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

[Mike McClurg <mike.mcclurg@xxxxxxxxxx>]

On 27/11/12 16:04, Rushikesh Jadhav wrote:
> On Tue, Nov 27, 2012 at 9:24 PM, Mike McClurg <mike.mcclurg@xxxxxxxxxx
> <mailto:mike.mcclurg@xxxxxxxxxx>> wrote:
> 
>     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
> 
> 
> Thank for the further info. So are you making a magic RO VDI which can
> hold the writes till its attached  ?
> That sounds good to me :)
> 
> Do you think that there is any usefulness to what I thought about
> on-boot=reset ?

Well, I think it would be very unsafe, because you'd basically be
working around the locking mechanism for the VDIs. Using resetvdis
manually is safer.

Mike

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