Update the default for zfs_txg_history

It's often useful to have access to txg history for debugging
purposes. This patch changes the default from 0 to 100 TXGs
worth of history preserved.

Reviewed by: Matthew Ahrens <[email protected]>
Reviewed-by: Giuseppe Di Natale <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed by: Richard Elling <[email protected]>
Reviewed by: Prakash Surya <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Signed-off-by: Alek Pinchuk <[email protected]>
Closes openzfs#6691

Author: alek-p

man/man5/zfs-module-parameters.5

@@ -12,7 +12,7 @@
 .\" CDDL HEADER, with the fields enclosed by brackets "[]" replaced with your
 .\" own identifying information:
 .\" Portions Copyright [yyyy] [name of copyright owner]
-.TH ZFS-MODULE-PARAMETERS 5 "Nov 16, 2013"
+.TH ZFS-MODULE-PARAMETERS 5 "Sept 28, 2017"
 .SH NAME
 zfs\-module\-parameters \- ZFS module parameters
 .SH DESCRIPTION
@@ -1766,7 +1766,7 @@ Default value: \fB32\fR.
 Historical statistics for the last N txgs will be available in
 \fB/proc/spl/kstat/zfs/<pool>/txgs\fR
 .sp
-Default value: \fB0\fR.
+Default value: \fB100\fR.
 .RE
 
 .sp

module/zfs/spa_stats.c

@@ -34,9 +34,9 @@ int zfs_read_history = 0;
 int zfs_read_history_hits = 0;
 
 /*
- * Keeps stats on the last N txgs, disabled by default.
+ * Keeps stats on the last 100 txgs by default.
  */
-int zfs_txg_history = 0;
+int zfs_txg_history = 100;
 
 /*
  * Keeps stats on the last N MMP updates, disabled by default.