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Is there a queue‑depth limit for iSCSI LUN?

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Applies to

ONTAP 9

Answer

  • No,ONTAP does not have a fixed queue depth limit per LUN for iSCSI, unlike Fibre Channel (FC) which has a per-LUN limit of 2048 commands.
  • iSCSI queue depth is controlled per session, not per LUN.
  • Default maximum commands per iSCSI session is 128.
  • This limit can be modified using ONTAP CLI:

vserver iscsi modify -max-ios-per-session <integer>

  • When multiple servers access the same LUN, each server’s iSCSI session can process up to the session limit (default 128 commands).
  • Aggregate queue depth equals the sum of concurrent IOs across all sessions.
  • Increasing the number of sessions (for example, by using multiple LIFs or multipath configurations) increases overall IO processing capacity.
  • Queue depth is not determined by LUN, virtual storage, or physical storage chassis—it is determined by iSCSI session.
  • Linux queue depth settings control the maximum IOs sent from the OS to storage, but do not affect the ONTAP iSCSI target’s session limit.
  • Setting a high queue depth on the OS does not guarantee higher performance.
  • Thorough validation and tuning are recommended, considering server count, session count, queue depth, network bandwidth, and ONTAP session limits.

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