NAME¶

multipath.conf - multipath daemon configuration file.

DESCRIPTION¶

/etc/multipath.conf is the configuration file for the multipath daemon. It is used to overwrite the built-in configuration table of multipathd. Any line whose first non-white-space character is a '#' is considered a comment line. Empty lines are ignored.

Currently used multipathd configuration can be displayed with the multipath -t or multipathd show config command.

SYNTAX¶

The configuration file contains entries of the form:

<section> {

<attribute> <value>
...
<subsection> {

<attribute> <value>
...

}

}

Each section contains one or more attributes or subsections. The recognized keywords for attributes or subsections depend on the section in which they occur.

<attribute> and <value> must be on a single line. <attribute> is one of the keywords listed in this man page. <value> is either a simple word (containing no whitespace and none of the characters '"', '#', and '!') or one string enclosed in double quotes ("..."). Outside a quoted string, text starting with '#', and '!' is regarded as a comment and ignored until the end of the line. Inside a quoted string, '#' and '!' are normal characters, and whitespace is preserved. To represent a double quote character inside a double quoted string, use two consecutive double quotes ('""'). Thus '2.5" SSD' can be written as "2.5"" SSD".

Opening braces ('{') must follow the (sub)section name on the same line. Closing braces ('}') that mark the end of a (sub)section must be the only non-whitespace character on the line. Whitespace is ignored except inside double quotes, thus the indentation shown in the above example is helpful for human readers but not mandatory.

Note on regular expressions: The multipath.conf syntax allows many attribute values to be specified as POSIX Extended Regular Expressions (see regex(7)). These regular expressions are case sensitive and not anchored, thus the expression "bar" matches "barbie", "rhabarber", and "wunderbar", but not "Barbie". To avoid unwanted substring matches, standard regular expression syntax using the special characters "^" and "$" can be used.

The following section keywords are recognized:

defaults

This section defines default values for attributes which are used whenever no values are given in the appropriate device or multipath sections.

blacklist

This section defines which devices should be excluded from the multipath topology discovery.

blacklist_exceptions

This section defines which devices should be included in the multipath topology discovery, despite being listed in the blacklist section.

multipaths

This section defines the multipath topologies. They are indexed by a World Wide Identifier(WWID). For details on the WWID generation see section WWID generation below. Attributes set in this section take precedence over all others.

devices

This section defines the device-specific settings. Devices are identified by vendor, product, and revision.

overrides

This section defines values for attributes that should override the device-specific settings for all devices.

defaults section¶

The defaults section recognizes the following keywords:

verbosity

Default verbosity. Higher values increase the verbosity level. Valid levels are between 0 and 6.

polling_interval

Interval between two path checks in seconds. For properly functioning paths, the interval between checks will gradually increase to max_polling_interval. This value will be overridden by the WatchdogSec setting in the multipathd.service definition if systemd is used.

max_polling_interval

Maximal interval between two path checks in seconds.

reassign_maps

Enable reassigning of device-mapper maps. With this option multipathd will remap existing device-mapper maps to always point to multipath device, not the underlying block devices. Possible values are yes and no.

multipath_dir

This option is deprecated, and will be removed in a future release. Directory where the dynamic shared objects are stored. Defined at compile time, commonly /lib64/multipath/ or /lib/multipath/.

path_selector

The default path selector algorithm to use; they are offered by the kernel multipath target:

path_grouping_policy

The default path grouping policy to apply to unspecified multipaths. Possible values are:

uid_attrs

Setting this option activates merging uevents by WWID, which may improve uevent processing effiency. Moreover, it's an alternative method to configure the udev properties to use for determining unique path identifiers (WWIDs).

uid_attribute

The udev attribute providing a unique path identifier (WWID). If uid_attribute is set to the empty string, WWID determination is done using the sysfs method rather then using udev (not recommended in production; see WWID generation below).

getuid_callout

(Superseded by uid_attribute) The default program and args to callout to obtain a unique path identifier. Should be specified with an absolute path.

prio

The name of the path priority routine. The specified routine should return a numeric value specifying the relative priority of this path. Higher number have a higher priority. "none" is a valid value. Currently the following path priority routines are implemented:

prio_args

Arguments to pass to to the prio function. This only applies to certain prioritizers:

features

Specify any device-mapper features to be used. Syntax is num list where num is the number, between 0 and 8, of features in list. Possible values for the feature list are:

path_checker

The default method used to determine the path's state. The synchronous checkers (all except tur and directio) will cause multipathd to pause most activity, waiting up to checker_timeout seconds for the path to respond. The asynchronous checkers (tur and directio) will not pause multipathd. Instead, multipathd will check for a response once per second, until checker_timeout seconds have elapsed. Possible values are:

alias_prefix

The user_friendly_names prefix.

failback

Tell multipathd how to manage path group failback. To select immediate or a value, it's mandatory that the device has support for a working prioritizer.

rr_min_io

Number of I/O requests to route to a path before switching to the next in the same path group. This is only for Block I/O(BIO) based multipath and only apply to round-robin path_selector.

rr_min_io_rq

Number of I/O requests to route to a path before switching to the next in the same path group. This is only for Request based multipath and only apply to round-robin path_selector.

max_fds

Specify the maximum number of file descriptors that can be opened by multipath and multipathd. This is equivalent to ulimit -n. A value of max will set this to the system limit from /proc/sys/fs/nr_open. If this is not set, the maximum number of open fds is taken from the calling process. It is usually 1024. To be safe, this should be set to the maximum number of paths plus 32, if that number is greated than 1024.

rr_weight

If set to priorities the multipath configurator will assign path weights as "path prio * rr_min_io". Possible values are priorities or uniform . Only apply to round-robin path_selector.

no_path_retry

Specify what to do when all paths are down. Possible values are:

queue_without_daemon

If set to no , when multipathd stops, queueing will be turned off for all devices. This is useful for devices that set no_path_retry. If a machine is shut down while all paths to a device are down, it is possible to hang waiting for I/O to return from the device after multipathd has been stopped. Without multipathd running, access to the paths cannot be restored, and the kernel cannot be told to stop queueing I/O. Setting queue_without_daemon to no , avoids this problem.

checker_timeout

Specify the timeout to use for path checkers and prioritizers, in seconds. Only prioritizers that issue scsi commands use checker_timeout. If a path does not respond to the checker command after checker_timeout seconds have elapsed, it is considered down.

allow_usb_devices

If set to no , all USB devices will be skipped during path discovery. If you intend to use multipath on USB attached devices, set this to yes.

flush_on_last_del

If set to yes , multipathd will disable queueing when the last path to a device has been deleted.

user_friendly_names

If set to yes , using the bindings file /etc/multipath/bindings to assign a persistent and unique alias to the multipath, in the form of mpath<n>. If set to no use the WWID as the alias. In either case this be will be overridden by any specific aliases in the multipaths section.

fast_io_fail_tmo

Specify the number of seconds the SCSI layer will wait after a problem has been detected on a FC remote port before failing I/O to devices on that remote port. This should be smaller than dev_loss_tmo. Setting this to off will disable the timeout.

dev_loss_tmo

Specify the number of seconds the SCSI layer will wait after a problem has been detected on a FC remote port before removing it from the system. This can be set to "infinity" which sets it to the max value of 2147483647 seconds, or 68 years. It will be automatically adjusted to the overall retry interval no_path_retry * polling_interval if a number of retries is given with no_path_retry and the overall retry interval is longer than the specified dev_loss_tmo value. The Linux kernel will cap this value to 600 if fast_io_fail_tmo is not set. See KNOWN ISSUES.

eh_deadline

Specify the maximum number of seconds the SCSI layer will spend doing error handling when scsi devices fail. After this timeout the scsi layer will perform a full HBA reset. Setting this may be necessary in cases where the rport is never lost, so fast_io_fail_tmo and dev_loss_tmo will never trigger, but (frequently do to load) scsi commands still hang. Note: when the scsi error handler performs the HBA reset, all target paths on that HBA will be affected. eh_deadline should only be set in cases where all targets on the affected HBAs are multipathed.

max_retries

Specify the maximum number of times the SCSI layer will retry IO commands for some types of SCSI errors before returning failure. Setting this can be helpful for cases where IO commands hang and timeout. By default, the SCSI layer will retry IOs 5 times. Reducing this value will allow multipath to retry the IO down another path sooner. Valid values are 0 through 5.

bindings_file

This option is deprecated, and will be removed in a future release. The full pathname of the binding file to be used when the user_friendly_names option is set.

wwids_file

This option is deprecated, and will be removed in a future release. The full pathname of the WWIDs file, which is used by multipath to keep track of the WWIDs for LUNs it has created multipath devices on in the past.

prkeys_file

This option is deprecated, and will be removed in a future release. The full pathname of the prkeys file, which is used by multipathd to keep track of the persistent reservation key used for a specific WWID, when reservation_key is set to file.

log_checker_err

If set to once , multipathd logs the first path checker error at logging level 2. Any later errors are logged at level 3 until the device is restored. If set to always , multipathd always logs the path checker error at logging level 2.

reservation_key

This is the service action reservation key used by mpathpersist. It must be set for all multipath devices using persistent reservations, and it must be the same as the RESERVATION KEY field of the PERSISTENT RESERVE OUT parameter list which contains an 8-byte value provided by the application client to the device server to identify the I_T nexus. If the --param-aptpl option is used when registering the key with mpathpersist, :aptpl must be appended to the end of the reservation key.

all_tg_pt

Set the 'all targets ports' flag when registering keys with mpathpersist. Some arrays automatically set and clear registration keys on all target ports from a host, instead of per target port per host. The ALL_TG_PT flag must be set to successfully use mpathpersist on these arrays. Setting this option is identical to calling mpathpersist with --param-alltgpt

retain_attached_hw_handler

(Obsolete for kernels >= 4.3) If set to yes and the SCSI layer has already attached a hardware_handler to the device, multipath will not force the device to use the hardware_handler specified by mutipath.conf. If the SCSI layer has not attached a hardware handler, multipath will continue to use its configured hardware handler.

detect_prio

If set to yes , multipath will try to detect if the device supports SCSI-3 ALUA. If so, the device will automatically use the sysfs prioritizer if the required sysf attributes access_state and preferred_path are supported, or the alua prioritizer if not. If set to no , the prioritizer will be selected as usual.

detect_checker

if set to yes , multipath will try to detect if the device supports SCSI-3 ALUA. If so, the device will automatically use the tur checker. If set to no , the checker will be selected as usual.

force_sync

If set to yes , multipathd will call the path checkers in sync mode only. This means that only one checker will run at a time. This is useful in the case where many multipathd checkers running in parallel causes significant CPU pressure.

strict_timing

If set to yes , multipathd will start a new path checker loop after exactly one second, so that each path check will occur at exactly polling_interval seconds. On busy systems path checks might take longer than one second; here the missing ticks will be accounted for on the next round. A warning will be printed if path checks take longer than polling_interval seconds.

deferred_remove

If set to yes , multipathd will do a deferred remove instead of a regular remove when the last path device has been deleted. This means that if the multipath device is still in use, it will be freed when the last user closes it. If path is added to the multipath device before the last user closes it, the deferred remove will be canceled.

partition_delimiter

This parameter controls how multipath chooses the names of partition devices of multipath maps if a multipath map is renamed (e.g. if a map alias is added or changed). If this parameter is set to a string other than "/UNSET/" (even the empty string), multipath inserts that string between device name and partition number to construct the partition device name. Otherwise (i.e. if this parameter is unset or has the value "/UNSET/"), the behavior depends on the map name: if it ends in a digit, a "p" is inserted between name and partition number; otherwise, the partition number is simply appended. Distributions may use a non-null default value for this option; in this case, the user must set it to "/UNSET/" to obtain the original <unset> behavior. Use multipath -T to check the current settings.

config_dir

This option is deprecated, and will be removed in a future release. If set to anything other than "", multipath will search this directory alphabetically for file ending in ".conf" and it will read configuration information from them, just as if it was in /etc/multipath.conf. config_dir must either be "" or a fully qualified directory name.

san_path_err_threshold

If set to a value greater than 0, multipathd will watch paths and check how many times a path has been failed due to errors.If the number of failures on a particular path is greater then the san_path_err_threshold, then the path will not reinstate till san_path_err_recovery_time. These path failures should occur within a san_path_err_forget_rate checks, if not we will consider the path is good enough to reinstantate. See "Shaky paths detection" below.

san_path_err_forget_rate

If set to a value greater than 0, multipathd will check whether the path failures has exceeded the san_path_err_threshold within this many checks i.e san_path_err_forget_rate . If so we will not reinstante the path till san_path_err_recovery_time. See "Shaky paths detection" below.

san_path_err_recovery_time

If set to a value greater than 0, multipathd will make sure that when path failures has exceeded the san_path_err_threshold within san_path_err_forget_rate then the path will be placed in failed state for san_path_err_recovery_time duration.Once san_path_err_recovery_time has timeout we will reinstante the failed path . san_path_err_recovery_time value should be in secs. See "Shaky paths detection" below.

marginal_path_double_failed_time

One of the four parameters of supporting path check based on accounting IO error such as intermittent error. When a path failed event occurs twice in marginal_path_double_failed_time seconds due to an IO error and all the other three parameters are set, multipathd will fail the path and enqueue this path into a queue of which members are sent a couple of continuous direct reading asynchronous IOs at a fixed sample rate of 10HZ to start IO error accounting process. See "Shaky paths detection" below.

marginal_path_err_sample_time

One of the four parameters of supporting path check based on accounting IO error such as intermittent error. If it is set to a value no less than 120, when a path fail event occurs twice in marginal_path_double_failed_time second due to an IO error, multipathd will fail the path and enqueue this path into a queue of which members are sent a couple of continuous direct reading asynchronous IOs at a fixed sample rate of 10HZ to start the IO accounting process for the path will last for marginal_path_err_sample_time. If the rate of IO error on a particular path is greater than the marginal_path_err_rate_threshold, then the path will not reinstate for marginal_path_err_recheck_gap_time seconds unless there is only one active path. After marginal_path_err_recheck_gap_time expires, the path will be requeueed for rechecking. If checking result is good enough, the path will be reinstated. See "Shaky paths detection" below.

marginal_path_err_rate_threshold

The error rate threshold as a permillage (1/1000). One of the four parameters of supporting path check based on accounting IO error such as intermittent error. Refer to marginal_path_err_sample_time. If the rate of IO errors on a particular path is greater than this parameter, then the path will not reinstate for marginal_path_err_recheck_gap_time seconds unless there is only one active path. See "Shaky paths detection" below.

marginal_path_err_recheck_gap_time

One of the four parameters of supporting path check based on accounting IO error such as intermittent error. Refer to marginal_path_err_sample_time. If this parameter is set to a positive value, the failed path of which the IO error rate is larger than marginal_path_err_rate_threshold will be kept in failed state for marginal_path_err_recheck_gap_time seconds. When marginal_path_err_recheck_gap_time seconds expires, the path will be requeueed for checking. If checking result is good enough, the path will be reinstated, or else it will keep failed. See "Shaky paths detection" below.

delay_watch_checks

This option is deprecated, and mapped to san_path_err_forget_rate. If this is set to a value greater than 0 and no san_path_err options are set, san_path_err_forget_rate will be set to the value of delay_watch_checks and san_path_err_threshold will be set to 1. See the san_path_err_forget_rate and san_path_err_threshold options, and "Shaky paths detection" below for more information.

delay_wait_checks

This option is deprecated, and mapped to san_path_err_recovery_time. If this is set to a value greater than 0 and no san_path_err options are set, san_path_err_recovery_time will be set to the value of delay_wait_checks times max_polling_interval. This will give approximately the same wait time as delay_wait_checks previously did. Also, san_path_err_threshold will be set to 1. See the san_path_err_recovery_time and san_path_err_threshold options, and "Shaky paths detection" below for more information.

marginal_pathgroups

If set to off, the delay_*_checks, marginal_path_*, and san_path_err_* options will keep marginal, or "shaky", paths from being reinstated until they have been monitored for some time. This can cause situations where all non-marginal paths are down, and no paths are usable until multipathd detects this and reinstates a marginal path. If the multipath device is not configured to queue IO in this case, it can cause IO errors to occur, even though there are marginal paths available. However, if this option is set to on, when one of the marginal path detecting methods determines that a path is marginal, it will be reinstated and placed in a seperate pathgroup that will only be used after all the non-marginal pathgroups have been tried first. This prevents the possibility of IO errors occuring while marginal paths are still usable. After the path has been monitored for the configured time, and is declared healthy, it will be returned to its normal pathgroup. If this option is set to fpin, multipathd will receive fpin notifications, set path states to "marginal" accordingly, and regroup paths as described for on. This option can't be used in combination with other options for "Shaky path detection" (see below). Note: If this is set to fpin, the marginal_path_* and san_path_err_* options are implicitly set to no. Also, this option cannot be switched either to or from fpin on a multipathd reconfigure. multipathd must be restarted for the change to take effect. See "Shaky paths detection" below for more information.

find_multipaths

This option controls whether multipath and multipathd try to create multipath maps over non-blacklisted devices they encounter. This matters a) when a device is encountered by multipath -u during udev rule processing (a device is blocked from further processing by higher layers - such as LVM - if and only if it´s considered a valid multipath device path), and b) when multipathd detects a new device. The following values are possible:

find_multipaths_timeout

Timeout, in seconds, to wait for additional paths after detecting the first one, if find_multipaths "smart" (see above) is set. If the value is positive, this timeout is used for all unknown, non-blacklisted devices encountered. If the value is negative (recommended), it's only applied to "known" devices that have an entry in multipath's hardware table, either in the built-in table or in a device section; other ("unknown") devices will use a timeout of only 1 second to avoid booting delays. The value 0 means "use the built-in default". If find_multipath has a value other than smart, this option has no effect.

uxsock_timeout

CLI receive timeout in milliseconds. For larger systems CLI commands might timeout before the multipathd lock is released and the CLI command can be processed. This will result in errors like "timeout receiving packet" to be returned from CLI commands. In these cases it is recommended to increase the CLI timeout to avoid those issues.

retrigger_tries

Sets the number of times multipathd will try to retrigger a uevent to get the WWID.

retrigger_delay

Sets the amount of time, in seconds, to wait between retriggers.

missing_uev_wait_timeout

Controls how many seconds multipathd will wait, after a new multipath device is created, to receive a change event from udev for the device, before automatically enabling device reloads. Usually multipathd will delay reloads on a device until it receives a change uevent from the initial table load.

skip_kpartx

If set to yes , kpartx will not automatically create partitions on the device.

disable_changed_wwids

This option is deprecated and ignored. If the WWID of a path suddenly changes, multipathd handles it as if it was removed and then added again.

remove_retries

This sets how may times multipath will retry removing a device that is in-use. Between each attempt, multipath will sleep 1 second.

max_sectors_kb

Sets the max_sectors_kb device parameter on all path devices and the multipath device to the specified value.

ghost_delay

Sets the number of seconds that multipath will wait after creating a device with only ghost paths before marking it ready for use in systemd. This gives the active paths time to appear before the multipath runs the hardware handler to switch the ghost paths to active ones. Setting this to 0 or no makes multipath immediately mark a device with only ghost paths as ready.

auto_resize

Controls when multipathd will automatically resize a multipath device. If set to never, multipath devices must always be manually resized by either running multipathd resize map <name>. If set to grow_only, when multipathd detects that all of a multipath device's paths have increased in size, it will automatically grow the multipath device to the new size. If set to grow_shrink, multipathd will also automatically shrink the device once it detects all of its paths have decreased in size.

enable_foreign

Enables or disables foreign libraries (see section FOREIGN MULTIPATH SUPPORT below). The value is a regular expression; foreign libraries are loaded if their name (e.g. "nvme") matches the expression. By default, no foreign libraries are enabled. Set this to "nvme" to enable NVMe native multipath support, or ".*" to enable all foreign libraries.

recheck_wwid

If set to yes, when a failed path is restored, its wwid is rechecked. If the wwid has changed, the path is removed from the current multipath device, and re-added as a new path. Multipathd will also recheck a path's wwid if it is manually re-added. This option only works for SCSI devices that are configured to use the default uid_attribute, ID_SERIAL, or sysfs for getting their wwid.

blacklist and blacklist_exceptions sections¶

The blacklist section is used to exclude specific devices from the multipath topology. It is most commonly used to exclude local disks or non-disk devices (such as LUNs for the storage array controller) from being handled by multipath-tools.

In the blacklist and blacklist_exceptions sections, starting a quoted value with an exclamation mark "!" will invert the matching of the rest of the regular expression. For instance, "!^sd[a-z]" will match all values that do not start with "sd[a-z]". The exclamation mark can be escaped "\!" to match a literal ! at the start of a regular expression. Note: The exclamation mark must be inside quotes, otherwise it will be treated as starting a comment.

The blacklist_exceptions section is used to revert the actions of the blacklist section. This allows one to selectively include ("whitelist") devices which would normally be excluded via the blacklist section. A common usage is to blacklist "everything" using a catch-all regular expression, and create specific blacklist_exceptions entries for those devices that should be handled by multipath-tools.

The following keywords are recognized in both sections. The defaults are empty unless explicitly stated.

devnode

Regular expression matching the device nodes to be excluded/included.

wwid

Regular expression for the World Wide Identifier of a device to be excluded/included.

device

Subsection for the device description. This subsection recognizes the vendor and product keywords. Both are regular expressions. For a full description of these keywords please see the devices section description.

property

Regular expression for an udev property. All devices that have matching udev properties will be excluded/included. The handling of the property keyword is special, because if a property blacklist_exception is set, devices must have at least one whitelisted udev property; otherwise they're treated as blacklisted, and the message "blacklisted, udev property missing" is displayed in the logs. For example, setting the property blacklist_exception to (SCSI_IDENT_|ID_WWN), will cause well-behaved SCSI devices and devices that provide a WWN (World Wide Number) to be included, and all others to be excluded. This works to exclude most non-multipathable devices.

protocol

Regular expression for the protocol of a device to be excluded/included.

For every device, these 5 blacklist criteria are evaluated in the the order "property, devnode, device, protocol, wwid". If a device turns out to be blacklisted by any criterion, it's excluded from handling by multipathd, and the later criteria aren't evaluated any more. For each criterion, the whitelist takes precedence over the blacklist if a device matches both.

Note: Besides the blacklist and whitelist, other configuration options such as find_multipaths have an impact on whether or not a given device is handled by multipath-tools.

multipaths section¶

The multipaths section allows setting attributes of multipath maps. The attributes that are set via the multipaths section (see list below) take precedence over all other configuration settings, including those from the overrides section.

The only recognized attribute for the multipaths section is the multipath subsection. If there are multiple multipath subsections matching a given WWID, the contents of these sections are merged, and settings from later entries take precedence.

The multipath subsection recognizes the following attributes:

wwid

(Mandatory) World Wide Identifier. Detected multipath maps are matched agains this attribute. Note that, unlike the wwid attribute in the blacklist section, this is not a regular expression or a substring; WWIDs must match exactly inside the multipaths section.

alias

Symbolic name for the multipath map. This takes precedence over a an entry for the same WWID in the bindings_file.

The following attributes are optional; if not set the default values are taken from the overrides, devices, or defaults section:

devices section¶

multipath-tools have a built-in device table with reasonable defaults for more than 100 known multipath-capable storage devices. The devices section can be used to override these settings. If there are multiple matches for a given device, the attributes of all matching entries are applied to it. If an attribute is specified in several matching device subsections, later entries take precedence. Thus, entries in files under config_dir (in reverse alphabetical order) have the highest precedence, followed by entries in multipath.conf; the built-in hardware table has the lowest precedence. Inside a configuration file, later entries have higher precedence than earlier ones.

The only recognized attribute for the devices section is the device subsection. Devices detected in the system are matched against the device entries using the vendor, product, and revision fields, which are all POSIX Extended regular expressions (see regex(7)).

The vendor, product, and revision fields that multipath or multipathd detect for devices in a system depend on the device type. For SCSI devices, they correspond to the respective fields of the SCSI INQUIRY page. In general, the command 'multipathd show paths format "%d %s"' command can be used to see the detected properties for all devices in the system.

The device subsection recognizes the following attributes:

vendor

(Mandatory) Regular expression to match the vendor name.

product

(Mandatory) Regular expression to match the product name.

revision

Regular expression to match the product revision. If not specified, any revision matches.

product_blacklist

Products with the given vendor matching this string are blacklisted. This is equivalent to a device entry in the blacklist section with the vendor attribute set to this entry's vendor, and the product attribute set to the value of product_blacklist.

alias_prefix

The user_friendly_names prefix to use for this device type, instead of the default "mpath".

vpd_vendor

The vendor specific vpd page information, using the vpd page abbreviation. The vpd page abbreviation can be found by running sg_vpd -e. multipathd will use this information to gather device specific information that can be displayed with the %g wilcard for the multipathd show maps format and multipathd show paths format commands. Currently only the hp3par vpd page is supported.

hardware_handler

The hardware handler to use for this device type. The following hardware handler are implemented:

The following attributes are optional; if not set the default values are taken from the defaults section:

overrides section¶

The overrides section recognizes the following optional attributes; if not set the values are taken from the devices or defaults sections:

The overrides section also recognizes the optional protocol subsection, and can contain multiple protocol subsections. Path devices are matched against the protocol subsection using the mandatory type attribute. Attributes in a matching protocol subsection take precedence over attributes in the rest of the overrides section. If there are multiple matching protocol subsections, later entries take precedence.

protocol subsection

The protocol subsection recognizes the following mandatory attribute:

WWID generation¶

Multipath uses a World Wide Identification (WWID) to determine which paths belong to the same device. Each path presenting the same WWID is assumed to point to the same device.

The WWID is generated by four methods (in the order of preference):

uid_attrs

The WWID is derived from udev attributes by matching the device node name; cf uid_attrs above.

getuid_callout

Use the specified external program; cf getuid_callout above. Care should be taken when using this method; the external program needs to be loaded from disk for execution, which might lead to deadlock situations in an all-paths-down scenario.

uid_attribute

Use the value of the specified udev attribute; cf uid_attribute above. This method is preferred to getuid_callout as multipath does not need to call any external programs here. However, under certain circumstances udev might not be able to generate the requested variable.

sysfs

Try to determine the WWID from sysfs attributes. For SCSI devices, this means reading the Vital Product Data (VPD) page "Device Identification" (0x83).

The default settings (using udev and uid_attribute configured from the built-in hardware table) should work fine in most scenarios. Users who want to enable uevent merging must set uid_attrs.

Shaky paths detection¶

A common problem in SAN setups is the occurence of intermittent errors: a path is unreachable, then reachable again for a short time, disappears again, and so forth. This happens typically on unstable interconnects. It is undesirable to switch pathgroups unnecessarily on such frequent, unreliable events. multipathd supports three different methods for detecting this situation and dealing with it. All methods share the same basic mode of operation: If a path is found to be "shaky" or "flipping", and appears to be in healthy status, it is not reinstated (put back to use) immediately. Instead, it is placed in the "delayed" state and watched for some time, and only reinstated if the healthy state appears to be stable. If the marginal_pathgroups option is set, the path will reinstated immediately, but placed in a special pathgroup for marginal paths. Marginal pathgroups will not be used until all other pathgroups have been tried. At the time when the path would normally be reinstated, it will be returned to its normal pathgroup. The logic of determining "shaky" condition, as well as the logic when to reinstate, differs between the three methods.

"delay_checks" failure tracking

This method is deprecated and mapped to the "san_path_err" method. See the delay_watch_checks and delay_wait_checks options above for more information.

"marginal_path" failure tracking

If a second failure event (good->bad transition) occurs within marginal_path_double_failed_time seconds after a failure, high-frequency monitoring is started for the affected path: I/O is sent at a rate of 10 per second. This is done for marginal_path_err_sample_time seconds. During this period, the path is not reinstated. If the rate of errors remains below marginal_path_err_rate_threshold during the monitoring period, the path is reinstated. Otherwise, it is kept in failed state for marginal_path_err_recheck_gap_time, and after that, it is monitored again. For this method, time intervals are measured in seconds.

"san_path_err" failure tracking

multipathd counts path failures for each path. Once the number of failures exceeds the value given by san_path_err_threshold, the path is not reinstated for san_path_err_recovery_time seconds. While counting failures, multipathd "forgets" one past failure every "san_path_err_forget_rate" ticks; thus if errors don't occur more often then once in the forget rate interval, the failure count doesn't increase and the threshold is never reached. Ticks are the time between path checks by multipathd, which is variable and controlled by the polling_interval and max_polling_interval parameters.

"FPIN " failure tracking

Fibre channel fabrics can notify hosts about fabric-level issues such as integrity failures or congestion with so-called Fabric Performance Impact Notifications (FPINs).On receiving the fpin notifications through ELS multipathd will move the affected path and port states to marginal.

See the documentation of the individual options above for details. It is strongly discouraged to use more than one of these methods for any given multipath map, because the two concurrent methods may interact in unpredictable ways. If the "marginal_path" method is active, the "san_path_err" parameters are implicitly set to 0.

FOREIGN MULTIPATH SUPPORT¶

multipath and multipathd can load "foreign" libraries to add support for other multipathing technologies besides the Linux device mapper. Currently this support is limited to printing detected information about multipath setup. In topology output, the names of foreign maps are prefixed by the foreign library name in square brackets, as in this example:

# multipath -ll
uuid.fedcba98-3579-4567-8765-123456789abc [nvme]:nvme4n9 NVMe,Some NVMe controller,FFFFFFFF
size=167772160 features='n/a' hwhandler='ANA' wp=rw
|-+- policy='n/a' prio=50 status=optimized
| `- 4:38:1    nvme4c38n1 0:0     n/a   optimized    live
`-+- policy='n/a' prio=50 status=optimized


  `- 4:39:1    nvme4c39n1 0:0     n/a   optimized    live

The "nvme" foreign library provides support for NVMe native multipathing in the kernel. It is part of the standard multipath package.

KNOWN ISSUES¶

The usage of queue_if_no_path option can lead to D state processes being hung and not killable in situations where all the paths to the LUN go offline. It is advisable to use the no_path_retry option instead.

The use of queue_if_no_path or no_path_retry might lead to a deadlock if the dev_loss_tmo setting results in a device being removed while I/O is still queued. The multipath daemon will update the dev_loss_tmo setting accordingly to avoid this deadlock. Hence if both values are specified the order of precedence is no_path_retry, queue_if_no_path, dev_loss_tmo.

SEE ALSO¶

udev(8), dmsetup(8), multipath(8), multipathd(8).

AUTHORS¶

multipath-tools was developed by Christophe Varoqui, <christophe.varoqui@opensvc.com> and others.