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Re: [et-mgmt-tools] Virtinst and blktap

Michael DeHaan wrote:
> I had a user question about how they can specify blktap for Xen in 
> Cobbler instead of the file driver (for performance reasons) ... well, 
> it turns out they can't (yet).
> Under what conditions can we use blktap, and is that advisable?   (I had 
> thought this might work like virtio, in which case, passing in the os 
> version uses a table, but it seems this may only actually depend on the 
> host and has no guest limitations?)

That's exactly right.  It's a property of how the host OS accesses the disk on
behalf of the domain, not of how the disk is presented to the domain.  In fact,
for Xen PV domains, you should *only* ever use blktap (i.e. tap:aio:); direct
file: access can corrupt your guest domains on host crash.  The situation with
FV domains is more complicated, but there, you can use file:, since it properly
does flushing.

> It looks like from the virtinst code I could just call 
> utils.is_blktap_capable() from my libvirt-using application and if so, 
> then pass the driver=DRIVER_TAP options to virstinst.
> Good idea?  Bad idea?

The only reasonable default for Xen PV domains, in my opinion.

Chris Lalancette

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