Re: [Qemu-devel] Disable image locking for snapshot drive?

[Andrew Baumann]

> From: Eric Blake [mailto:address@hidden
> Sent: Tuesday, 18 July 2017 8:07
> On 07/17/2017 07:33 PM, John Snow wrote:
> > On 07/17/2017 07:30 PM, Andrew Baumann via Qemu-devel wrote:
> >> I'm running a recent Linux build of qemu on Windows Subsystem for Linux
> (WSL) which doesn't appear to implement file locking:
> >>
> >> $ qemu-system-aarch64 ... -drive file=test.vhdx,if=none,id=hd0 -device
> virtio-blk-pci,drive=hd0
> >> qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to
> unlock byte 100
> 
> Does WSL implement fcntl(F_SETLK) but not fcntl(F_OFD_SETLK)?

Yes, this appears to be the case (there's also one report that it's broken):
https://github.com/Microsoft/BashOnWindows/issues/1927

> We
> already have code in place for compiling when F_OFD_SETLK is not
> supported (which makes lock=auto do nothing, and issues a warning that
> F_SETLK locks may be ineffective when locks are explicitly requested),
> do we need to just expand that code into a runtime test of whether
> F_OFD_SETLK appears to be unsupported?

That would be a nice fix, and it would avoid the need for yet another flag. On 
the other hand, WSL is aiming for ABI compatibility, so they should get around 
to implementing F_OFD_SETLK et al eventually.

Even if this were fixed in QEMU or implemented in WSL, shouldn't there to be a 
way to turn snapshot file locking off on a per-drive basis?

Andrew