Expose zfs_vdev_open_timeout_ms as a tunable

Some of our customers have been occasionally hitting zfs import failures
in Linux because udevd doesn't create the by-id symbolic links in time
for zpool import to use them. The main issue is that the
systemd-udev-settle.service that zfs-import-cache.service and other
services depend on is racy. There is also an openzfs issue filed (see
https://github.com/openzfs/zfs/issues/10891) outlining the problem and
potential solutions.

With the proper solutions being significant in terms of complexity and
the priority of the issue being low for the time being, this patch
exposes `zfs_vdev_open_timeout_ms` as a tunable so people that are
experiencing this issue often can increase it as a workaround.

Reviewed-by: Matthew Ahrens <[email protected]>
Reviewed-by: Richard Yao <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Don Brady <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: Serapheim Dimitropoulos <[email protected]>
Closes #14133 

1 files changed, 4 insertions, 1 deletions

diff --git a/module/os/linux/zfs/vdev_disk.c b/module/os/linux/zfs/vdev_disk.c
index 11ed3ea6f..84d191abb 100644
--- a/module/os/linux/zfs/vdev_disk.c
+++ b/module/os/linux/zfs/vdev_disk.c
@@ -56,7 +56,7 @@ static void *zfs_vdev_holder = VDEV_HOLDER;
  * device is missing. The missing path may be transient since the links
  * can be briefly removed and recreated in response to udev events.
  */
-static unsigned zfs_vdev_open_timeout_ms = 1000;
+static uint_t zfs_vdev_open_timeout_ms = 1000;
 
 /*
  * Size of the "reserved" partition, in blocks.
@@ -1042,3 +1042,6 @@ param_set_max_auto_ashift(const char *buf, zfs_kernel_param_t *kp)
 
 	return (0);
 }
+
+ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, open_timeout_ms, UINT, ZMOD_RW,
+	"Timeout before determining that a device is missing");