Re: [Qemu-devel] [Xen-devel] [PATCH RFC 1/3] xen_disk: handle disk files on ramfs/tmpfs

[Roger Pau Monné]

On 04/01/13 15:54, Stefano Stabellini wrote:
> On Thu, 3 Jan 2013, Ian Campbell wrote:
>> On Mon, 2012-12-31 at 12:16 +0000, Roger Pau Monne wrote:
>>> Files that reside on ramfs or tmpfs cannot be opened with O_DIRECT,
>>> if first call to bdrv_open fails with errno = EINVAL, try a second
>>> call without BDRV_O_NOCACHE.
>>
>> Doesn't that risk spuriously turning of NOCACHE on other sorts of
>> devices as well which (potentially) opens up a data loss issue?
> 
> I agree, we shouldn't have this kind of critical configuration changes
> behind the user's back.
> 
> I would rather let the user set the cache attributes, QEMU has already a
> command line option for it, but we can't use it directly because
> xen_disk gets the configuration solely from xenstore at the moment.
> 
> I guess we could add a key pair cache=foobar to the xl disk
> configuration spec, that gets translated somehow to a key on xenstore.
> Xen_disk would read the key and sets qflags accordingly.
> We could use the same cache parameters supported by QEMU, see
> bdrv_parse_cache_flags.
> 
> As an alternative, we could reuse the already defined "access" key, like
> this:
> 
> access=rw|nocache
> 
> or
> 
> access=rw|unsafe

I needed this patch to be able to perform the benchmarks for the
persistent grants implementation, but I realize this is not the best way
to solve this problem.

It might be worth to think of a good way to pass more information to the
qdisk backend (not only limited to whether O_DIRECT should be used or
not), so we can take advantage in the future of all the possible file
backends that Qemu supports, like GlusterFS or SheepDog.