Introduce snapshot recycling.

When snapshotting large file systems whose contents do not change much between
snapshots, we end up removing large amounts of files just to recreate (hard
links to) most of them afterwards. This patch changes snapshot creation so that
outdated, redundant and orphaned snapshots are reused as the basis for new
snapshots. Only if no existing snapshot is suitable for recycling, a new one is
created.

diff --git a/dss.c b/dss.c
index bd7ead875eb3c982b08d4c26aef1cff32838665d..223f80e086e66b6ae3367e84292de91c62a9195a 100644 (file)
--- a/dss.c
+++ b/dss.c
@@ -1043,22 +1043,48 @@ static int use_rsync_locally(char *logname)
 static int rename_resume_snap(int64_t creation_time)
 {
        struct snapshot_list sl = {.num_snapshots = 0};
-       struct snapshot *s;
+       struct snapshot *s = NULL;
        char *new_name = incomplete_name(creation_time);
        int ret;
+       const char *why;
 
        ret = 0;
        if (conf.no_resume_given)
                goto out;
        dss_get_snapshot_list(&sl);
+       /*
+        * Snapshot recycling: We first look at the newest snapshot. If this
+        * snapshot happens to be incomplete, the last rsync process was
+        * aborted and we reuse this one. Otherwise we look at snapshots which
+        * could be removed (outdated and redundant snapshots) as candidates
+        * for recycling. If no outdated/redundant snapshot exists, we check if
+        * there is an orphaned snapshot, which likely is useless anyway.
+        *
+        * Only if no existing snapshot is suitable for recycling, we bite the
+        * bullet and create a new one.
+        */
        s = get_newest_snapshot(&sl);
-       if (!s)
+       if (!s) /* no snapshots at all */
                goto out;
-       if ((s->flags & SS_COMPLETE) != 0) /* complete */
+       /* re-use last snapshot if it is incomplete */
+       why = "aborted";
+       if ((s->flags & SS_COMPLETE) == 0)
                goto out;
-       DSS_INFO_LOG("resuming: reusing %s as destination dir\n", s->name);
-       ret = dss_rename(s->name, new_name);
+       why = "outdated";
+       s = find_outdated_snapshot(&sl);
+       if (s)
+               goto out;
+       why = "redundant";
+       s = find_redundant_snapshot(&sl);
+       if (s)
+               goto out;
+       why = "orphaned";
+       s = find_orphaned_snapshot(&sl);
 out:
+       if (s) {
+               DSS_INFO_LOG("reusing %s snapshot %s\n", why, s->name);
+               ret = dss_rename(s->name, new_name);
+       }
        if (ret >= 0)
                DSS_NOTICE_LOG("creating new snapshot %s\n", new_name);
        free(new_name);