[PATCH v2] btrfs-progs: btrfs: implement 'start-sync' and 'wait-sync' commands

[Sage Weil <sage@newdream.net>]

The 'start-sync' command initiates a sync, but does not wait for it to
complete.  A transaction is printed that can be fed to 'wait-sync', which
will wait for it to commit.

'wait-sync' can also be used in combination with 'async-snapshot' to wait
for an async snapshot creation to commit.

Updates the man page too.

Signed-off-by: Sage Weil <sage@newdream.net>
---
 btrfs.c        |    9 +++++++++
 btrfs_cmds.c   |   49 +++++++++++++++++++++++++++++++++++++++++++++++++
 btrfs_cmds.h   |    2 ++
 man/btrfs.8.in |   14 ++++++++++++++
 4 files changed, 74 insertions(+), 0 deletions(-)

diff --git a/btrfs.c b/btrfs.c
index 46314cf..c871f4a 100644
--- a/btrfs.c
+++ b/btrfs.c
@@ -77,6 +77,15 @@ static struct Command commands[] = {
 	  "filesystem sync", "<path>\n"
 		"Force a sync on the filesystem <path>."
 	},
+	{ do_start_sync, 1,
+	  "filesystem start-sync", "<path>\n"
+	        "Start a sync on the filesystem <path>, and print the resulting\n"
+	        "transaction id."
+	},
+	{ do_wait_sync, 2,
+	  "filesystem wait-sync", "<path> <transid>\n"
+	        "Wait for the transaction <transid> on the filesystem at <path> to commit."
+	},
 	{ do_resize, 2,
 	  "filesystem resize", "[+/-]<newsize>[gkm]|max <filesystem>\n"
 		"Resize the file system. If 'max' is passed, the filesystem\n"
diff --git a/btrfs_cmds.c b/btrfs_cmds.c
index 8031c58..736437d 100644
--- a/btrfs_cmds.c
+++ b/btrfs_cmds.c
@@ -526,6 +526,55 @@ int do_fssync(int argc, char **argv)
 	return 0;
 }
 
+int do_start_sync(int argc, char **argv)
+{
+	int fd, res;
+	char	*path = argv[1];
+	__u64 transid;
+
+	fd = open_file_or_dir(path);
+	if (fd < 0) {
+		fprintf(stderr, "ERROR: can't access to '%s'\n", path);
+		return 12;
+	}
+
+	printf("StartSync '%s'\n", path);
+	res = ioctl(fd, BTRFS_IOC_START_SYNC, &transid);
+	close(fd);
+	if( res < 0 ){
+		fprintf(stderr, "ERROR: unable to fs-syncing '%s'\n", path);
+		return 16;
+	} else {
+		printf("transid %llu\n", (unsigned long long)transid);
+	}
+
+	return 0;
+}
+
+int do_wait_sync(int argc, char **argv)
+{
+	int fd, res;
+	char	*path = argv[1];
+	__u64 transid = atoll(argv[2]);
+
+	fd = open_file_or_dir(path);
+	if (fd < 0) {
+		fprintf(stderr, "ERROR: can't access to '%s'\n", path);
+		return 12;
+	}
+
+	printf("WaitSync '%s' transid %llu\n", path, (unsigned long long)transid);
+	res = ioctl(fd, BTRFS_IOC_WAIT_SYNC, &transid);
+	close(fd);
+	if( res < 0 ){
+		fprintf(stderr, "ERROR: unable to wait-sync on '%s' transid %llu: %s\n", path,
+			(unsigned long long)transid, strerror(errno));
+		return 16;
+	}
+
+	return 0;
+}
+
 int do_scan(int argc, char **argv)
 {
 	int	i, fd;
diff --git a/btrfs_cmds.h b/btrfs_cmds.h
index 7bde191..84c489f 100644
--- a/btrfs_cmds.h
+++ b/btrfs_cmds.h
@@ -19,6 +19,8 @@ int do_clone(int nargs, char **argv);
 int do_delete_subvolume(int nargs, char **argv);
 int do_create_subvol(int nargs, char **argv);
 int do_fssync(int nargs, char **argv);
+int do_start_sync(int nargs, char **argv);
+int do_wait_sync(int nargs, char **argv);
 int do_defrag(int argc, char **argv);
 int do_show_filesystem(int nargs, char **argv);
 int do_add_volume(int nargs, char **args);
diff --git a/man/btrfs.8.in b/man/btrfs.8.in
index 26ef982..e87b5fe 100644
--- a/man/btrfs.8.in
+++ b/man/btrfs.8.in
@@ -19,6 +19,10 @@ btrfs \- control a btrfs filesystem
 .PP
 \fBbtrfs\fP \fBfilesystem sync\fP\fI <path> \fP
 .PP
+\fBbtrfs\fP \fBfilesystem start-sync\fP\fI <path> \fP
+.PP
+\fBbtrfs\fP \fBfilesystem wait-sync\fP\fI <path> <transid>\fP
+.PP
 \fBbtrfs\fP \fBfilesystem resize\fP\fI [+/\-]<size>[gkm]|max <filesystem>\fP
 .PP
 \fBbtrfs\fP \fBdevice scan\fP\fI [<device> [<device>..]]\fP
@@ -115,6 +119,16 @@ all the block devices.
 Force a sync for the filesystem identified by \fI<path>\fR.
 .TP
 
+\fBfilesystem start-sync\fR\fI <path> \fR
+Start a sync operation for the filesystem identified by \fI<path>\fR.  A transaction id
+is printed that can be waited on using the \fBfilesystem wait-sync\fR command.
+.TP
+
+\fBfilesystem wait-sync\fR\fI <path> <transid>\fR
+Wait for a the transaction \fI<transid>\fR to commit to disk.  If \fI<transid>\fR is zero,
+wait for any currently committing transaction to commit.
+.TP
+
 .\"
 .\" Some wording are extracted by the resize2fs man page
 .\"
-- 
1.7.0