Bug 788716

Summary:	Write-perf reports useless results due to use of /dev/urandom
Product:	[Community] GlusterFS	Reporter:	Jeff Darcy <jdarcy>
Component:	glusterd	Assignee:	Jeff Darcy <jdarcy>
Status:	CLOSED CURRENTRELEASE	QA Contact:	Anush Shetty <ashetty>
Severity:	low	Docs Contact:
Priority:	unspecified
Version:	mainline	CC:	gluster-bugs, rfortier
Target Milestone:	---
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Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:	glusterfs-3.4.0	Doc Type:	Bug Fix
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Clone Of:		Environment:
Last Closed:	2013-07-24 14:01:43 EDT	Type:	---
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oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---
Bug Depends On:
Bug Blocks:	817967

Description Jeff Darcy 2012-02-08 16:35:43 EST

Contrary to popular belief, urandom is not free.  In generic testing, a simple dd using /dev/urandom performs at about half the performance compared to /dev/zero for fully synchronous I/O, about 1/20 for buffered I/O (i.e. the difference on the input side is mostly masked when waiting for disks on the output side).  There's probably we can do about read-perf without platform-specific flushing of kernel buffers, but for write-perf we can at least get more accurate and useful numbers by using /dev/zero instead.

Comment 1 Anand Avati 2012-02-09 01:51:12 EST

CHANGE: http://review.gluster.com/2728 (Use /dev/zero instead of /dev/urandom to get realistic numbers.) merged in master by Anand Avati (avati@gluster.com)