Re: [kvm-devel] [PATCH] qemu: use statically allocate 512 byte buffer in the stack for sector in bdrv_commit

[Javier G. <ja...@gu...>]

On 1/7/08, Laurent Vivier <Lau...@bu...> wrote:
> "cache=off" means files is opened with "O_DIRECT" and thus there is no
> cache in the kernel memory on the host side.
> IMO, "cache=off" and "snapshot=on" are incompatible because a snapshot
> can be seen like a cache.
>
> > so far, the only way is to setup a network block device (iSCSI, AoE,
> > nbd). i'd like to simply specify the same backing file for two
> > instances' hdb parameter.
>
> I'm sorry but I don't understand this part.

to test a cluster filesystem, i need two (virtual) machines with some
shared storage.  i tried long ago something like this (with (k)qemu):

- create a disk image, call it hda-1.img
- boot and install linux on it, shutdown
- copy to hda-2.img
- boot it (with new MAC) and change IP, hostname, little things, shutdown
- boot both with the same bridge, check that network works between them
- create a new disk image, call id hdb-shr.img
- boot both VMs, sharing hdb-shr.img:

qemu -hda=hda-1.img -hdb-shr.img
qemu -hda=hda-2.img -hdb-shr.img

- try to setup a cluster filesystem on hdb

it almost worked... but some writes didn't propagate to the other
until some extra writes to hdb; so i guessed that each qemu instance
had some caching on file I/O

hopefully, it would now work with "-cache=off", don't you think?

> > and snapshots help a lot to go back after blowing up the on-disk structures
>
> But I think if you use a snapshot there is no reason to use "cache=off"

in the above case, if both KVM instances share the snapshot without
cacheing it, the cluster should still work, and have some rollback
capability at the same time

or is it too much wishful thinking?

-- 
Javier