[Bug 1634161] Re: Default multipath policy is round-robin, shows considerably worse read throughput compared to using service-time

[ChristianEhrhardt]

FYI the minimal bits I checked in the background when we talked about it
yesterday.

Done in debian/patches/path_selector.patch

Related Changelog:
  * debian/patches/path_selector.patch: switch the default path selector             
    back to round-robin while service-time isn't available to the installer          
    multipath-modules.

Likely starts by fully understanding the old dependencies that got to
the decision to add that.

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Title:
  Default multipath policy is round-robin, shows considerably worse read
  throughput compared to using service-time

Status in Ubuntu on IBM z Systems:
  New
Status in multipath-tools package in Ubuntu:
  New

Bug description:
  ---Problem Description---
  Default multipath policy is round-robin, shows considerably worse read throughput compared to using service-time
  Contact Information = Barbara Mundle, barbara.mundle at de.ibm.com 
  ---uname output---
  4.4.0-21-generic
  Machine Type = z systems: Type 2964, Model 701 NC9 
  ---Debugger---
  A debugger is not configured
  ---Steps to Reproduce---
   Conducting throughput measurements with both policies shows the difference

  Userspace tool common name: multipath-tools 
  The userspace tool has the following bit modes: 64-bit 
  Userspace rpm: multipath-tools_0.5.0+git1.656f8865-5ubuntu2.2_s390x.deb 
  Userspace tool obtained from project website:  na 
   
  *Additional Instructions for Barbara Mundle, barbara.mundle at de.ibm.com:
  -Attach ltrace and strace of userspace application.

  Throughput numbers with FIO for sequential read (in KBPS) with
  multipath policies service.time/round-robin (varying with number of
  jobs from 1 - 64):

            service-time   round-robin

  1        3193030         2917220 
  2        5248430         5020375
  4        8453570         6438140
  8        9708690         6444175
  16      9712100         6448080
  32      9714035         6450905
  64      9718355         6461650

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