Re: [Xen-API] Move blktap2.reset_leaf call to new SR command "epoch_begin"

[Mike McClurg <mike.mcclurg@xxxxxxxxxx>]

On 27/11/12 15:44, Rushikesh Jadhav wrote:
> Hi Mike and Andrei,
> 
> Thanks for making the discussion open about this famous issue.
> 
> Many times we face this issue. If a host is hang then the VMs from that
> host are unable to boot on any other host until the database VDI RW flag
> is reset. Usual error is "The VDI is already attached in RW mode".  The
> solution procedure is to reboot the hang host or resetvdis manually.
> 
> If we can do on-boot=reset then this will help to quickly boot failed
> VMs on new host without errors. As mentioned this will apply to start
> and restart operation but will "xe vm-reset-powerstate" be affected ?
> 
> Regards,
> Rushikesh

Hi Rushikesh,

I don't think that on-boot=reset does what you think it does. Setting
on-boot=reset on a VDI will force it to "reset" on attach, meaning that
it will delete the leaf node of the VHD tree that makes up the VDI.

Basically, this is a feature which lets you create a VM which will roll
its disks back to a specified state every time you reboot. All of the
writes that you made to the disk before the reboot (and after setting
on-boot=reset) will be gone.

The problem is that there is a bug where this action happens on VM
migrate! This is what we're trying to fix.

Mike

> On Tue, Nov 27, 2012 at 5:45 PM, Andrei Lifchits
> <Andrei.Lifchits@xxxxxxxxxx <mailto:Andrei.Lifchits@xxxxxxxxxx>> wrote:
> 
>     Hi Mike,
> 
>     Sounds good to me. Should be pretty easy to change.
> 
>     Andrei
> 
>     ________________________________________
>     From: Mike McClurg
>     Sent: Monday, November 26, 2012 4:13 PM
>     To: xen-api@xxxxxxxxxxxxx <mailto:xen-api@xxxxxxxxxxxxx>; Andrei
>     Lifchits
>     Subject: Move blktap2.reset_leaf call to new SR command "epoch_begin"
> 
>     Hi Andrei,
> 
>     (Note that I'm CC'ing the xen-api list to encourage more open
>     development discussions.)
> 
>     I'm working on the VDI reset bug we talked about last week, where
>     setting on-boot=reset on a VDI, and then migrating the VM, triggers the
>     VDI reset after the migration is complete, because VDI reset is called
>     on VDI attach.
> 
>     The fix for this should be to only call blktap2.reset_leaf when the VDI
>     attach operation is performed during a VM start or reboot. There is a
>     set of new SMAPIv2 calls in Xapi: begin_epoch, end_epoch, which are
>     called by Xapi when a VM is started and stopped (respectively), and also
>     during a VM reboot. These functions are not called during a VM migrate.
> 
>     My proposal is that we add two new SR commands, begin_epoch and
>     end_epoch, and that we move the VDI reset code into begin_epoch. The two
>     corresponding Xapi functions would then call these SR commands.
> 
>     Does this sound like a good solution to the migrate triggers VDI reset
>     bug? Did I miss anything here? Thanks,
> 
>     Mike
> 
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> 
> 


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