Re: path_grouping_policy group_by_serial prio question

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Hi,

the path_grouping_policy is only responsible for the path grouping.
The "emc" prioritizer here, attributes a prio to each path.
Priorities of paths in the same group are aggregated into a pathgroup priority.
The pathgroup with the highest priority is activated.

Best regards,
Christophe Varoqui
www.opensvc.com


On Tue, Mar 8, 2016 at 1:03 PM, Bernd Broermann <bernd@xxxxxxxxxxxxx> wrote:
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
}

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