[LU-15213] improve MDT QOS space balance

• Type: Bug
• Resolution: Duplicate
• Priority: Minor
• Severity: 3
• Assignee: Andreas Dilger
• Reporter: Andreas Dilger

Description

The MDT space balance heuristics in lmv_locate_tgt_qos() has an unstable behavior.

If the MDTs are balanced, then it returns -EAGAIN and the normal round-robin code is in effect and working properly. However, once the MDTs are imbalanced more than lq_threshold_rr, then the QOS code becomes active. It has an extra check that tries to keep subdirectory creation local to the same MDT when it is deep in the directory tree, to avoid creating too many remote directories:

static struct lu_tgt_desc *lmv_locate_tgt_qos(struct lmv_obd *lmv, __u32 *mdt,
                                              unsigned short dir_depth)
{

        /* if current MDT has above-average space, within range of the QOS
         * threshold, stay on the same MDT to avoid creating needless remote
         * MDT directories. It's more likely for low level directories.
         */
        rand = total_avail * (256 - lmv->lmv_qos.lq_threshold_rr) /
               (total_usable * 256 * (1 + dir_depth / 4));
        if (cur && cur->ltd_qos.ltq_avail >= rand) {
                tgt = cur;
                GOTO(unlock, tgt);
        }

There are three factors that make up "rand":

• total_avail / total_usable is the average (mean) free space across all MDTs
• (256 - lmv->lmv_qos.lq_threshold_rr) / 256 reduces the average free space slightly (e.g. 95% by default) so the MDT is still considered "balanced" if within qos_threshold_rr of the average MDT free space
• (1 + dir_depth / 4) is to keep deeper subdirectories on the same MDT as the parent

I think the dir_depth factor makes "rand" too small (reduced by 1/2 when dir_depth >= 4), which results in ltq_avail > average / 2 and the client always selects the parent MDT until it has half as much free space as the other MDTs. This factor is even stronger for subdirectories created below that level. The "(1 + dir_depth / 4)" factor should only reduce rand slightly for each increase in subdirectory depth (e.g. (16 / (dir_depth/2 + 13) (16/13 = 123% of average at root (i.e. no preference for parent unless it has 23% more than average free space, 16/16 = 100% of average free space at 6 levels deep, and 16/32 = 50% of average at 38 levels deep).

Also, rather than always returning "tgt = cur" in the "prefer parent" case, it should return -EAGAIN and leave it up to lmv_locate_tgt() to decide to use round-robin or same-MDT allocation should be used. However, that would need to teach lmv_locate_tgt_rr() to skip MDTs with more than average free space if ltd_qos_is_usable() is true, which is a more complex change. Something along the lines of "skip MDT N times if it has N times less free space than (most_free - average)", but this needs more thought.

I think the "(1 + dir_depth / 4)" factor should only reduces rand slightly for each increase in depth (e.g. (16 / (dir_depth + 16) (16/20 = 75% of average at 4 levels deep, and 16/32 = 50% of average at 16 levels deep).

I also wonder if, rather than always returning tgt = cur in this case, it should return -EAGAIN and leave it up to the caller to decide if round-robin or same-MDT allocation should be used?

Issue Links

• duplicates: LU-15216 improve MDT QOS space balance