[dm-devel] path_grouping_policy group_by_serial prio question

[Christophe Varoqui]

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 at broermann.com> 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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>
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