log xattr=sa create/remove/update to ZIL

As such, there are no specific synchronous semantics defined for
the xattrs. But for xattr=on, it does log to ZIL and zil_commit() is
done, if sync=always is set on dataset. This provides sync semantics
for xattr=on with sync=always set on dataset.

For the xattr=sa implementation, it doesn't log to ZIL, so, even with
sync=always, xattrs are not guaranteed to be synced before xattr call
returns to caller. So, xattr can be lost if system crash happens, before
txg carrying xattr transaction is synced.

This change adds xattr=sa logging to ZIL on xattr create/remove/update
and xattrs are synced to ZIL (zil_commit() done) for sync=always.
This makes xattr=sa behavior similar to xattr=on.

Implementation notes:
The actual logging is fairly straight-forward and does not warrant
additional explanation.
However, it has been 14 years since we last added new TX types
to the ZIL [1], hence this is the first time we do it after the
introduction of zpool features. Therefore, here is an overview of the
feature activation and deactivation workflow:

1. The feature must be enabled. Otherwise, we don't log the new
    record type. This ensures compatibility with older software.
2. The feature is activated per-dataset, since the ZIL is per-dataset.
3. If the feature is enabled and dataset is not for zvol, any append to
    the ZIL chain will activate the feature for the dataset. Likewise
    for starting a new ZIL chain.
4. A dataset that doesn't have a ZIL chain has the feature deactivated.

We ensure (3) by activating on the first zil_commit() after the feature
was enabled. Since activating the features requires waiting for txg
sync, the first zil_commit() after enabling the feature will be slower
than usual. The downside is that this is really a conservative
approximation: even if we never append a 'TX_SETSAXATTR' to the ZIL
chain, we pay the penalty for feature activation. The upside is that the
user is in control of when we pay the penalty, i.e., upon enabling the
feature.

We ensure (4) by hooking into zil_sync(), where ZIL destroy actually
happens.

One more piece on feature activation, since it's spread across
multiple functions:

zil_commit()
  zil_process_commit_list()
    if lwb == NULL // first zil_commit since zil_open
      zil_create()
        if no log block pointer in ZIL header:
          if feature enabled and not active:
	    // CASE 1
            enable, COALESCE txg wait with dmu_tx that allocated the
	    log block
         else // log block was allocated earlier than this zil_open
          if feature enabled and not active:
	    // CASE 2
            enable, EXPLICIT txg wait
    else // already have an in-DRAM LWB
      if feature enabled and not active:
        // this happens when we enable the feature after zil_create
	// CASE 3
        enable, EXPLICIT txg wait

[1] https://github.com/illumos/illumos-gate/commit/da6c28aaf62fa55f0fdb8004aa40f88f23bf53f0

Reviewed-by: Matthew Ahrens <[email protected]>
Reviewed-by: Christian Schwarz <[email protected]>
Reviewed-by: Ahelenia Ziemiańska <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Jitendra Patidar <[email protected]>
Closes #8768 
Closes #9078 

1 files changed, 18 insertions, 0 deletions

diff --git a/man/man7/zpool-features.7 b/man/man7/zpool-features.7
index d5148fb48..9a202ca8a 100644
--- a/man/man7/zpool-features.7
+++ b/man/man7/zpool-features.7
@@ -778,6 +778,24 @@ by user and group.
 \*[instant-never]
 \*[remount-upgrade]
 .
+.feature org.openzfs zilsaxattr yes extensible_dataset
+This feature enables
+.Sy xattr Ns = Ns Sy sa
+extended attribute logging in the ZIL.
+If enabled, extended attribute changes
+.Pq both Sy xattrdir Ns = Ns Sy dir No and Sy xattr Ns = Ns Sy sa
+are guaranteed to be durable if either the dataset had
+.Sy sync Ns = Ns Sy always
+set at the time the changes were made, or
+.Xr sync 2
+is called on the dataset after the changes were made.
+.Pp
+This feature becomes
+.Sy active
+when a ZIL is created for at least one dataset and will be returned to the
+.Sy enabled
+state when it is destroyed for all datasets that use this feature.
+.
 .feature com.delphix zpool_checkpoint yes
 This feature enables the
 .Nm zpool Cm checkpoint