Upcall prioritizer for multipath

[Oliver Mangold <Oliver.Mangold@EMEA.NEC.COM>]

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I'm working with JBODs and would like to access the disks inside through 
dm-multipath. For performance reasons I would like to use to prioritizer 
to pin individual disks to different paths. For testing purposes this 
can be done with the weighted_path prioritizer, but for production use 
this is unsatisfactory, as neither the device name nor the SCSI address 
are persistent over reboots. Thus I wrote a new, simple prioritizer to 
call an external program to determine what the prio should be. I would 
like to ask if there is interest to include this (or something similar) 
upstream (see attached patch).

Remark: After I did this, I've seen the wwn regex mode in the upcoming 
0.6 version, which would also provide a solution to the problem I 
started out with. But using this would still means one had to recreate 
multipath.conf each time one replaces a failed disk, so I still believe 
having an external upcall makes sense in some situations.

Any thoughts?


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From 124b54531dd1859755175ca5e9e4882d15cdd87d Mon Sep 17 00:00:00 2001
From: Oliver Mangold <oliver.mangold@emea.nec.com>
Date: Fri, 22 Apr 2016 12:53:55 +0200
Subject: [PATCH] libmultipath: adding new prioritizer 'upcall' to call
 external program

---
 libmultipath/prioritizers/Makefile |  1 +
 libmultipath/prioritizers/upcall.c | 25 +++++++++++++++++++++++++
 multipath/multipath.conf.5         |  6 ++++++
 3 files changed, 32 insertions(+)
 create mode 100644 libmultipath/prioritizers/upcall.c

diff --git a/libmultipath/prioritizers/Makefile b/libmultipath/prioritizers/Makefile
index 6cfac88..1b32d00 100644
--- a/libmultipath/prioritizers/Makefile
+++ b/libmultipath/prioritizers/Makefile
@@ -15,6 +15,7 @@ LIBS = \
 	libpriodatacore.so \
 	libpriohds.so \
 	libprioweightedpath.so \
+	libprioupcall.so \
 	libprioiet.so
 
 CFLAGS += -I..
diff --git a/libmultipath/prioritizers/upcall.c b/libmultipath/prioritizers/upcall.c
new file mode 100644
index 0000000..175b8fe
--- /dev/null
+++ b/libmultipath/prioritizers/upcall.c
@@ -0,0 +1,25 @@
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+#include <prio.h>
+#include <structs.h>
+
+int getprio (struct path * pp, char * args)
+{
+    char* cmd;
+    int ret = asprintf(&cmd, "%s %s", args, pp->dev);
+    if (ret == -1)
+      return -1;
+    FILE* pipe = popen( cmd, "r" );
+    free(cmd);
+    int prio;
+    int count = fscanf(pipe, "%i", &prio);
+    ret = pclose(pipe);
+    if (ret != 0)
+      return -1;
+    if (count != 1)
+        return -1;
+    return prio;
+}
diff --git a/multipath/multipath.conf.5 b/multipath/multipath.conf.5
index 0d4df0f..d8d6ad6 100644
--- a/multipath/multipath.conf.5
+++ b/multipath/multipath.conf.5
@@ -217,6 +217,9 @@ Generate a random priority between 1 and 10.
 Generate the path priority based on the regular expression and the 
 priority provided as argument. requires prio_args keyword.
 .TP
+.B upcall
+Call an external program to calculate the priority.
+.TP
 Default value is \fBnone\fR.
 .RE
 .TP
@@ -243,6 +246,9 @@ If
 .I exclusive_pref_bit
 is set, paths with the TPGS pref bit set will always be in their own path
 group.
+.TP
+.B upcall
+Cmdline to execute to determine device priority. The name of the component device of interest is added as the last argument.
 .RE
 .TP
 .B features
-- 
2.8.0


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