Linux Config.fm
Draft Document for Review March 28, 2011 12:24 pm
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IBM System Storage DS3500: Introduction and Implementation Guide
list|show config
list|show blacklist
list|show devices
list|show wildcards
add path $path
remove|del path $path
add map|multipath $map
remove|del map|multipath $map
switch|switchgroup map|multipath $map group $group
reconfigure
suspend map|multipath $map
resume map|multipath $map
resize map|multipath $map
reload map|multipath $map
reinstate path $path
fail path $path
multipathd>
20.4.2 Comparing RDAC(MPP) to DMM
While MPP is actually a driver, and performs its virtualization functions at a lower level, much
of Device Mapper’s virtualization occurs at a higher level, using a service.
If you run “SMdevices”, the system will show every path to each device. These will usually be
named something like /dev/sdb, /dev/sdc, etc. Each of these are called a “block device”, to
which smash can write IO.
Device mapper takes these underlying block devices and maps all the paths leading to a
single volume to a single virtualized block device. These will usually be named something like
/dev/dm-0, /dev/dm-1, etc. These are also block devices, and will be our smash targets
instead of the /dev/sd* devices.
Because device mapper performs its virtualization above the SCSI midlayer (which acts as an
intermediary layer between the lower and upper layers), the system will still see each and
every path to the volume. Therefore, if a path that is running IO is lost, the host will report an
“IO error”. This, in itself, does not indicate a test failure, but rather that the host tried to send
IO down the path and the path was not present. The SCSI midlayer will show that there was
an IO error, and will report that to the device mapper multipath service, which will fail the path
and transfer IO to the alternate path.
This is different from MPP where two paths are virtualized into a single device in such a way
that the SCSI midlayer only sees one device. In that case, the SCSI midlayer would never
report the IO error if the system was running MPP.
20.4.3 Planning for the installation
Follow the below guidelines for planning:
Multiple combinations of systems, AIX versions, Host Bus Adapters (HBA’s), firmware
levels and multipath drivers can be used in order to connect a POWER® system to a
DS3500 storage subsystem. For detailed HBA and systems information regarding
combinations, see the following Web site to access the System Storage Interoperation
Center (SSIC):
http://www.ibm.com/systems/support/storage/config/ssic
Summary of Contents for DS3500
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