Add -c to zpool iostat & status to run command

This patch adds a command (-c) option to zpool status and zpool iostat.  The
-c option allows you to run an arbitrary command on each vdev and display
the first line of output in zpool status/iostat.  The environment vars
VDEV_PATH and VDEV_UPATH are set to the vdev's path and "underlying path"
before running the command.  For device mapper, multipath, or partitioned
vdevs, VDEV_UPATH is the actual underlying /dev/sd* disk.  This can be useful
if the command you're running requires a /dev/sd* device.

The patch also uses /sys/block/<dev>/slaves/ to lookup the underlying device
instead of using libdevmapper.  This not only removes the libdevmapper
requirement at build time, but also allows you to resolve device mapper
devices without being root.  This means that UDEV_UPATH get set correctly
when running zpool status/iostat as an unprivileged user.

Example:

$ zpool status -c 'echo I am $VDEV_PATH, $VDEV_UPATH'

NAME        STATE     READ WRITE CKSUM
mypool      ONLINE       0     0     0
  mirror-0  ONLINE       0     0     0
    mpatha  ONLINE       0     0     0  I am /dev/mapper/mpatha, /dev/sdc
    sdb     ONLINE       0     0     0  I am /dev/sdb1, /dev/sdb

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #5368

cmd/zpool/zpool_iter.c

@@ -33,6 +33,7 @@
 #include <strings.h>
 
 #include <libzfs.h>
+#include <sys/zfs_context.h>
 
 #include "zpool_util.h"
 
@@ -316,3 +317,162 @@ for_each_vdev(zpool_handle_t *zhp, pool_vdev_iter_f func, void *data)
 	}
 	return (for_each_vdev_cb(zhp, nvroot, func, data));
 }
+
+/* Thread function run for each vdev */
+static void
+vdev_run_cmd_thread(void *cb_cmd_data)
+{
+	vdev_cmd_data_t *data = cb_cmd_data;
+	char *pos = NULL;
+	FILE *fp;
+	size_t len = 0;
+	char cmd[_POSIX_ARG_MAX];
+
+	/* Set our VDEV_PATH and VDEV_UPATH env vars and run command */
+	if (snprintf(cmd, sizeof (cmd), "VDEV_PATH=%s && VDEV_UPATH=%s && %s",
+	    data->path, data->upath ? data->upath : "\"\"", data->cmd) >=
+	    sizeof (cmd)) {
+		/* Our string was truncated */
+		return;
+	}
+
+	fp = popen(cmd, "r");
+	if (fp == NULL)
+		return;
+
+	data->line = NULL;
+
+	/* Save the first line of output from the command */
+	if (getline(&data->line, &len, fp) != -1) {
+		/* Success.  Remove newline from the end, if necessary. */
+		if ((pos = strchr(data->line, '\n')) != NULL)
+		    *pos = '\0';
+	} else {
+		data->line = NULL;
+	}
+	pclose(fp);
+}
+
+/* For each vdev in the pool run a command */
+static int
+for_each_vdev_run_cb(zpool_handle_t *zhp, nvlist_t *nv, void *cb_vcdl)
+{
+	vdev_cmd_data_list_t *vcdl = cb_vcdl;
+	vdev_cmd_data_t *data;
+	char *path = NULL;
+	int i;
+
+	if (nvlist_lookup_string(nv, ZPOOL_CONFIG_PATH, &path) != 0)
+		return (1);
+
+	/* Spares show more than once if they're in use, so skip if exists */
+	for (i = 0; i < vcdl->count; i++) {
+		if ((strcmp(vcdl->data[i].path, path) == 0) &&
+		    (strcmp(vcdl->data[i].pool, zpool_get_name(zhp)) == 0)) {
+			/* vdev already exists, skip it */
+			return (0);
+		}
+	}
+
+	/*
+	 * Resize our array and add in the new element.
+	 */
+	if (!(vcdl->data = realloc(vcdl->data,
+	    sizeof (*vcdl->data) * (vcdl->count + 1))))
+		return (ENOMEM);	/* couldn't realloc */
+
+	data = &vcdl->data[vcdl->count];
+
+	data->pool = strdup(zpool_get_name(zhp));
+	data->path = strdup(path);
+	data->upath = zfs_get_underlying_path(path);
+	data->cmd = vcdl->cmd;
+
+	vcdl->count++;
+
+	return (0);
+}
+
+/* Get the names and count of the vdevs */
+static int
+all_pools_for_each_vdev_gather_cb(zpool_handle_t *zhp, void *cb_vcdl)
+{
+	return (for_each_vdev(zhp, for_each_vdev_run_cb, cb_vcdl));
+}
+
+/*
+ * Now that vcdl is populated with our complete list of vdevs, spawn
+ * off the commands.
+ */
+static void
+all_pools_for_each_vdev_run_vcdl(vdev_cmd_data_list_t *vcdl)
+{
+	taskq_t *t;
+	int i;
+	/* 5 * boot_ncpus selfishly chosen since it works best on LLNL's HW */
+	int max_threads = 5 * boot_ncpus;
+
+	/*
+	 * Under Linux we use a taskq to parallelize running a command
+	 * on each vdev.  It is therefore necessary to initialize this
+	 * functionality for the duration of the threads.
+	 */
+	thread_init();
+
+	t = taskq_create("z_pool_cmd", max_threads, defclsyspri, max_threads,
+	    INT_MAX, 0);
+	if (t == NULL)
+		return;
+
+	/* Spawn off the command for each vdev */
+	for (i = 0; i < vcdl->count; i++) {
+		(void) taskq_dispatch(t, vdev_run_cmd_thread,
+		    (void *) &vcdl->data[i], TQ_SLEEP);
+	}
+
+	/* Wait for threads to finish */
+	taskq_wait(t);
+	taskq_destroy(t);
+	thread_fini();
+}
+
+/*
+ * Run command 'cmd' on all vdevs in all pools.  Saves the first line of output
+ * from the command in vcdk->data[].line for all vdevs.
+ *
+ * Returns a vdev_cmd_data_list_t that must be freed with
+ * free_vdev_cmd_data_list();
+ */
+vdev_cmd_data_list_t *
+all_pools_for_each_vdev_run(int argc, char **argv, char *cmd)
+{
+	vdev_cmd_data_list_t *vcdl;
+	vcdl = safe_malloc(sizeof (vcdl));
+	vcdl->cmd = cmd;
+
+	/* Gather our list of all vdevs in all pools */
+	for_each_pool(argc, argv, B_TRUE, NULL,
+	    all_pools_for_each_vdev_gather_cb, vcdl);
+
+	/* Run command on all vdevs in all pools */
+	all_pools_for_each_vdev_run_vcdl(vcdl);
+
+	return (vcdl);
+}
+
+/*
+ * Free the vdev_cmd_data_list_t created by all_pools_for_each_vdev_run()
+ */
+void
+free_vdev_cmd_data_list(vdev_cmd_data_list_t *vcdl)
+{
+	int i;
+	for (i = 0; i < vcdl->count; i++) {
+		free(vcdl->data[i].path);
+		free(vcdl->data[i].pool);
+		free(vcdl->data[i].upath);
+		free(vcdl->data[i].line);
+	}
+	free(vcdl->data);
+	free(vcdl);
+}