[dm-devel] path_grouping_policy group_by_serial prio question

[Bernd Broermann]

Hello,


Does "group_by_serial" activate the "low latency"  path ?

There are two servers which have a separate path to a EMC VPLEX box in 
different Data centers ( DC1 , DC2 ) .

Because of latency the paths are grouped by the serial number of the 
VPLEX box.




        DC1                DC2

    +------------+    +------------+
    | Server 1   |    | Server 2   |
    +------------+    +------------+
       |   \              /  |
       |     \          /    |
       |       \      /      |
       |         \  /        |
       |          X          |
       |        /   \        |
       |      /       \      |
       |    /           \    |
       |  /              \   |
    +------------+    +------------+
    |disk VPLEX A|    |disk VPLEX B|
    +------------+    +------------+


As I expected the paths are grouped and the path to the "nearest" disk( 
VPLEX) is active.

Is this an coincidence or does it work as designed ?

I didn't find something enlightening in the dm documentation.

Thank you for your help.

Bernd



partitial output of  "multipathd show config"
....
device {
     vendor "EMC"
     product "Invista"
     product_blacklist "LUNZ"
     path_grouping_policy group_by_serial
     getuid_callout "/lib/udev/scsi_id --whitelisted --device=/dev/%n"
     path_selector "round-robin 0"
     path_checker tur
     features "0"
     hardware_handler "1 emc"
     prio emc
     rr_weight uniform
     no_path_retry fail
     rr_min_io 1000
     rr_min_io_rq 1
     detect_prio yes
}