Re: [Xen-users] Is using w! safe to share data between domains?

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> Even if you eliminated the race condition, in general, it's probably not
> a good idea to open a file system (even if it's read-only) while someone
> else has it open read-write.  There's no guarantee the meta-data is
> going to be in a consistent state.

If you open a filesystem read only whilst there's another writer, the read 
only domain will detect filesystem corruption and get upset.  It's not 
inconceivable that the reader would actually crash but I haven't actually 
heard of this happening.

Cheers,
Mark

> Your best bet is probably to use networking to share data between
> partitions.
>
> Regards,
>
> Anthony Liguori
>
> John A. Sullivan III wrote:
> >I have a slightly unusual situation where I need to pass data from one
> >domain to another but, for security reasons, one of the domains will not
> >be on the network.  I would like to pass the data via a shared disk
> >partition.  I would like to know if what I have done is safe.
> >
> >I created a disk partition named /dev/VG1/pkipass.
> >Each domU disk definition contains: 'phy:VG1/pkipass,sda3,w!'
> >None of the domUs automatically mounts this device.
> >
> >When one domU needs to deposit data for another domU to pick up (this
> >exchange is always a manual effort to first deposit the shared data and
> >then retrieve the shared data), we do the following:
> >
> >The depositing domU mounts sda3 read-only.
> >It looks for a tag file -- this tag file is created when a domU has
> >mounted the partition as read-write
> >If the tag-file exists {
> >     it unmounts the partition
> >     it sleeps briefly
> >     it retries
> >}
> >else {
> >     it remounts the partition read-write
> >     it creates the tag file
> >     it deposits the data
> >     it deletes the tag fie
> >     it unmounts the partition
> >}
> >
> >A similar process is used to retrieve the data.
> >
> >Is this safe?
> >
> >Thanks - John
>
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