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Potential performance impact due to low aggregate free space

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Category:
ontap-9
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perf
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Applies to

  • Clustered Data ONTAP
  • ONTAP 9

Issue

  • Storage controllers running Data ONTAP with used blocks exceeding 90% of any available aggregate's available space might experience performance degradation due to increased overhead for WRITE operations.
  • Under low free space conditions, disk operation will be higher as a result of increased CP-reads (calculate parity read) relative to WRITE operations.
    • Under high workloads, this can manifest itself as degradation in performance, such as increased protocol latencies.
    • A CP-read consists of blocks that are read to satisfy a write.
  • Aggregate disk statistics detailing cp-read and write operations can be seen from statit on the node shell of a CDOT controller or 7-mode controller. 
::> set advanced
::*> node run -node node_1 statit -b
/* wait 60s */
::*> node run -node node_1 statit -e
...
disk             ut%  xfers  ureads--chain-usecs writes--chain-usecs cpreads-chain-usecs greads--chain-usecs gwrites-chain-usecs
/aggr11/plex0/rg0:
1d.11.23          72 165.41    0.19   1.00 11500  80.93  38.04   421   84.29  5.72  2299   0.00   ....     .   0.00   ....     .
1a.12.21          71 166.58    0.19   1.00 12000  82.10  37.54   444   84.29  5.69  2417   0.00   ....     .   0.00   ....     .
1d.10.22          88 225.73   10.66   1.23 44511 107.01  20.88   896  108.06  5.57  2424   0.00   ....     .   0.00   ....     . 
1a.12.22          90 240.50   11.05   1.88 31453 114.08  19.32   948  115.37  5.35  2497   0.00   ....     .   0.00   ....     .
1d.10.23          86 225.61   10.37   2.19 21496 106.33  21.37   892  108.91  5.51  2472   0.00   ....     .   0.00   ....     . 
1a.12.23          86 231.36   11.05   1.66 34169 109.43  20.53   974  110.88  5.02  2586   0.00   ....     .   0.00   ....     . 
1d.10.20          95 234.87   13.86   1.69 35062 109.43  20.20  1180  111.58  6.13  2663   0.00   ....     .   0.00   ....     .
1a.12.17          93 248.97   10.76   1.61 32754 118.35  18.98  1071  119.86  5.24  2778   0.00   ....     .   0.00   ....     .
1d.11.19          90 240.82   13.67   1.60 39547 113.31  19.04   980  113.85  5.95  2320   0.00   ....     .   0.00   ....     . 
1a.12.18          93 229.39    9.60   1.36 50467 109.43  20.38  1016  110.36  5.80  2644   0.00   ....     .   0.00   ....     . 
  • A small number of CP-read operations relative to write operations are normal.
    • However, in this example, the CP-read operations almost double the operational load on the aggregate.

 

 

 

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