mirrors/scst
1
0
Fork 0
mirror of https://github.com/SCST-project/scst.git synced 2026-08-20 06:06:23 +00:00
Code Issues Packages Projects Releases Wiki Activity

Missed piece from the previous commit

Browse Source 
git-svn-id: http://svn.code.sf.net/p/scst/svn/trunk@6577 d57e44dd-8a1f-0410-8b47-8ef2f437770f
This commit is contained in:
Vladislav Bolkhovitin
2015-11-06 03:31:04 +00:00
parent e50f240d33
commit 9ce3d2989a
1 changed files with 44 additions and 29 deletions
Download Patch File Download Diff File Expand all files Collapse all files
scst/README_in-tree
+44 -29
View File
@@ -1300,16 +1300,18 @@ persistent reservations from this device are released, upon reconnect
the initiators will see it.
Implicit ALUA Support
---------------------
ALUA Support
------------
SCST supports implicit asymmetric logical unit access (ALUA). Implicit ALUA is
a feature defined by the ANSI T10 SCSI committee that allows a target to tell
the initiator which path to use in a multipath setup. The redundant paths
between initiator and target can be used either for redundancy or for load
sharing purposes. The target can either be a single target system running SCST
with multiple communication interfaces or two target systems each running SCST
and configured in a high availability setup.
SCST supports both implicit and explicit asymmetric logical unit access
(ALUA). ALUA is a feature defined by the ANSI T10 SCSI committee. It
allows a target to tell the initiator which path to use in a multipath
setup plus, in the explicit case, control state of each path via SET
TARGET PORT GROUPS SCSI command. The redundant paths between initiator
and target can be used either for redundancy or for load sharing
purposes. The target can either be a single target system running SCST
with multiple communication interfaces or two target systems each
running SCST and configured in a high availability setup.
In the SPC-4 standard the following concepts are defined related to ALUA:
* Relative target port ID. A number between 1 and 65535 that uniquely
@@ -1330,17 +1332,17 @@ ANSI T10 standard called SPC-4.
ALUA support in SCST
....................
SCST allows to define implicit ALUA settings for each unique combination of
SCST device and SCST target. An initiator however queries ALUA settings by
sending an appropriate SCSI command to a specific LUN of an SCST target. Each
such LUN maps uniquely to an SCST device. For hardware SCST target drivers,
e.g. ib_srpt, there is a one-to-one correspondence between SCST target and
SCSI target port. With other SCST targets, e.g. iSCSI-SCST, by default the
only relationship between SCST targets and SCSI target ports is that all SCST
targets defined on a system are visible via all SCSI target ports. See also
the iSCSI-SCST documentation about the allowed_portal attribute for
information about how to associate iSCSI targets with a single physical
interface.
SCST allows to define ALUA settings for each unique combination of SCST
device and SCST target. An initiator however queries ALUA settings by
sending an appropriate SCSI command to a specific LUN of an SCST target.
Each such LUN maps uniquely to an SCST device. For hardware SCST target
drivers, e.g. ib_srpt, there is a one-to-one correspondence between SCST
target and SCSI target port. With other SCST targets, e.g. iSCSI-SCST,
by default the only relationship between SCST targets and SCSI target
ports is that all SCST targets defined on a system are visible via all
SCSI target ports. See also the iSCSI-SCST documentation about the
allowed_portal attribute for information about how to associate iSCSI
targets with a single physical interface.
Notes:
- In a H.A. setup it is the responsibility of the user to synchronize ALUA
@@ -1363,7 +1365,7 @@ Notes:
Configuring ALUA in SCST
........................
SCST allows to configure the following settings related to implicit ALUA
SCST allows to configure the following settings related to ALUA
for each unique combination of SCST target and virtual SCST device
(vdisk_fileio, vdisk_blockio, vcdrom, ...):
* The target port group asymmetric access state. SCST supports all ALUA port
@@ -1461,15 +1463,26 @@ DEVICE_GROUP dgroup2 {
}
}
Explicit ALUA
.............
To enable explicit ALUA you need in addition to the above settings set
expl_alua device attribute to 1 (by default it is 0). Also you need to
run stpgd and supply to it path to a script or program, which will
perform actual path state switching on SET TARGET PORT GROUPS command,
for instance, by calling drbdadm. For more information see stpgd README
as well as sample script scst_on_stpg.
Checking the Target Configuration
.................................
One way to verify the implicit ALUA configuration from a Linux initiator is
via the commands provided in the sg3_utils package. The first step is to
verify whether for a certain LUN implicit ALUA has been configured on the
target. This is possible by checking whether the TPGS=1 text appears in the
sg_inq output, where /dev/sdb is a device node created by the ib_srp initiator:
One way to verify the ALUA configuration from a Linux initiator is via
the commands provided in the sg3_utils package. The first step is to
verify whether for a certain LUN ALUA has been configured on the target.
This is possible by checking whether the TPGS=1 text appears in the
sg_inq output, where /dev/sdb is a device node created by the ib_srp
initiator:
# sg_inq /dev/sdb
standard INQUIRY:
@@ -1529,9 +1542,11 @@ Device Identification VPD page:
Initiator Support
.................
On Linux systems implicit ALUA support is provided by the scsi_dh_alua kernel
driver in combination with the user space multipathd daemon. You will have to
modify at least the following in /etc/multipath.conf to enable implicit ALUA:
On Linux systems ALUA support is provided by the scsi_dh_alua kernel
driver in combination with the user space multipathd daemon. You will
have to modify at least the following in /etc/multipath.conf to enable
ALUA:
* hardware_handler "1 alua"
* prio alua
* path_grouping_policy group_by_prio