MPIO global settings - Revision history

Da-F at 09:54, 8 September 2023

2023-09-08T09:54:20Z

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	Setting that tells multipath how to check state of path.		Setting that tells multipath how to check state of path.

	#'''Direct I/O'''~~(default)~~ - read first sector of disk without using any cache.		#'''Direct I/O''' - read first sector of disk without using any cache.
	#'''Test Unit Ready''' - Use SCSI command “Test Unit Ready” to check if disk is available. On response to this command device return if it is accessible by client application.		#'''Test Unit Ready''' (default) - Use SCSI command “Test Unit Ready” to check if disk is available. On response to this command device return if it is accessible by client application.
	#'''EMC Clariion, RDAC storage controller and HP storage array''' - this settings are specific for particular hardware. Some hardware vendors provides custom path checker options. Those options can be used with specified hardware.		#'''EMC Clariion, RDAC storage controller and HP storage array''' - this settings are specific for particular hardware. Some hardware vendors provides custom path checker options. Those options can be used with specified hardware.

Ma-W at 14:20, 15 March 2019

2019-03-15T14:20:49Z

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Ma-W at 09:14, 15 March 2019

2019-03-15T09:14:19Z

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migrate>Mi-S at 12:52, 8 February 2017

2017-02-08T12:52:59Z

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	</ol>		</ol>

	<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">Most probably Path weight is used in weighted round robin algorithm that calculates how much data should be send through particular path in active path group. Path with higher weight is considered to be faster than path with lower weight. Algorithm simply send more data through path with higher weight in order to better balance load. In case of uniform setting use of weighted round robin algorithm is technically disabled because each patch has same weight. In case of priorities weight makes actual impact only if multipath set different priorities to particular paths. Because if priorities have the same value then also weight of each path is different and rr_min_io_rq can be set for whole multipath only but not for single path. See </span>[https://docs.google.com/document/d/1Y15ap4EfeMnfSZc4e77InGkSTymN9Q52eDnm6DXXdqk/edit#heading=h.eqta6ifvcs0x <span style="font-size: 11pt; font-family: Arial; color: rgb(17, 85, 204); background-color: transparent; text-decoration: underline; vertical-align: baseline;">path priority routine</span>]<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">to check how multipath can assign priorities to paths.</span>		<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">Most probably Path weight is used in weighted round robin algorithm that calculates how much data should be send through particular path in active path group. Path with higher weight is considered to be faster than path with lower weight. Algorithm simply send more data through path with higher weight in order to better balance load. In case of uniform setting use of weighted round robin algorithm is technically disabled because each patch has same weight. In case of priorities weight makes actual impact only if multipath set different priorities to particular paths. Because if priorities have the same value then also weight of each path is different and rr_min_io_rq can be set for whole multipath only but not for single path.</span>

migrate>Mi-S at 12:52, 8 February 2017

2017-02-08T12:52:39Z

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	== <span style="font-size: 16pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; font-weight: 400; vertical-align: baseline;">Path weight</span> ==		== <span style="font-size: 16pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; font-weight: 400; vertical-align: baseline;">Path weight</span> ==

	<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">This option allows to select method used to assign weight to paths in a group~~. At the beginning available settings are explained and after that is actual explanation what this option is for (to some extent it is assumption)~~. Setting allows two values:</span>		<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">This option allows to select method used to assign weight to paths in a group. Setting allows two values:</span>
	<ol style="margin-top:0pt;margin-bottom:0pt;">		<ol style="margin-top:0pt;margin-bottom:0pt;">
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Uniform</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">(default) - all paths has same weight.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Uniform</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">(default) - all paths has same weight.</span></li>
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	</ol>		</ol>

	<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">~~Now it is required to explain what is a purpose of path weight.~~ Most probably it is used in weighted round robin algorithm that calculates how much data should be send through particular path in active path group. Path with higher weight is considered to be faster than path with lower weight. Algorithm simply send more data through path with higher weight in order to better balance load. In case of uniform setting use of weighted round robin algorithm is technically disabled because each patch has same weight. In case of priorities weight makes actual impact only if multipath set different priorities to particular paths. Because if priorities have the same value then also weight of each path is different and rr_min_io_rq can be set for whole multipath only but not for single path. See </span>[https://docs.google.com/document/d/1Y15ap4EfeMnfSZc4e77InGkSTymN9Q52eDnm6DXXdqk/edit#heading=h.eqta6ifvcs0x <span style="font-size: 11pt; font-family: Arial; color: rgb(17, 85, 204); background-color: transparent; text-decoration: underline; vertical-align: baseline;">path priority routine</span>]<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">to check how multipath can assign priorities to paths.</span>		<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">Most probably Path weight is used in weighted round robin algorithm that calculates how much data should be send through particular path in active path group. Path with higher weight is considered to be faster than path with lower weight. Algorithm simply send more data through path with higher weight in order to better balance load. In case of uniform setting use of weighted round robin algorithm is technically disabled because each patch has same weight. In case of priorities weight makes actual impact only if multipath set different priorities to particular paths. Because if priorities have the same value then also weight of each path is different and rr_min_io_rq can be set for whole multipath only but not for single path. See </span>[https://docs.google.com/document/d/1Y15ap4EfeMnfSZc4e77InGkSTymN9Q52eDnm6DXXdqk/edit#heading=h.eqta6ifvcs0x <span style="font-size: 11pt; font-family: Arial; color: rgb(17, 85, 204); background-color: transparent; text-decoration: underline; vertical-align: baseline;">path priority routine</span>]<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">to check how multipath can assign priorities to paths.</span>

migrate>Mi-S at 12:49, 8 February 2017

2017-02-08T12:49:25Z

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 == <span style="font-size: 16pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; font-weight: 400; vertical-align: baseline;">No. of I/O request (rr_min_io and rr_min_io_rq)</span> ==
-<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">This section discusses two settings rr_min_io and rr_min_io_rq because those settings are connected with each other. On GUI those settings are called “No. of I/O request for BIO based multipath” (rr_min_io) and “No. of I/O request for request based multipath” (rr_min_io_rq). Basically multipath manual describes rr_min_io as minimum number of I/O that have to be performed before it can switch to next path in same group and this value applies only for block based multipaths. Second setting rr_min_io_rq is described as minimum number of requests that have to be routed before it can switch to next path, this setting applies only to request based multipaths. Generally those settings looks more or less simple, by using them it is possible to set minimum amount of data that have to be send through one path before multipath switches to the next one. But there is a gotcha because we have no knowledge if particular multipath is request or block based. I have no idea from where can I fetch such information. According to that user also have no idea about type of used multipath. There is assumption that it might be related to underlying device but it is only assumption that doesn’t give any clue how to actually check it.</span>
+<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">This section discusses two settings rr_min_io and rr_min_io_rq because those settings are connected with each other. On GUI those settings are called “No. of I/O request for BIO based multipath” (rr_min_io) and “No. of I/O request for request based multipath” (rr_min_io_rq). We can describe rr_min_io as minimum number of I/O that have to be performed before it can switch to next path in same group and this value applies only for block based multipaths. Second setting rr_min_io_rq is a minimum number of requests that have to be routed before it can switch to next path, this setting applies only to request based multipaths. By using those settings it is possible to set minimum amount of data that have to be send through one path before multipath switches to the next one.</span>
 <span style="background-color: transparent; color: rgb(0, 0, 0); font-family: Arial; font-size: 11pt;">Default value for rr_min_io_rq is 1 and for rr_min_io is 1000.</span>
 == <span style="font-size: 16pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; font-weight: 400; vertical-align: baseline;">Path weight</span> ==

migrate>Mi-S at 12:42, 8 February 2017

2017-02-08T12:42:35Z

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	== <span style="font-size: 16pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; font-weight: 400; vertical-align: baseline;">Queue disabling (flush_on_last_del)</span> ==		== <span style="font-size: 16pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; font-weight: 400; vertical-align: baseline;">Queue disabling (flush_on_last_del)</span> ==

	<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">~~Manual says that this~~ option will disable queueing when last path to device is removed. I made some tests by writing data (using dd) to multipath device and disconnecting paths to device or device itself. In both available settings “yes” and “no” (default) it behaved in the same way - as expected transfer was simply broken. Some additional search in google revealed that this option was introduced as a </span>[https://bugzilla.redhat.com/show_bug.cgi?id=430494 <span style="font-size: 11pt; font-family: Arial; color: rgb(17, 85, 204); background-color: transparent; text-decoration: underline; vertical-align: baseline;">bug fix</span>]<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">. In short summary there was reported that without this option (when it is set to no) LVM commands might hang infinitely when underlying disk is removed (which causes that all paths are removed).</span>		<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">This option will disable queueing when last path to device is removed.</span>

	== <span style="font-size: 16pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; font-weight: 400; vertical-align: baseline;">Path retry (no_path_retry)</span> ==		== <span style="font-size: 16pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; font-weight: 400; vertical-align: baseline;">Path retry (no_path_retry)</span> ==

migrate>Mi-S at 12:34, 8 February 2017

2017-02-08T12:34:27Z

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	<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">This setting specifies how multipath handles path group that recovers from failure. According to setting multipath may do nothing or evaluate particular path group priority and switch to group if it has higher priority than currently used group.</span>		<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">This setting specifies how multipath handles path group that recovers from failure. According to setting multipath may do nothing or evaluate particular path group priority and switch to group if it has higher priority than currently used group.</span>
	<ol style="margin-top:0pt;margin-bottom:0pt;">		<ol style="margin-top:0pt;margin-bottom:0pt;">
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Manual</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">(default) - In this setting multipath do not switches back automatically to path group that recovered. However if some path group recovered and all other failed then multipath will switch to this last available path group that previously was unavailable. This setting won’t cause any transfer break.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Manual </span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">(default) - In this setting multipath do not switches back automatically to path group that recovered. However if some path group recovered and all other failed then multipath will switch to this last available path group that previously was unavailable. This setting won’t cause any transfer break.</span></li>
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Immediate</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- failed path groups are being monitored and as soon as multipath realises that path group recovered it is enabled immediately. But when particular path group recovers it is enabled only if it has higher priority than the one that multipath switched to. If priorities are the same then multipath has no reason to switch path group.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Immediate </span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- failed path groups are being monitored and as soon as multipath realises that path group recovered it is enabled immediately. But when particular path group recovers it is enabled only if it has higher priority than the one that multipath switched to. If priorities are the same then multipath has no reason to switch path group.</span></li>
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Custom value</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- number of seconds after path group recovery that have to pass before multipath can switch to it. This setting is similar to immediate but instead of immediately switching path group multipath wait given number of seconds until it is allowed to switch path group. Basically if path group with higher priority than currently used recovers then multipath will switch to it only if it is available for specified time.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Custom value </span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- number of seconds after path group recovery that have to pass before multipath can switch to it. This setting is similar to immediate but instead of immediately switching path group multipath wait given number of seconds until it is allowed to switch path group. Basically if path group with higher priority than currently used recovers then multipath will switch to it only if it is available for specified time.</span></li>
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Follower</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- ~~manual says that~~ this setting allows automatic failback only if first path in path group becomes active.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Follower </span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- this setting allows automatic failback only if first path in path group becomes active.</span></li>
	</ol>		</ol>

migrate>Mi-S at 12:31, 8 February 2017

2017-02-08T12:31:00Z

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	<ol style="margin-top:0pt;margin-bottom:0pt;">		<ol style="margin-top:0pt;margin-bottom:0pt;">
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Multibus</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">(default) - in this policy only one path group is created using all available paths.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Multibus</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">(default) - in this policy only one path group is created using all available paths.</span></li>
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Failover</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- every path is put in separate group.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Failover </span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- every path is put in separate group.</span></li>
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Group by node name</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- 1 group is created for each target node name~~. According to documentation this setting has something to with FC targets and paths are grouped using names of nodes that exports particular target (this is what I understand)~~.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Group by node name </span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- 1 group is created for each target node name.</span></li>
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Group by priority value</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- paths with same priority are added to the same group. Each group has paths with same priority.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Group by priority value </span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- paths with the same priority are added to the same group. Each group has paths with same priority.</span></li>
	<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Group by serial number</span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- paths that has same serial number are put in one group. I assume that paths can have different serial numbers because it is modified or reported by some controllers or other devices that are used to connect disks. Those devices can report different serial numbers (modified) for actually same disk. This may make sense to put this kind of paths in different group because for example particular path goes physically by different route.</span></li>		<li><span style="font-size: 11pt; background-color: transparent; font-weight: 700; vertical-align: baseline;">Group by serial number </span><span style="font-size: 11pt; background-color: transparent; vertical-align: baseline;">- paths that has same serial number are put in one group. We assume that paths can have different serial numbers because it is modified or reported by some controllers or other devices that are used to connect disks. Those devices can report different serial numbers (modified) for actually same disk. This may make sense to put this kind of paths in different group because for example particular path goes physically by different route.</span></li>
	</ol>		</ol>

migrate>Mi-S at 12:25, 8 February 2017

2017-02-08T12:25:38Z

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	= <span style="background-color: transparent; color: rgb(0, 0, 0); font-family: Arial; font-size: 16pt; font-weight: 400;">Path grouping policy</span> =		== <span style="background-color: transparent; color: rgb(0, 0, 0); font-family: Arial; font-size: 16pt; font-weight: 400;">Path grouping policy</span> ==

	<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">In order to understand this option it is required to explain how paths are organized by multipath. When disk is connected using multiple paths and multipath device is created those paths can be organized in groups of paths. Sometimes it might be just one group with all paths but it is not always the case. Multipath can group paths in a few ways and use them to transfer data. At particular point in time data is transferred through one path group that is selected according to its priority. Within selected path group data is transferred in round-robin fashion by default to increase throughput. Multipath has a few policies that are used to create groups of paths.</span>		<span style="font-size: 11pt; font-family: Arial; color: rgb(0, 0, 0); background-color: transparent; vertical-align: baseline;">In order to understand this option it is required to explain how paths are organized by multipath. When disk is connected using multiple paths and multipath device is created those paths can be organized in groups of paths. Sometimes it might be just one group with all paths but it is not always the case. Multipath can group paths in a few ways and use them to transfer data. At particular point in time data is transferred through one path group that is selected according to its priority. Within selected path group data is transferred in round-robin fashion by default to increase throughput. Multipath has a few policies that are used to create groups of paths.</span>