Bug 677808

Summary: significant I/O performance decrease over previous release
Product: Red Hat Enterprise MRG Reporter: Guy Streeter <streeter>
Component: realtime-kernelAssignee: Steven Rostedt <srostedt>
Status: CLOSED NOTABUG QA Contact: David Sommerseth <davids>
Severity: high Docs Contact:
Priority: medium    
Version: 1.3CC: bhu, lgoncalv, nobody, ovasik, wgomerin, williams
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Target Release: ---   
Hardware: x86_64   
OS: Linux   
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Fixed In Version: Doc Type: Bug Fix
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Clone Of: Environment:
Last Closed: 2012-01-12 15:04:15 UTC Type: ---
Regression: --- Mount Type: ---
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:

Description Guy Streeter 2011-02-15 21:45:48 UTC 
- 2.6.24.7-108.el5rt

# echo 3 > /proc/sys/vm/drop_caches
# sync; time `dd if=/dev/zero of=ddfile2 oflag=sync bs=4k count=20000; sync`
20000+0 records in
20000+0 records out
81920000 bytes (82 MB) copied, 16.956 seconds, 4.8 MB/s

real    0m17.273s
user    0m0.022s
sys     0m0.882s


- 2.6.33.7-rt29.47.el5rt

# echo 3 > /proc/sys/vm/drop_caches
# sync; time `dd if=/dev/zero of=ddfile2 oflag=sync bs=4k count=20000; sync`
20000+0 records in
20000+0 records out
81920000 bytes (82 MB) copied, 315.311 seconds, 260 kB/s

real    5m15.559s
user    0m0.007s
sys     0m1.037s


This is using the cfq I/O scheduler.
There is not a similar drop in performance when the deadline scheduler is used.
Comment 5 Steven Rostedt 2011-04-25 16:29:15 UTC 
Talking with the fs folks, this is not a bug. It was a bug fix. Doing a sync write with dd is _suppose_ to be slow. The old kernel was not doing the proper sync with the hard drive, which is why it performed so well. But that good performance was a symptom of a bug, not a feature.

Fixing the bug caused performance to take a hit, and the slow sync dd is correct.

Perhaps we should look at why the deadline scheduler is performing so well. Perhaps it is not working either.