[LU-15216] improve MDT QOS space balance

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.15.0
• Severity: 3
• Assignee: Lai Siyao
• 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. For example, (16 / (dir_depth + 10)), gives 16/10 = 160% of average at root (i.e. no preference for parent MDT unless it has 60% more than average free space), 16/16 = 100% of average free space at 6 levels deep, 16/22=72% of average at 12 levels deep, and 16/32 = 50% of average at 22 levels deep).

Also, rather than always returning "tgt = cur" in the "prefer parent" case, it could return -EAGAIN and leave it up to lmv_locate_tgt() to decide to use round-robin or same-MDT allocation for the directory. 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)", preferably taking qos_prio_free into account, but this needs more thought and should be implemented in a second patch.

Issue Links

• is duplicated by: LU-15213 improve MDT QOS space balance
• is duplicated by: LU-15214 improve MDT QOS space balance
• is duplicated by: LU-15215 improve MDT QOS space balance
• is related to: LU-13417 DNE3: mkdir() automatically create remote directory on MDS which has more space
• is related to: LU-9 Optimize weighted QOS Round-Robin allocator