Merge ~cjwatson/launchpad-mojo-specs:git-hard-mount into launchpad-mojo-specs:master

[Colin Watson]

git: Use "hard" NFS client recovery

From nfs(5):

  soft / hard Determines the recovery behavior of the NFS client after
                an NFS request times out. If neither option is
                specified (or if the hard option is specified), NFS
                requests are retried indefinitely. If the soft option
                is specified, then the NFS client fails an NFS request
                after retrans retransmissions have been sent, causing
                the NFS client to return an error to the calling
                application.

                NB: A so-called "soft" timeout can cause silent data
                corruption in certain cases. As such, use the soft
                option only when client responsiveness is more important
                than data integrity. Using NFS over TCP or increasing
                the value of the retrans option may mitigate some of the
                risks of using the soft option.

I think I initially went for "soft" because retrying forever rather than failing seemed undesirable, and because the code writing to this NFS mount is very limited and known to have reasonable error handling. However, it seems that under load this can also cause the client to
consider the NFS mount dead after relatively short periods of unresponsiveness when it might otherwise be able to recover. Let's see if using the default of "hard" works more gracefully in practice.

[Ioana Lasc (ilasc)]

review: Approve

diff --git a/mojo-lp-git/services b/mojo-lp-git/services
index 64d2fb9..76362ac 100644
--- a/mojo-lp-git/services
+++ b/mojo-lp-git/services
@@ -150,7 +150,7 @@ turnip:
 {%- else %}
           - ppa:nfs-ganesha-charmers/ubuntu/backports-staging
 {%- endif %}
-        mount_options: "nfsvers=4.2,soft,lookupcache=positive"
+        mount_options: "nfsvers=4.2,lookupcache=positive"
         nagios_context: "{{ nagios_context }}"
     turnip-pack-backend:
       charm: turnip-pack-backend