High Latency on disks with no HDD errors

• ONTAP 9
• FAS systems
• Cloud Volumes ONTAP systems with hard disk drives (HDD)

• Increased latency to end NAS/SAN users.
• qos statistics volume latency show  points to seconds of latency in disk, but statistics on the disk histogram shows 8ms or less

cluster1::> qos statistics volume latency show -vserver vs0 -volume vs0_vol0
Workload            ID    Latency    Network    Cluster       Data       Disk    Qos Max    Qos Min      NVRAM      Cloud  FlexCache    SM Sync         VA
--------------- ------ ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ---------- ----------
-total-              -   455.00us   158.00us        0ms   297.00us        0ms        0ms        0ms        0ms        0ms        0ms        0ms        0ms
vs0_vol0-wid1..  15658   109.00ms   155.00us        0ms   273.00us    108.2ms        0ms        0ms        0ms        0ms        0ms        0ms        0ms

• High io_queued is seen in the disk object for only one disk or a few disks, generally only one disk in most cases
• There are no hardware errors for the single disk in the event log nor any other hardware issues on the shelf or stack that could explain i/o queuing on a single drive
  • Failing a drive may move the i/o queuing to another disk

Example: The statistics command disk object shows high queuing on a disk and statit shows 100% busy disk i/o on disk 0c.23.13

Cluster::> set -privilege diag

Warning: These diagnostic commands are for use by NetApp personnel only.
Do you want to continue? {y|n}: y

Cluster::*> statistics start -object disk -counter io_queued
Statistics collection is being started for sample-id: sample_148


Cluster::*> statistics show -filter "io_queued>100"


Object: disk
Instance: 0d.23.13
Start-time: 12/5/2022 16:48:26
End-time: 12/5/2022 16:51:58
Elapsed-time: 212s
Scope: node1
Number of Constituents: 1 (complete_aggregation)
    Counter                                                     Value
    -------------------------------- --------------------------------
    io_queued                                                     818

1 entry was displayed.

Cluster::*> node run -node node1 -command statit -b

Note: Post 30 seconds observed queuing on 0d.23.13, the hot disk is seen using statit

Cluster::*> node run -node node1 -command statit -e
...
disk             ut%  xfers  ureads--chain-usecs writes--chain-usecs cpreads-chain-usecs greads--chain-usecs gwrites-chain-usecs
/aggr1_node1/plex0/rg2:
0d.23.18          25  69.19    0.01   2.00  2033  31.50  60.67   175  37.69  51.19   169   0.00   ....     .   0.00   ....     .
0a.21.16          24  69.77    0.01   2.00  6964  32.11  59.56   194  37.66  51.18    90   0.00   ....     .   0.00   ....     .
0d.22.17          57 231.26  133.56   5.45  2075  26.91  29.41   573  70.79   9.87   641   0.00   ....     .   0.00   ....     .
0d.23.22          57 230.68  132.96   5.46  1845  26.83  29.56   646  70.90   9.74   604   0.00   ....     .   0.00   ....     .
0d.23.13          95 295.63  198.16   4.10  5472  26.83  29.76  1371  70.63   9.91  1975   0.01   ....     .   0.00   ....     .
0d.22.18          57 231.26  133.55   5.38  2080  26.84  29.60   561  70.86   9.73   634   0.00   ....     .   0.00   ....     .
0a.20.18          57 231.69  133.54   5.42  1846  27.00  29.46   647  71.15   9.78   608   0.00   ....     .   0.00   ....     .
0a.20.16          57 233.00  134.49   5.48  1879  27.08  30.09   634  71.43   9.83   594   0.00   ....     .   0.00   ....     .
0d.22.19          57 231.98  134.18   5.41  2099  26.87  29.67   567  70.93   9.87   646   0.00   ....     .   0.00   ....     .

Cluster::*> set admin