Disk cache defaults

Disk cache defaults

[anthony smith]

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Having spent a while searching for answers on this I am making a last-ditch
effort by contacting the devel group.

Could someone let me know if it is possible to change the default disk
cache setting in qemu globally?

Currently it is set to writeback, in many if not most use cases "none" is
optimal.

I am aware I can change it per instance however I really need to find a
global method so that unless specified otherwise the default becomes
cache=none

It does seem a little strange that options appear to exist:

qemu-img -h | grep writeback
    options are: 'none', 'writeback' (default, except for convert),
'writethrough',

However I can't find any documentation that hints at a method of changing
the default, rather all documentation provides is a method per use/instance.

Appreciate any assistance even if the answer is just "Not possible" so I
can at least drop this search for answers.

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Re: Disk cache defaults

[Stefan Hajnoczi]

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On Wed, Jul 22, 2020 at 07:14:59AM +0100, anthony smith wrote:
> Appreciate any assistance even if the answer is just "Not possible" so I
> can at least drop this search for answers.

You can create an /etc/qemu/qemu.conf file but I don't remember if there
is syntax to set -drive cache=none.

CCing Kevin Wolf and Markus Armbruster for ideas.

The hacky "solution" is to write a wrapper script (e.g. a shell script)
that modifies QEMU command-line arguments to use cache=none if no
explicit cache= option was given:

  $ cat qemu-wrapper.sh
  #!/bin/bash
  # Add QEMU -drive cache=none

  declare -a opts

  i=0
  is_drive=0
  for opt in "$@"; do
    if [ "$is_drive" -eq 1 ]; then
      opt="$opt,cache=none" # TODO skip if cache= is already in opts?
    fi
    opts[$i]="$opt"
    if [ "$opt" = "-drive" ]; then
      is_drive=1
    else
      is_drive=0
    fi
    i=$((i + 1))
  done

  exec /usr/bin/qemu-system-x86_64 "${opts[@]}"

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Re: Disk cache defaults

[Markus Armbruster]

Stefan Hajnoczi <stefanha@gmail.com> writes:

> On Wed, Jul 22, 2020 at 07:14:59AM +0100, anthony smith wrote:
>> Appreciate any assistance even if the answer is just "Not possible" so I
>> can at least drop this search for answers.
>
> You can create an /etc/qemu/qemu.conf file but I don't remember if there
> is syntax to set -drive cache=none.
>
> CCing Kevin Wolf and Markus Armbruster for ideas.

I'm not aware of a way to modify defaults (except for device propeties
with -global, which doesn't help you, and is usually a bad idea anyway).

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