| |
|
 |
|
 |
|
|
|
| |
|
|
xen-devel
[Xen-devel] Re: xl create should refuse to share block devices RW betwee
On 07/27/2010 08:33 AM, Ian Jackson wrote:
Jeremy Fitzhardinge writes ("Re: xl create should refuse to share block devices RW
between domains"):
Well, my specific use case is that I have pairs of domain configs, one
PV, one HVM, referring to the same set of resources. I want xl create
to catch when I try and create the PV version of a domain while the HVM
is still running.
Mmm. Of course an HVM domain needs to open the underlying device
twice, once for blktap and once for qemu.
Good point. For stubdoms that's pretty obvious if you consider the
stubdom to be part of the main domain. For dom0-resident qemu it you
need a more subtle check which can match a domain to its corresponding qemu.
A more comprehensive check would be nice, but just this would be
useful. But whatever it does check should be 100% reliable.
Well, I guess I meant:
1. Do we have to catch every possible conflict ? If so then
your e2fsprogs example is one we need to consider, and we
will have to add a new kernel feature which can prevent e2fsprogs
from opening the block device, or simulate "mounting" it or
something.
1b. If not, then which conflicts are we trying to detect ?
I think domU vs domU conflicts are the most important, since they can
come about from normal operation of the tools. dom0 vs domU requires
someone doing something non-tools-related.
2. If we catch a particular combination (eg, start two domains at
once using the same storage resources) does our check have to be
race-free ? That may make it more complicated - and if the answer
to my first question is "no" there will be some things which are
inherently racy (eg, spotting mounting a domain's disk
vs. starting a domain with a disk which is mounted).
Yes, it needs to be race-free. In general I think that specific
invocations of "xl" should be considered atomic operations. For
something like domain creation, it should be able to gather and reserve
all the resources required before actually starting the domain, and if
it can't fail and release everything with a useful message. A test
which can fail in certain racy circumstances is useless.
NB that when we fix the bug that you can start multiple domains with
the same name, you'll be able to avoid your PV/HVM accident by
specifying the same name in each config file.
BTW, that's another bug. xl create doesn't prevent two domains with the
same name from being created...
J
_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel
|
|
|
| |
|
Home