Kevin Wolf <kwolf@xxxxxxxxxx> wrote:
> Am 10.01.2011 14:32, schrieb Juan Quintela:
>> Juan Quintela <quintela@xxxxxxxxxx> wrote:
>>> Juan Quintela <quintela@xxxxxxxxxx> wrote:
>>>
>>> Now sent it to the right kvm list. Sorry for the second sent.
>>>
>>>> Please send any agenda items you are interested in covering.
>>>>
>>>> - KVM Forum 2011 (Jes).
>>>>
>>>> thanks, Juan.
>>
>> - migration and block devices: a mess.
>> * patches I sent last week: only work for root (for some definition of
>> work)
>> * qemu is used as non-root user.
>> * forcing to have cache=none solves the issue
>
> I need to have a look at the specific problem, but it's hard to imagine
> that cache=none fixes anything reliably.
It uses O_DIRECT, that means that we don't have buffering problems.
I state the problem again:
machine A read 1st block of device.
<and stays without doing anything else>
machine B reads writes lots of places including 1st block
now guest from machine A migrates to machine B
machine A re-reads the 1st block, and lo and behold, it reads the old
contents, not the new ones.
Solutions:
- invalidate all buffers for that block device on machine A after
migration.
* with NFS, just close + reopen the file (and pray that nobody else
has it also opened)
* with block devices: use BLKFLBLK ioctl, and pray that nobody else is
using the device, that device is not a ramdisk, and some more
things. To add injury to insult, you need to be root to be able
to issue that ioctl (technically have CAP_SYS_ADMIN).
O_DIRECT fixes this problem altogether, because there is no buffering,
and if there are not buffers, they can't be invalid O:-)
Notice the "pray" part in the other solutions, we are basically trying
to do a "poor man" DLM, and that is not trivial to do. (althougth our
problem is not the general one, the principles are the same).
Later, Juan.
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