[PATCH v2] btrfs-progs: filesystem usage: add avail info from statfs()

[Sidong Yang <realwakka@gmail.com>]

dd available space information from statfs(). This can be different from
'Free (estimated)' in some cases. This patch provide more information
about filesystem usage like below. and update document for this.

Overall:
    Device size:                   5.00GiB
    Device allocated:              1.02GiB
    Device unallocated:            3.98GiB
    Device missing:                  0.00B
    Used:                         88.00KiB
    Free (estimated):              4.48GiB      (min: 2.49GiB)
    Avail:                         4.48GiB
    Data ratio:                       1.00
    Metadata ratio:                   2.00
    Global reserve:              832.00KiB      (used: 0.00B)
    Multiple profiles:                  no

Issue: #306
Signed-off-by: Sidong Yang <realwakka@gmail.com>
---
 Documentation/btrfs-filesystem.asciidoc |  3 +++
 cmds/filesystem-usage.c                 | 10 ++++++++++
 2 files changed, 13 insertions(+)

diff --git a/Documentation/btrfs-filesystem.asciidoc b/Documentation/btrfs-filesystem.asciidoc
index 6a5561ed..18623afe 100644
--- a/Documentation/btrfs-filesystem.asciidoc
+++ b/Documentation/btrfs-filesystem.asciidoc
@@ -276,6 +276,7 @@ Overall:
     Device missing:                  0.00B
     Used:                          1.14TiB
     Free (estimated):            692.57GiB      (min: 692.57GiB)
+    Avail:                       692.57GiB
     Data ratio:                       1.00
     Metadata ratio:                   1.00
     Global reserve:              512.00MiB      (used: 0.00B)
@@ -295,6 +296,8 @@ including the reserved space
 data, including currently allocated space and estimating the usage of the
 unallocated space based on the block group profiles, the 'min' is the lower bound
 of the estimate in case multiple profiles are present
+* 'Avail' -- the amount of space available for data. it's get by statfs() system
+call that can be different from 'Free (estimated)' in some cases
 * 'Data ratio' -- ratio of total space for data including redundancy or parity to
 the effectively usable data space, eg. single is 1.0, RAID1 is 2.0 and for RAID5/6
 it depends on the number of devices
diff --git a/cmds/filesystem-usage.c b/cmds/filesystem-usage.c
index ab60d769..ed743a61 100644
--- a/cmds/filesystem-usage.c
+++ b/cmds/filesystem-usage.c
@@ -19,6 +19,7 @@
 #include <string.h>
 #include <unistd.h>
 #include <sys/ioctl.h>
+#include <sys/vfs.h>
 #include <errno.h>
 #include <stdarg.h>
 #include <getopt.h>
@@ -430,6 +431,7 @@ static int print_filesystem_usage_overall(int fd, struct chunk_info *chunkinfo,
 	u64 free_min = 0;
 	double max_data_ratio = 1.0;
 	int mixed = 0;
+	struct statfs statfs_buf;
 
 	sargs = load_space_info(fd, path);
 	if (!sargs) {
@@ -556,6 +558,12 @@ static int print_filesystem_usage_overall(int fd, struct chunk_info *chunkinfo,
 	if (unit_mode != UNITS_HUMAN)
 		width = 18;
 
+	ret = statfs(path, &statfs_buf);
+	if (ret) {
+		warning("cannot get space info with statfs() on '%s': %m", path);
+		ret = 0;
+	}
+
 	printf("Overall:\n");
 
 	printf("    Device size:\t\t%*s\n", width,
@@ -572,6 +580,8 @@ static int print_filesystem_usage_overall(int fd, struct chunk_info *chunkinfo,
 		width,
 		pretty_size_mode(free_estimated, unit_mode));
 	printf("min: %s)\n", pretty_size_mode(free_min, unit_mode));
+	printf("    Avail:\t\t\t%*s\n", width,
+		pretty_size_mode(statfs_buf.f_bavail * statfs_buf.f_bsize, unit_mode));
 	printf("    Data ratio:\t\t\t%*.2f\n",
 		width, data_ratio);
 	printf("    Metadata ratio:\t\t%*.2f\n",
-- 
2.25.1