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Friday 16 May 2008

Comparison: EMC PowerPath vs. GNU/Linux dm-multipath

By Julien Gabel on Friday 16 May 2008, 21:10 - GNU/Linux

I will present some notes about the use of multipath solutions on Red Hat systems: EMC PowerPath and GNU/Linux dm-multipath. Along those notes, keep in mind that they were based on tests done when pressure was very high to put new systems in production, so lack of time resulted in less complete tests than expected. These tests were done more than a year ago, and so before the release of RHEL4 Update 5 and some of RHBA related to both LVM and dm-multipath technologies.

Keep in mind that without purchasing an appropriate EMC license, PowerPath can only be used in failover mode (active-passive mode). Multiple paths accesses are not supported in this case: no round-robin, and no I/O load balancer for example.

EMC PowerPath

Advantages

  1. Not specific to the SAN Host Bus Adapter (HBA).
  2. Support for multiple and heterogeneous SAN storage provider.
  3. Support for most UNIX and Unix-like platforms.
  4. Without a valid license, can only work in degraded mode (failover).
  5. Is not sensible to a change in the SCSI LUN renumbering. Adapt accordingly the corresponding multiple sd devices (different paths to a given device) with its multipath definition of the emcpower device.
  6. Provide easily the ID of the SAN storage.

Drawbacks

  1. Not integrated with the operating system (which generally has its own solution).
  2. The need to force a RPM re-installation in case of a kernel upgrade on RHEL systems (due to the fact that kernel modules are stored in a path containing the exact major and minor versions of the installed (booted) kernel.
  3. Non-automatic update procedure.

GNU/Linux device-mapper-multipath

Advantages

  1. Not specific to the SAN Host Bus Adapter (HBA).
  2. Support for multiple and heterogeneous SAN storage provider.
  3. Well integrated with the operating system.
  4. Automatic update using RHN (you must be a licensed and registered user in this case).
  5. No additional license cost.

Drawbacks

  1. Only available on GNU/Linux systems.
  2. Configuration (files and keywords) very tedious and difficult.
  3. Without the use of LVM (Logical Volume Management), it has not the ability to follow SCSI LUN renumbering! Even in this case, be sure not to have blacklisted the newly discovered SCSI devices (sd).

Last, please find some interesting documentation on the subject below:

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Saturday 9 February 2008

Deleting SCSI Device Paths For A Multipath SAN LUN

By Julien Gabel on Saturday 9 February 2008, 12:28 - GNU/Linux

When releasing a multipath device under RHEL4, different SCSI devices corresponding to different paths must be cleared properly before removing the SAN LUN effectively. When the LUN was delete before to clean up the paths at the OS level, it is always possible to remove them afterwards. In the following example, it is assume that the freeing LVM manipulations were already done, and that the LUN is managed by EMC PowerPath.

  1. First, get and verify the SCSI devices corresponding to the multipath LUN: 
    # grep "I/O error on device" /var/log/messages | tail -2
Feb  4 00:20:47 beastie kernel: Buffer I/O error on device sdo, \
 logical block 12960479
Feb  4 00:20:47 beastie kernel: Buffer I/O error on device sdp, \
 logical block 12960479
# powermt display dev=sdo
Bad dev value sdo, or not under Powerpath control.
# powermt display dev=sdp
Bad dev value sdp, or not under Powerpath control.
  2. Then, get the appropriate scsi#:channel#:id#:lun# informations: 
    # find /sys/devices -name "*block" -print | \
 xargs \ls -l | awk -F\/ '$NF ~ /sdo$/ || $NF ~ /sdp$/ \
 {print "HBA: "$7"\tscsi#:channel#:id#:lun#: "$9}'
HBA: host0      scsi#:channel#:id#:lun#: 0:0:0:9
HBA: host0      scsi#:channel#:id#:lun#: 0:0:1:9
  3. When the individual SCSI paths are known, remove them from the system: 
    # echo 1 > /sys/bus/scsi/devices/0\:0\:0\:9/delete
# echo 1 > /sys/bus/scsi/devices/0\:0\:1\:9/delete
# dmesg | grep "Synchronizing SCSI cache"
Synchronizing SCSI cache for disk sdp:
Synchronizing SCSI cache for disk sdo:

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