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NDMP backups to fibre channel tape exhibit sporadic low throughput

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ndmp
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dp
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Applies to

  • Two or more clustered Data ONTAP nodes, with each node connected to fibre channel tape.
  • NDMP is running in SVM-scope mode.
  • Data Management Application (DMA) configuration contains all nodes' tape device paths.
  • DMA is not configured to prefer local device paths, but will instead use any available tape device to complete a backup or restore.

Issue

  • NDMP backup or restore takes longer than usual.
  • Compared to previous jobs, the "slow" job exhibits lower throughput.
  • The low throughput is sporadic -- The same job will run at acceptable throughput during one attempt, and then exhibit low throughput on another attempt.
  • All backups, both fast and slow jobs, are being written to SAN tape devices connected to the NetApp cluster nodes.

A "dump to null" test of the volume associated with the slow job does not indicate a read throughput bottleneck.

Example:

The volume "data1" on SVM "nasdata" usually backs up at a rate of ~200 MB/sec, but will occasionally back up at ~75 MB/sec.

If the backup log is examined, the jobs that run at higher throughput have b=60 in the dump options:

dmp Sat May 11 17:10:50 IDT 2019 /nasdata/data1(3) Start (Level 0, NDMP:96285)
dmp Sat May 11 17:10:50 IDT 2019 /nasdata/data1(3) Options (b=60, u)

The jobs that run at lower throughput have b=0 in the dump options, as well as TCP receive and send buffer sizes:

dmp Sat May 11 01:49:54 IDT 2019 /nasdata/data1(7) Start (Level 0, NDMP:36047)
dmp Sat May 11 01:49:54 IDT 2019 /nasdata/data1(7) Options (b=0, u, TCP recv buffer size = 33580, TCP send buffer size = 33580) 

 

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