[libvirt PATCH 1/4] util: add a helper method for controlling the COW flag on btrfs
Jim Fehlig
jfehlig at suse.com
Thu Jul 23 20:22:44 UTC 2020
On 7/23/20 8:59 AM, Daniel P. Berrangé wrote:
> On Thu, Jul 23, 2020 at 03:08:41PM +0200, Peter Krempa wrote:
>> On Thu, Jul 23, 2020 at 14:00:40 +0100, Daniel Berrange wrote:
>>> On Thu, Jul 23, 2020 at 02:57:32PM +0200, Peter Krempa wrote:
>>>> On Mon, Jul 20, 2020 at 18:33:19 +0100, Daniel Berrange wrote:
>>>>> btrfs defaults to performing copy-on-write for files. This is often
>>>>> undesirable for VM images, so we need to be able to control whether this
>>>>> behaviour is used.
>>>>>
>>>>> The virFileSetCOW() will allow for this. We use a tristate, since out of
>>>>> the box, we want the default behaviour attempt to disable cow, but only
>>>>> on btrfs, silently do nothing on non-btrfs. If someone explicitly asks
>>>>> to disable/enable cow, then we want to raise a hard error on non-btrfs.
>>>>>
>>>>> Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
>>>>> ---
>>>>> src/libvirt_private.syms | 1 +
>>>>> src/util/virfile.c | 76 ++++++++++++++++++++++++++++++++++++++++
>>>>> src/util/virfile.h | 3 ++
>>>>> 3 files changed, 80 insertions(+)
>>
>> [...]
>>
>>>>> + if (buf.f_type != BTRFS_SUPER_MAGIC) {
>>>>> + if (state == VIR_TRISTATE_BOOL_ABSENT) {
>>>>
>>>> Can't we handle the _ABSENT case before even attempting to open the
>>>> file?
>>>
>>> This would require us to use statfs() instad of fstatfs() in order to
>>> check the super magic. I'm not seeing that improves things.
>>
>> I definitely agree. But adding a function which does non-obvious things
>> without any comment pointing to the non-obvious behaviour isn't good
>> practice either.
I was reviewing this series Monday before getting distracted with other things
and had to read this function a few times to get my head around it.
> I'll add this
>
> /**
> * virFileSetCow:
> * @path: file or directory to control the COW flag on
> * @state: the desired state of the COW flag
> *
> * When @state is VIR_TRISTATE_BOOL_ABSENT, some helpful
> * default logic will be used. Specifically if the filesystem
> * containing @path is 'btrfs', then it will attempt to
> * disable the COW flag, but errors will be ignored. For
> * any other filesystem no change will be made.
> *
> * When @state is VIR_TRISTATE_BOOL_YES or VIR_TRISTATE_BOOL_NO,
> * it will attempt to set the COW flag state to that explicit
> * value, and always return an error if it fails. Note this
> * means it will always return error if the filesystem is not
> * 'btrfs'.
> */
But that definitely helps! Thanks for adding it.
Regards,
Jim
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