How does multipath work

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How is a Repository Organized? How is a Repository Created? Configuration File Blacklist. The blacklist section of the multipath configuration file specifies the devices that will not be used when the system configures multipath devices. Devices that are blacklisted will not be grouped into a multipath device. In older releases of Red Hat Enterprise Linux, multipath always tried to create a multipath device for every path that was not explicitly blacklisted.

There are at least two paths that are not blacklisted with the same WWID. The user manually forces the creation of the device by specifying a device with the multipath command. A path has the same WWID as a multipath device that was previously created even if that multipath device does not currently exist. Whenever a multipath device is created, multipath remembers the WWID of the device so that it will automatically create the device again as soon as it sees a path with that WWID.

This allows you to have multipath automatically choose the correct paths to make into multipath devices, without have to edit the multipath blacklist. Because of this, it will generally not be necessary to blacklist devices. If you do need to blacklist devices, you can do so according to the following criteria:.

By device name, as described in Section 4. By device type, as described in Section 4. By udev property, as described in Section 4. By device protocol, as described in Section 4. By default, a variety of device types are blacklisted, even after you comment out the initial blacklist section of the configuration file.

For information, see Section 4. Blacklisting by WWID. You can specify individual devices to blacklist by their World-Wide IDentification with a wwid entry in the blacklist section of the configuration file. The following example shows the lines in the configuration file that would blacklist a device with a WWID of f Blacklisting By Device Name. You can blacklist device types by device name so that they will not be grouped into a multipath device by specifying a devnode entry in the blacklist section of the configuration file.

You can use a devnode entry in the blacklist section of the configuration file to specify individual devices to blacklist rather than all devices of a specific type. This is not recommended, however, since unless it is statically mapped by udev rules, there is no guarantee that a specific device will have the same name on reboot. By default, the following devnode entries are compiled in the default blacklist; the devices that these entries blacklist do not generally support DM Multipath.

Blacklisting By Device Type. You can specify specific device types in the blacklist section of the configuration file with a device section.

This parameter allows users to blacklist certain types of devices. The property parameter takes a regular expression string that is matched against the udev environment variable name for the device. You can specify the protocol for a device to be excluded from multipathing in the blacklist section of the configuration file with a protocol section. The protocol strings that multipath recognizes are scsi:fcp, scsi:spi, scsi:ssa, scsi:sbp, scsi:srp, scsi:iscsi, scsi:sas, scsi:adt, scsi:ata, scsi:unspec, ccw, cciss, nvme, and undef.

The following example blacklists all devices with an undefined protocol or an unknown SCSI transport type. Blacklist Exceptions. For example, a WWID exception will not apply to devices specified by a devnode blacklist entry, even if the blacklisted device is associated with that WWID. Similarly, devnode exceptions apply only to devnode entries, and device exceptions apply only to device entries. If the parameter is set, the device must have a udev variable that matches.

Otherwise, the device is blacklisted. Configuration File Defaults. The configuration file includes a template of configuration defaults. This section is commented out, as follows. To overwrite the default value for any of the configuration parameters, you can copy the relevant line from this template into the defaults section and uncomment it. These values are used by DM Multipath unless they are overwritten by the attributes specified in the devices and multipaths sections of the multipath.

The default value is 5. If this parameter is set to yes , then multipath will not try to create a device for every path that is not blacklisted. Instead multipath will create a device only if one of three conditions are met: - There are at least two paths that are not blacklisted with the same WWID. This allows you to have multipath automatically choose the correct paths to make into multipath devices, without having to edit the multipath blacklist.

The default value is no. The default multipath. With this option, The multipathd daemon will remap existing device-mapper maps to always point to the multipath device, not the underlying block devices.

Possible values are yes and no. The default value is yes. Higher values increase the verbosity level. Valid levels are between 0 and 6. The default value is 2. The default value is service-time 0. Possible values include: failover : 1 path per priority group.

Priorities are determined by callout programs specified as global, per-controller, or per-multipath options. The default value is failover. Possible values include: const : Set a priority of 1 to all paths. The default value is const. For information on issues that may arise when using this feature, see Section 5. If the SCSI layer has not attached a hardware handler, multipath will continue to use its configured hardware handler as usual. Possible values include: readsector0 : Read the first sector of the device.

The default value is directio. A value of immediate specifies immediate failback to the highest priority path group that contains active paths. A value of manual specifies that there should not be immediate failback but that failback can happen only with operator intervention. A value of followover specifies that automatic failback should be performed when the first path of a path group becomes active.

This keeps a node from automatically failing back when another node requested the failover. A numeric value greater than zero specifies deferred failback, expressed in seconds. The default value is manual. This setting is only for systems running kernels older than 2. The default value is This setting should be used on systems running current kernels.

On systems running kernels older than 2. The default value is 1. If set to uniform , all path weights are equal. The default value is uniform. A value of fail indicates immediate failure, without queuing. A value of queue indicates that queuing should not stop until the path is fixed.

The default value is 0. If set to no , specifies that the system should use the WWID as the alias for the multipath. In either case, what is specified here will be overridden by any device-specific aliases you specify in the multipaths section of the configuration file. This is equivalent to the ulimit -n command. As of the Red Hat Enterprise Linux 6.

For earlier releases, if this is not set the maximum number of open file descriptors is taken from the calling process; it is usually To be safe, this should be set to the maximum number of paths plus 32, if that number is greater than Setting this to off will disable the timeout.

The default value is determined by the OS. For details, see Section 4. Setting this to infinity will set this to seconds, or 68 years. Otherwise, the user's device configuration is treated as a new configuration. If the device does not support ALUA, it will determine the prioritizer as it always does.

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. This allows you to keep paths that may be unreliable from immediately being put back into use as soon as they come back online.

This feature can be used for booting up during installation, when multipath would otherwise attempt to set itself up on devices that it did not claim when they first appeared by means of the udev rules. This parameter can be set to yes or no. If unset, it defaults to no. Both of these options accept numbers greater than or equal to zero.

When this parameter is set to no multipath will not create any new bindings in the initramfs file system. This allows you to have one main configuration that you share between machines in addition to a separate machine-specific configuration file or files.

This ensures that if a multipathed device is in use when a regular remove is performed and the remove fails, the device will automatically be removed when the last user closes the device.

Any later errors are logged at verbosity level 3 until the device is restored. If it is set to always , multipathd always logs the path checker error at verbosity level 2. The default value is always. This allows users to create a multipath device without creating partitions, even if the device has a partition table. The default value of this option is no. If this parameter is not set by the user, the path devices have it set by their device driver, and the multipath device inherits it from the path devices.

Between each attempt, multipath will sleep 1 second. The default value is 0, which means that multipath will not retry the remove. The default value is no , which does not check if a path's WWID has changed.

If so, the device will automatically use the tur path checker. If the --param-aptpl option is used when registering the key with mpathpersist , :aptpl must be appended to the end of the reservation key. The multipathd daemon will then use this key to register additional paths as they appear. It is unset by default. This must be set to yes to successfully use mpathpersist on arrays that automatically set and clear registration keys on all target ports from a host, instead of per target port per host.

Multipaths Device Configuration Attributes. These attributes apply only to the one specified multipath. These defaults are used by DM Multipath and override attributes set in the defaults and devices sections of the multipath. This parameter is mandatory for this section of the multipath. This setting is only for systems running kernels older that 2. If set to no , specifies that the system should use use the WWID as the alias for the multipath. The following example shows multipath attributes specified in the configuration file for two specific multipath devices.

The first device has a WWID of bdb and a symbolic name of yellow. Configuration File Devices. These attributes are used by DM Multipath unless they are overwritten by the attributes specified in the multipaths section of the multipath. These attributes override the attributes set in the defaults section of the multipath.

Many devices that support multipathing are included by default in a multipath configuration. You probably will not need to modify the values for these devices, but if you do you can overwrite the default values by including an entry in the configuration file for the device that overwrites those values.