spa data structure to track the time of the last
  configuration cache write failure. This is referenced in
  spa_async_dispatch() to effect the rate limiting.

uts/common/sys/fm/fs/zfs.h:

• Add FM_EREPORT_ZFS_CONFIG_CACHE_WRITE as an ereport class.