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Bug 1561960 - 'writeback' attribute is missing in 'BlockdevCacheOptions'
Summary: 'writeback' attribute is missing in 'BlockdevCacheOptions'
Alias: None
Product: Red Hat Enterprise Linux 7
Classification: Red Hat
Component: qemu-kvm-rhev
Version: 7.5
Hardware: Unspecified
OS: Unspecified
Target Milestone: rc
: ---
Assignee: Kevin Wolf
QA Contact: CongLi
Depends On:
Blocks: 760547 1558125
TreeView+ depends on / blocked
Reported: 2018-03-29 09:18 UTC by Peter Krempa
Modified: 2018-04-03 09:26 UTC (History)
11 users (show)

Fixed In Version:
Doc Type: If docs needed, set a value
Doc Text:
Clone Of:
Last Closed: 2018-04-03 09:26:46 UTC
Target Upstream Version:

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Description Peter Krempa 2018-03-29 09:18:03 UTC
Description of problem:
The legacy cache modes 'none', 'writeback' and 'unsafe' require setting 'cache.writeback' to true, but the QAPI schema does not provide this attribute when adding the image via blockdev/blockdev add.

Version-Release number of selected component (if applicable):
Current upstream git, so downstream versions are affected as well.

How reproducible:

Steps to Reproduce:
1. Look for 'writeback' attribute in  'BlockdevCacheOptions' or the equivalent traversal in the schema returned by 'query-qmp-schema' (blockdev-add->arg-type->cache)

Actual results:
only 'direct' and 'no-flush' attributes are present

Expected results:
'writeback' is present as well.

Additional info:
This is necessary for libvirt to use blockdev-add, since we support all the cache modes as described by this table (taken from qemu-options.hx)

              │ cache.writeback                 
 writeback    │ on                off            off                             
 none         │ on                on             off                             
 writethrough │ off               off            off                             
 directsync   │ off               on             off                             
 unsafe       │ on                off            on

Comment 1 Kevin Wolf 2018-04-03 09:26:46 UTC
"cache.writeback" is not a backend option, but a frontend one that can differ between multiple users of a backend (e.g. the guest device needs writethrough, but a block job doesn't). Consequently, it can't be set in -blockdev, but when configuring the users of the node.

The option that you're looking for is write-cache=on|off in -device.

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