Add TXG timestamp database

This feature enables tracking of when TXGs are committed to disk,
providing an estimated timestamp for each TXG.

With this information, it becomes possible to perform scrubs based
on specific date ranges, improving the granularity of data
management and recovery operations.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Signed-off-by: Mariusz Zaborski <mariusz.zaborski@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Closes #16853

man/man4/zfs.4

@@ -2246,6 +2246,21 @@ Defer frees starting in this pass.
 Maximum memory used for prefetching a checkpoint's space map on each
 vdev while discarding the checkpoint.
 .
+.It Sy zfs_spa_note_txg_time Ns = Ns Sy 600 Pq uint
+This parameter defines, in seconds, how often the TXG time database will record
+a new TXG if it has changed.
+After the specified time interval has passed, and if the TXG number has changed,
+the new value is recorded in the database.
+These timestamps can later be used for more granular operations, such as
+scrubbing.
+.
+.It Sy zfs_spa_flush_txg_time Ns = Ns Sy 600 Pq uint
+This parameter defines, in seconds, how often the ZFS will flush
+the TXG time database to disk.
+It ensures that the data is actually written to persistent storage, which helps
+preserve the database in case of unexpected shutdown.
+The database is also automatically flushed during the export sequence.
+.
 .It Sy zfs_special_class_metadata_reserve_pct Ns = Ns Sy 25 Ns % Pq uint
 Only allow small data blocks to be allocated on the special and dedup vdev
 types when the available free space percentage on these vdevs exceeds this