Bug 689252 - Write caching for virtio-blk disk is ENABLED and cannot be disabled (within a 2008R2 guest)
Summary: Write caching for virtio-blk disk is ENABLED and cannot be disabled (within a...
Keywords:
Status: CLOSED NOTABUG
Alias: None
Product: Red Hat Enterprise Linux 5
Classification: Red Hat
Component: kvm
Version: 5.6
Hardware: x86_64
OS: Windows
unspecified
medium
Target Milestone: rc
: ---
Assignee: Vadim Rozenfeld
QA Contact: Virtualization Bugs
URL:
Whiteboard:
Depends On:
Blocks: Rhel5KvmTier2
TreeView+ depends on / blocked
 
Reported: 2011-03-20 15:27 UTC by Yaniv Kaul
Modified: 2013-07-04 02:26 UTC (History)
5 users (show)

Fixed In Version:
Doc Type: Bug Fix
Doc Text:
Clone Of:
Environment:
Last Closed: 2011-08-11 16:13:48 UTC


Attachments (Terms of Use)

Description Yaniv Kaul 2011-03-20 15:27:58 UTC
Description of problem:
On my 2008R2 VM, the write cache settings for a virtio-blk enabled disk is ENABLED. Moreover - it cannot be disabled ("This device does not allow its write-caching setting to be changed" - with an exclamation mark next to it).
I can, however, uncheck 'turn off Windows write cache buffer flushing on the device'. Not sure if it's good enough. 

I'm running the VM via RHEVM 2.2, with a QCOW2 image, cache=none, against a Netapp iSCSI with target with write-through cache settings (as determined via /sys/class/scsi_disk/N:N:N:N/cache_type)

Version-Release number of selected component (if applicable):
kernel-2.6.18-194.26.1.el5
kvm-83-164.el5_5.26 

How reproducible:
Always

Steps to Reproduce:
1. Via the device manager, locate the virtio-blk based disk, right click on its properties and try to change the caching parameter.
2.
3.
  
Actual results:


Expected results:


Additional info:

Comment 1 Yaniv Kaul 2011-03-20 15:34:25 UTC
Note: it seems it can be changed in a 2003 guest.

Comment 2 Vadim Rozenfeld 2011-08-11 16:13:48 UTC
(In reply to comment #0)
> Description of problem:
> On my 2008R2 VM, the write cache settings for a virtio-blk enabled disk is
> ENABLED. Moreover - it cannot be disabled ("This device does not allow its
> write-caching setting to be changed" - with an exclamation mark next to it).

In our situation it is not possible to change WCE on the working system. WCE can be turned down by specifying cache=writethrough.


> I can, however, uncheck 'turn off Windows write cache buffer flushing on the
> device'. Not sure if it's good enough. 

This way one can throttle down or even disable sending of flush requests from a file system driver.  

I'm going to close this bug as NOTABUG. Please feel free to reopen this bug in 6.3 if you think different.


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