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scst: update ALUA documentation (Active/Non-Optimized)

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git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@7206 d57e44dd-8a1f-0410-8b47-8ef2f437770f
This commit is contained in:
Vladislav Bolkhovitin
2017-06-07 01:13:58 +00:00
parent 64d07d25cf
commit e174d00f08
2 changed files with 162 additions and 4 deletions
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83
scst/README
View File

@@ -360,7 +360,8 @@ in/out in Makefile and scst.h:
commands, which normally processed locally by the SCST core, not
processed locally, but passed to the backend device. Intended to be
used to implement NON-OPTIMIZED ALUA state together with "forwarding"
target attribute on the remote node. Disabled by default for safety.
target attribute on the remote node. See below for more details.
Disabled by default for safety.
HIGHMEM kernel configurations are fully supported, but not recommended
for performance reasons, except for scst_user, where they are not
@@ -1679,6 +1680,12 @@ DEVICE_GROUP dgroup2 {
}
}
Note, if you are using "active" BLOCKIO device attribute to prevent open
of the backend block device on the passive node, it is not recommended
to set both active ("active", "nonoptimized") and passive ("standby",
etc.) ALUA states for the same device as shown above to keep internal
"active" state of the BLOCKIO device consistent.
Explicit ALUA
.............
@@ -1699,7 +1706,9 @@ secondary transition, if this device is open.
SCST BLOCKIO handler has necessary support for such behavior:
1. If you need to prevent an SCST BLOCKIO device from opening its block
device, you need to create it with parameter "active=0"
device, you need to create it with parameter "active=0". In case of DRBD
it would be done automatically, you don't have to use the "active"
attribute.
2. If you write new ALUA state in the "state" attribute, SCST BLOCKIO
handler before transition closes open handles on all affected SCST
@@ -1851,6 +1860,76 @@ information about ALUA support in Windows Server, see also:
* Microsoft, ALUA MPIO Logo Test, MSDN
(http://msdn.microsoft.com/en-us/library/gg607458%28v=vs.85%29.aspx).
Active/Non-Optimized via internal redirection
...............................................
The Active-Standby configuration is simple to understand and setup,
however, it might have serious interoperability issues, because not all
initiators handle Standby state correctly. For instance, some versions
of VMware reported to have such issues. Same for Windows.
Hence, it is better to use Non-Optimized state on the passive node
instead of Standby with internal commands redirection to the active
node. This is what the vast majority of storage vendors are doing, which
is, actually, the reason why Standby and Unavailable states have all
those initiator issues. Simply, they have had too few testing, because
only marginally used.
SCST has necessary support for such redirection, it just needs to be
configured it correctly. It's a little bit of effort, especially to
understand how it's going to function, but then it would work MUCH more
reliable for full range of initiators. Ever poor initiators, who have no
idea about ALUA (boot from SAN, e.g.) would work now.
1. Build SCST with CONFIG_SCST_FORWARD_MODE_PASS_THROUGH enabled in scst.h
2. Setup on active node internal redirect target, which is going to
accept redirected commands from the passive node. It must be visible
only to the passive node.
3. Set "forwarding" attribute for this target to 1. This is necessary to
correctly handle PRs.
4. Export through this target the SAME backend SCST device as being
served to initiator(s) (consider for simplicity that there is only one
served device)
5. Connect to this SCST device through this internal target from the
passive node. Now you have a local SCSI device on the passive side
pointing to the active node.
6. Export this local device to the initiator(s) using SCST
*path-through* handler (scst_disk). Pass-though is needed to redirect
non-block commands as well: ATS, XCOPY, etc.
7. Set ALUA state to this target as "nonoptimized".
That's it on the normal path. Now the initiator(s) would see 2 paths:
OPTIMIZED going to the active node and NON-OPTIMIZED going to the
passive node, then redirected to the active node.
On failover (i.e. switching active and passive states):
1. Setup similar redirect target on the new active node.
2. Setup connectivity to that new redirect target from the new passive
node
3. Start ALUA change (see above) on both nodes
4. !! Exchange in the sysfs security group(s) for the initiator(s) *LUN*
from old SCST device to the new one (blockio -> pass-through on the new
passive and pass-through -> blockio on the new active) using "replace_no_ua"
SCST command. You need to do it directly in the sysfs interface,
scstadmin can't do it.
5. Set ALUA states to "active" on the new active node and "nonoptimized"
on the new passive node.
6. Finish ALUA states change.
If you have any questions, please read this above text at least 3 times
before asking. It might be tricky to understand.
VAAI
----

83
scst/README_in-tree
View File

@@ -250,7 +250,8 @@ your favorite kernel configuration Makefile target, e.g. "make xconfig":
commands, which normally processed locally by the SCST core, not
processed locally, but passed to the backend device. Intended to be
used to implement NON-OPTIMIZED ALUA state together with "forwarding"
target attribute on the remote node. Disabled by default for safety.
target attribute on the remote node. See below for more details.
Disabled by default for safety.
HIGHMEM kernel configurations are fully supported, but not recommended
for performance reasons.
@@ -1532,6 +1533,12 @@ DEVICE_GROUP dgroup2 {
}
}
Note, if you are using "active" BLOCKIO device attribute to prevent open
of the backend block device on the passive node, it is not recommended
to set both active ("active", "nonoptimized") and passive ("standby",
etc.) ALUA states for the same device as shown above to keep internal
"active" state of the BLOCKIO device consistent.
Explicit ALUA
.............
@@ -1552,7 +1559,9 @@ secondary transition, if this device is open.
SCST BLOCKIO handler has necessary support for such behavior:
1. If you need to prevent an SCST BLOCKIO device from opening its block
device, you need to create it with parameter "active=0"
device, you need to create it with parameter "active=0". In case of DRBD
it would be done automatically, you don't have to use the "active"
attribute.
2. If you write new ALUA state in the "state" attribute, SCST BLOCKIO
handler before transition closes open handles on all affected SCST
@@ -1704,6 +1713,76 @@ information about ALUA support in Windows Server, see also:
* Microsoft, ALUA MPIO Logo Test, MSDN
(http://msdn.microsoft.com/en-us/library/gg607458%28v=vs.85%29.aspx).
Active/Non-Optimized via internal redirection
...............................................
The Active-Standby configuration is simple to understand and setup,
however, it might have serious interoperability issues, because not all
initiators handle Standby state correctly. For instance, some versions
of VMware reported to have such issues. Same for Windows.
Hence, it is better to use Non-Optimized state on the passive node
instead of Standby with internal commands redirection to the active
node. This is what the vast majority of storage vendors are doing, which
is, actually, the reason why Standby and Unavailable states have all
those initiator issues. Simply, they have had too few testing, because
only marginally used.
SCST has necessary support for such redirection, it just needs to be
configured it correctly. It's a little bit of effort, especially to
understand how it's going to function, but then it would work MUCH more
reliable for full range of initiators. Ever poor initiators, who have no
idea about ALUA (boot from SAN, e.g.) would work now.
1. Build SCST with CONFIG_SCST_FORWARD_MODE_PASS_THROUGH enabled in scst.h
2. Setup on active node internal redirect target, which is going to
accept redirected commands from the passive node. It must be visible
only to the passive node.
3. Set "forwarding" attribute for this target to 1. This is necessary to
correctly handle PRs.
4. Export through this target the SAME backend SCST device as being
served to initiator(s) (consider for simplicity that there is only one
served device)
5. Connect to this SCST device through this internal target from the
passive node. Now you have a local SCSI device on the passive side
pointing to the active node.
6. Export this local device to the initiator(s) using SCST
*path-through* handler (scst_disk). Pass-though is needed to redirect
non-block commands as well: ATS, XCOPY, etc.
7. Set ALUA state to this target as "nonoptimized".
That's it on the normal path. Now the initiator(s) would see 2 paths:
OPTIMIZED going to the active node and NON-OPTIMIZED going to the
passive node, then redirected to the active node.
On failover (i.e. switching active and passive states):
1. Setup similar redirect target on the new active node.
2. Setup connectivity to that new redirect target from the new passive
node
3. Start ALUA change (see above) on both nodes
4. !! Exchange in the sysfs security group(s) for the initiator(s) *LUN*
from old SCST device to the new one (blockio -> pass-through on the new
passive and pass-through -> blockio on the new active) using "replace_no_ua"
SCST command. You need to do it directly in the sysfs interface,
scstadmin can't do it.
5. Set ALUA states to "active" on the new active node and "nonoptimized"
on the new passive node.
6. Finish ALUA states change.
If you have any questions, please read this above text at least 3 times
before asking. It might be tricky to understand.
VAAI
----