Bug 788716

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