Re: [Qemu-devel][PATCH] Qemu image over raw devices

[Kevin Wolf]

Shahar Frank schrieb:
> The following patch enables QEMU to create and use images with any
> format on top of a raw device. Note that -f <format> is not enough for
> bcking files support.

When would I need to explicitly specify the type of a backing file?

> The patch includes the following:
> 
> 1. The check for block devices is weaken so you can override it by
> specifying a protocol
> 2. If a protocol exists but not found in the protocols list, the logic
> falls back to image type probing. This means use can write
> "probe:filename" or just ":filename"

IIUC, on qemu side this is just another syntax for -drive format=xyz?
Wouldn't it be better to add a parameter to qemu-img then instead of
inventing new ways of specifying the format?

> Note that if regular file/device path names are used, the previous
> behavior is kept.
> 
> lvcreate -L 5G -n base store
> dd bs=32k if=win.qcow2 of=/dev/store/base
> ./qemu-img info :/dev/store/base
> lvcreate -L 2G -n l2 store
> ./qemu-img create -b :/dev/store/base -f qcow2 /dev/store/l2
> ./x86_64-softmmu/qemu-system-x86_64 -hda :/dev/store/l2 -L pc-bios/
> lvcreate -L 2G -n l3 store
> ./qemu-img create -b :/dev/store/l2 -f qcow2 /dev/store/l3
> ./x86_64-softmmu/qemu-system-x86_64 -hda :/dev/store/l3 -L pc-bios/

Does it even make sense to store qcow2 images on raw block devices?
qcow2 are usually growing whereas devices tend to not change their size.

Kevin