Bug 545194 - Discrepancy between man page and source code for qcow2 with regards to default value used when no explicit caching is specified
Summary: Discrepancy between man page and source code for qcow2 with regards to defaul...
Keywords:
Status: CLOSED ERRATA
Alias: None
Product: Red Hat Enterprise Linux 5
Classification: Red Hat
Component: kvm
Version: 5.4
Hardware: x86_64
OS: Linux
medium
medium
Target Milestone: rc
: 5.4
Assignee: Kevin Wolf
QA Contact: Virtualization Bugs
URL:
Whiteboard:
Depends On:
Blocks:
TreeView+ depends on / blocked
 
Reported: 2009-12-07 20:25 UTC by Nandini Chandra
Modified: 2018-10-27 15:10 UTC (History)
8 users (show)

Fixed In Version: kvm-83-143.el5
Doc Type: Bug Fix
Doc Text:
Clone Of:
Environment:
Last Closed: 2010-03-30 07:55:13 UTC
Target Upstream Version:
Embargoed:


Attachments (Terms of Use)


Links
System ID Private Priority Status Summary Last Updated
Red Hat Product Errata RHSA-2010:0271 0 normal SHIPPED_LIVE Important: kvm security, bug fix and enhancement update 2010-03-29 13:19:48 UTC

Description Nandini Chandra 2009-12-07 20:25:18 UTC
Description of problem:
While explaining kvm's specification on block I/O data integrity to customer,we found some discrepancy between man page and how the code works for qcow2 disk images.

According to man page (man qemu-kvm),if no explicit caching is specified for a qcow2 disk image, cache=writeback will be used.

From the source code,
./qemu/block-qcow2.c

   277     /* We default to NO CACHE if nothing is defined */
   278     if ((flags & BDRV_O_CACHE_DEF)) {
   279         flags |= BDRV_O_NOCACHE;
   280         flags &= ~BDRV_O_CACHE_DEF;
   281     }

From the source code,it seems like a default value of NO CACHE is used for qcow2 disk images.


Version-Release number of selected component (if applicable):
RHEL5
kvm-83-105.el5_4.9


How reproducible:
NA

Steps to Reproduce:
1.
2.
3.
  
Actual results:
There is a discrepancy between man pages and how the code works for qcow2 images
with regards to default value used when no explicit caching is specified.


Expected results:
There should be no discrepancy between man pages and how the code works for qcow2 images with regards to default value used when no explicit caching is specified.

Additional info:

Comment 2 Dor Laor 2009-12-13 08:51:30 UTC
You're right, we fixed the code but not the documentation

Comment 8 juzhang 2010-01-18 08:54:44 UTC
Tested on kvm-83-147.el5.  pass.


steps:

1.man qemu-kvm:
By default, if no explicit caching is specified for a qcow2 disk image, cache=none will be used.  For all other disk types,cache=writethrough is the default.

Comment 9 juzhang 2010-01-20 05:36:06 UTC
Reproduced on kvm-83-135.el5.fail.
Tested on kvm-83-147.el5. pass.

1), Reproduced on on kvm-83-135.el5. fail.
steps:
1.man qemu-kvm:
By default, if no explicit caching is specified for a qcow2 disk image, cache=writeback will be used. For all other disk types, cache=writethrough is the default.

2),Tested on kvm-83-147.el5. pass.
steps:
1.man qemu-kvm:
By default, if no explicit caching is specified for a qcow2 disk image,cache=none will be used. For all other disk types,cache=writethrough is the default

Comment 13 errata-xmlrpc 2010-03-30 07:55:13 UTC
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.

http://rhn.redhat.com/errata/RHSA-2010-0271.html


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