device usage based on devices file

The LVM devices file lists devices that lvm can use.  The default
file is /etc/lvm/devices/system.devices, and the lvmdevices(8)
command is used to add or remove device entries.  If the file
does not exist, or if lvm.conf includes use_devicesfile=0, then
lvm will not use a devices file.  When the devices file is in use,
the regex filter is not used, and the filter settings in lvm.conf
or on the command line are ignored.

LVM records devices in the devices file using hardware-specific
IDs, such as the WWID, and attempts to use subsystem-specific
IDs for virtual device types.  These device IDs are also written
in the VG metadata.  When no hardware or virtual ID is available,
lvm falls back using the unstable device name as the device ID.
When devnames are used, lvm performs extra scanning to find
devices if their devname changes, e.g. after reboot.

When proper device IDs are used, an lvm command will not look
at devices outside the devices file, but when devnames are used
as a fallback, lvm will scan devices outside the devices file
to locate PVs on renamed devices.  A config setting
search_for_devnames can be used to control the scanning for
renamed devname entries.

Related to the devices file, the new command option
--devices <devnames> allows a list of devices to be specified for
the command to use, overriding the devices file.  The listed
devices act as a sort of devices file in terms of limiting which
devices lvm will see and use.  Devices that are not listed will
appear to be missing to the lvm command.

Multiple devices files can be kept in /etc/lvm/devices, which
allows lvm to be used with different sets of devices, e.g.
system devices do not need to be exposed to a specific application,
and the application can use lvm on its own set of devices that are
not exposed to the system.  The option --devicesfile <filename> is
used to select the devices file to use with the command.  Without
the option set, the default system devices file is used.

Setting --devicesfile "" causes lvm to not use a devices file.

An existing, empty devices file means lvm will see no devices.

The new command vgimportdevices adds PVs from a VG to the devices
file and updates the VG metadata to include the device IDs.
vgimportdevices -a will import all VGs into the system devices file.

LVM commands run by dmeventd not use a devices file by default,
and will look at all devices on the system.  A devices file can
be created for dmeventd (/etc/lvm/devices/dmeventd.devices)  If
this file exists, lvm commands run by dmeventd will use it.

Internal implementaion:

- device_ids_read - read the devices file
  . add struct dev_use (du) to cmd->use_devices for each devices file entry
- dev_cache_scan - get /dev entries
  . add struct device (dev) to dev_cache for each device on the system
- device_ids_match - match devices file entries to /dev entries
  . match each du on cmd->use_devices to a dev in dev_cache, using device ID
  . on match, set du->dev, dev->id, dev->flags MATCHED_USE_ID
- label_scan - read lvm headers and metadata from devices
  . filters are applied, those that do not need data from the device
  . filter-deviceid skips devs without MATCHED_USE_ID, i.e.
    skips /dev entries that are not listed in the devices file
  . read lvm label from dev
  . filters are applied, those that use data from the device
  . read lvm metadata from dev
  . add info/vginfo structs for PVs/VGs (info is "lvmcache")
- device_ids_find_renamed_devs - handle devices with unstable devname ID
  where devname changed
  . this step only needed when devs do not have proper device IDs,
    and their dev names change, e.g. after reboot sdb becomes sdc.
  . detect incorrect match because PVID in the devices file entry
    does not match the PVID found when the device was read above
  . undo incorrect match between du and dev above
  . search system devices for new location of PVID
  . update devices file with new devnames for PVIDs on renamed devices
  . label_scan the renamed devs
- continue with command processing

2 files changed, 31 insertions, 0 deletions

diff --git a/lib/config/config_settings.h b/lib/config/config_settings.h
index d61b707e9..ee5d97721 100644
--- a/lib/config/config_settings.h
+++ b/lib/config/config_settings.h
@@ -288,6 +288,32 @@ cfg_array(devices_preferred_names_CFG, "preferred_names", devices_CFG_SECTION, C
 	"preferred_names = [ \"^/dev/mpath/\", \"^/dev/mapper/mpath\", \"^/dev/[hs]d\" ]\n"
 	"#\n")
 
+cfg(devices_use_devicesfile_CFG, "use_devicesfile", devices_CFG_SECTION, CFG_DEFAULT_COMMENTED, CFG_TYPE_BOOL, DEFAULT_USE_DEVICES_FILE, vsn(2, 3, 12), NULL, 0, NULL,
+	"Enable or disable the use of a devices file.\n"
+	"When enabled, lvm will only use devices that\n"
+	"are lised in the devices file. A devices file will\n"
+	"be used, regardless of this setting, when the --devicesfile\n"
+	"option is set to a specific file name.\n")
+
+cfg(devices_devicesfile_CFG, "devicesfile", devices_CFG_SECTION, CFG_DEFAULT_COMMENTED, CFG_TYPE_STRING, DEFAULT_DEVICES_FILE, vsn(2, 3, 12), NULL, 0, NULL,
+	"The name of the system devices file, listing devices that LVM should use.\n"
+	"This should not be used to select a non-system devices file.\n"
+	"The --devicesfile option is intended for alternative devices files.\n")
+
+cfg(devices_search_for_devnames_CFG, "search_for_devnames", devices_CFG_SECTION, CFG_DEFAULT_COMMENTED, CFG_TYPE_STRING, DEFAULT_SEARCH_FOR_DEVNAMES, vsn(2, 3, 12), NULL, 0, NULL,
+	"Look outside of the devices file for missing devname entries.\n"
+	"A devname entry is used for a device that does not have a stable\n"
+	"device id, e.g. wwid, so the unstable device name is used as\n"
+	"the device id. After reboot, or if the device is reattached,\n"
+	"the device name may change, in which case lvm will not find\n"
+	"the expected PV on the device listed in the devices file.\n"
+	"This setting controls whether lvm will search other devices,\n"
+	"outside the devices file, to look for the missing PV on a\n"
+	"renamed device. If \"none\", lvm will not look at other devices,\n"
+	"and the PV may appear to be missing. If \"auto\", lvm will look\n"
+	"at other devices, but only those that are likely to have the PV.\n"
+	"If \"all\", lvm will look at all devices on the system.\n")
+
 cfg_array(devices_filter_CFG, "filter", devices_CFG_SECTION, CFG_DEFAULT_COMMENTED, CFG_TYPE_STRING, "#Sa|.*|", vsn(1, 0, 0), NULL, 0, NULL,
 	"Limit the block devices that are used by LVM commands.\n"
 	"This is a list of regular expressions used to accept or reject block\n"
diff --git a/lib/config/defaults.h b/lib/config/defaults.h
index bcc20cc97..2870dee12 100644
--- a/lib/config/defaults.h
+++ b/lib/config/defaults.h
@@ -323,4 +323,9 @@
 
 #define DEFAULT_MD_COMPONENT_CHECKS "auto"
 
+#define DEFAULT_USE_DEVICES_FILE 0
+#define DEFAULT_DEVICES_FILE "system.devices"
+
+#define DEFAULT_SEARCH_FOR_DEVNAMES "auto"
+
 #endif				/* _LVM_DEFAULTS_H */