The recomputation of file system summary at mount time can be a

very slow process, especially for large file systems that is just
recovered from a crash.

Since the summary is already re-sync'ed every 30 second, we will
not lag behind too much after a crash.  With this consideration
in mind, it is more reasonable to transfer the responsibility to
background fsck, to reduce the delay after a crash.

Add a new sysctl variable, vfs.ffs.compute_summary_at_mount, to
control this behavior.  When set to nonzero, we will get the
"old" behavior, that the summary is computed immediately at mount
time.

Add five new sysctl variables to adjust ndir, nbfree, nifree,
nffree and numclusters respectively.  Teach fsck_ffs about these
API, however, intentionally not to check the existence, since
kernels without these sysctls must have recomputed the summary
and hence no adjustments are necessary.

This change has eliminated the usual tens of minutes of delay of
mounting large dirty volumes.

Reviewed by:	mckusick
MFC After:	1 week

2 files changed, 60 insertions, 0 deletions

diff --git a/sbin/fsck_ffs/fsck.h b/sbin/fsck_ffs/fsck.h
index 487dcd42a669..55a57bc6959f 100644
--- a/sbin/fsck_ffs/fsck.h
+++ b/sbin/fsck_ffs/fsck.h
@@ -252,6 +252,11 @@ long countdirs;			/* number of directories we actually found */
 #define MIBSIZE	3		/* size of fsck sysctl MIBs */
 int	adjrefcnt[MIBSIZE];	/* MIB command to adjust inode reference cnt */
 int	adjblkcnt[MIBSIZE];	/* MIB command to adjust inode block count */
+int	adjndir[MIBSIZE];	/* MIB command to adjust number of directories */
+int	adjnbfree[MIBSIZE];	/* MIB command to adjust number of free blocks */
+int	adjnifree[MIBSIZE];	/* MIB command to adjust number of free inodes */
+int	adjnffree[MIBSIZE];	/* MIB command to adjust number of free frags */
+int	adjnumclusters[MIBSIZE];	/* MIB command to adjust number of free clusters */
 int	freefiles[MIBSIZE];	/* MIB command to free a set of files */
 int	freedirs[MIBSIZE];	/* MIB command to free a set of directories */
 int	freeblks[MIBSIZE];	/* MIB command to free a set of data blocks */
diff --git a/sbin/fsck_ffs/pass5.c b/sbin/fsck_ffs/pass5.c
index c3c564e7700c..796b546b9516 100644
--- a/sbin/fsck_ffs/pass5.c
+++ b/sbin/fsck_ffs/pass5.c
@@ -42,6 +42,7 @@ __FBSDID("$FreeBSD$");
 #include <ufs/ffs/fs.h>
 
 #include <err.h>
+#include <inttypes.h>
 #include <limits.h>
 #include <string.h>
 
@@ -344,6 +345,60 @@ pass5(void)
 		fs->fs_fmod = 0;
 		sbdirty();
 	}
+
+	/*
+	 * When doing background fsck on a snapshot, figure out whether
+	 * the superblock summary is inaccurate and correct it when
+	 * necessary.
+	 */
+	if (cursnapshot != 0) {
+		cmd.size = 1;
+
+		cmd.value = cstotal.cs_ndir - fs->fs_cstotal.cs_ndir;
+		if (cmd.value != 0) {
+			if (debug)
+				printf("adjndir by %+" PRIi64 "\n", cmd.value);
+			if (sysctl(adjndir, MIBSIZE, 0, 0,
+			    &cmd, sizeof cmd) == -1)
+				rwerror("ADJUST NUMBER OF DIRECTORIES", cmd.value);
+		}
+
+		cmd.value = cstotal.cs_nbfree - fs->fs_cstotal.cs_nbfree;
+		if (cmd.value != 0) {
+			if (debug)
+				printf("adjnbfree by %+" PRIi64 "\n", cmd.value);
+			if (sysctl(adjnbfree, MIBSIZE, 0, 0,
+			    &cmd, sizeof cmd) == -1)
+				rwerror("ADJUST NUMBER OF FREE BLOCKS", cmd.value);
+		}
+
+		cmd.value = cstotal.cs_nifree - fs->fs_cstotal.cs_nifree;
+		if (cmd.value != 0) {
+			if (debug)
+				printf("adjnifree by %+" PRIi64 "\n", cmd.value);
+			if (sysctl(adjnifree, MIBSIZE, 0, 0,
+			    &cmd, sizeof cmd) == -1)
+				rwerror("ADJUST NUMBER OF FREE INODES", cmd.value);
+		}
+
+		cmd.value = cstotal.cs_nffree - fs->fs_cstotal.cs_nffree;
+		if (cmd.value != 0) {
+			if (debug)
+				printf("adjnffree by %+" PRIi64 "\n", cmd.value);
+			if (sysctl(adjnffree, MIBSIZE, 0, 0,
+			    &cmd, sizeof cmd) == -1)
+				rwerror("ADJUST NUMBER OF FREE FRAGS", cmd.value);
+		}
+
+		cmd.value = cstotal.cs_numclusters - fs->fs_cstotal.cs_numclusters;
+		if (cmd.value != 0) {
+			if (debug)
+				printf("adjnumclusters by %+" PRIi64 "\n", cmd.value);
+			if (sysctl(adjnumclusters, MIBSIZE, 0, 0,
+			    &cmd, sizeof cmd) == -1)
+				rwerror("ADJUST NUMBER OF FREE CLUSTERS", cmd.value);
+		}
+	}
 }
 
 static void