[PATCH] doc: add some recommendations around slow HDDs

[Eric Wong <e@yhbt.net>]

grok-pull is still painful with serialization on an old USB 2.0
HDD, but at least it can finish with flock(1) and disabling
parallelization.  While parallel "git fetch" doesn't seem so
bad, slow seeks are exacerbated by parallel reads in Xapian.
That means some updates can take days instead of hours.  The
same updates take only seconds or minutes on an SSD.
---
 Documentation/public-inbox-index.pod   | 10 ++++++++++
 examples/grok-pull.post_update_hook.sh |  6 ++++++
 2 files changed, 16 insertions(+)

diff --git a/Documentation/public-inbox-index.pod b/Documentation/public-inbox-index.pod
index b1b24917b..ff2e54867 100644
--- a/Documentation/public-inbox-index.pod
+++ b/Documentation/public-inbox-index.pod
@@ -32,6 +32,16 @@ normal search functionality.
 
 =over
 
+=item --jobs=JOBS, -j
+
+Control the number of Xapian indexing jobs in a
+(L<public-inbox-v2-format(5)>) inbox.
+
+C<--jobs=0> is accepted as of public-inbox 1.6.0 (PENDING)
+to disable parallel indexing.
+
+Default: the number of existing Xapian shards
+
 =item --compact / -c
 
 Compacts the Xapian DBs after indexing.  This is recommended
diff --git a/examples/grok-pull.post_update_hook.sh b/examples/grok-pull.post_update_hook.sh
index 3ead39440..ec4ae93e8 100755
--- a/examples/grok-pull.post_update_hook.sh
+++ b/examples/grok-pull.post_update_hook.sh
@@ -1,4 +1,9 @@
 #!/bin/sh
+
+# use flock(1) from util-linux to avoid seek contention on slow HDDs
+# when using multiple `pull_threads' with grok-pull:
+# [ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock "$0" "$0" "$@" || :
+
 # post_update_hook for repos.conf as used by grok-pull, takes a full
 # git repo path as it's first and only arg.
 full_git_dir="$1"
@@ -119,6 +124,7 @@ then
 		: v2 inboxes may be init-ed with an empty msgmap
 		;;
 	*)
+		# if on HDD and limited RAM, add `-j0' w/ public-inbox 1.6.0+
 		$EATMYDATA public-inbox-index -v "$inbox_dir"
 		;;
 	esac