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Feature #4334

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Improve ZFS N-way mirror read performance

Added by Steven Hartland almost 8 years ago.

Status:
New
Priority:
Normal
Category:
zfs - Zettabyte File System
Start date:
2013-11-17
Due date:
% Done:

0%

Estimated time:
Difficulty:
Medium
Tags:
needs-triage
Gerrit CR:

Description

Improve ZFS N-way mirror read performance by using load and locality information.

The existing algorithm selects a preferred leaf vdev based on offset of the zio request modulo the number of members in the mirror. It assumes the devices are of equal performance and that spreading the requests randomly over both drives will be sufficient to saturate them. In practice this results in the leaf vdevs being under utilized.

The new algorithm takes into the following additional factors:
  • Load of the vdevs (number outstanding I/O requests)
  • The locality of last queued I/O vs the new I/O request.

Within the locality calculation additional knowledge about the underlying vdev is considered such as; is the device backing the vdev a rotating media device.

This results in performance increases across the board as well as significant increases for predominantly streaming loads and for configurations which don't
have evenly performing devices.

References:
http://svnweb.freebsd.org/base?view=revision&revision=256956
http://open-zfs.org/wiki/Features#Improve_N-way_mirror_read_performance

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