Disk I/O to Sun ZFS 7120 via iSCSI is Slow (Doc ID 1395545.1)

Last updated on MARCH 08, 2017

Applies to:

Linux OS - Version: Oracle Linux 5.6 with Unbreakable Enterprise Kernel [2.6.32] and later   [Release: OL5U6 and later ]
Generic Linux

Symptoms

Experiencing slow disk I/O to Sun ZFS 7120 (with SATA disks installed) via iSCSI.

After various I/O performance tests (including vdbench benchmark tool) the average service time (svctm) is good (5-7 ms) but the await (average wait time for an I/O request) is very high (900 ms). All statistics are collected by iostat.

According the Linux man pages, the await is the average time (in milliseconds) for I/O requests issued to the device to be served. This includes the time spent by the requests in queue and the time spent servicing them.

The servers are connected via a 10Gb connection to the ZFS appliance running iSCSI.

Cause

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