Re: [Xen-devel] VBD extents

[Mark Williamson <Mark.Williamson@xxxxxxxxxxxx>]

> So in XEN 2.0 is VBD grow/shrink used?

Yes, Xend uses them when creating a VBD.  Xend first creates a given VBD, 
which starts off with zero size.  Xend then adds exactly one segment to the 
VBD using a grow message - this segment describes the device the VBD is 
mapping.

Shrink messages probably aren't used anywhere at all.

> 0).  If indeed these messages will eventually be removed, i take it XEND
> will need to send a message to blkback identifying the disk device driver

That sounds right - basically just removing the vestiges of multi-extent 
support in blkback so that each VBD has exactly one extent.

Cheers
Mark

> Eric
>
> Ian Pratt <m+Ian.Pratt@xxxxxxxxxxxx> wrote:
> > Can anyone tell me what exactly the VBD extents are used for
> > in the XEN back-end block device driver? It almost looks
> > like each virtual disk is made up of multiple physical drives
> > if I read the code correctly (for each VBD grow message
> > received by blkback, it opens a device and retreives various
> > info such as its size).
>
> The extent stuff is left over from Xen 1.2, where we effectively had to
> do our own version of LVM inside Xen. We'd like to give the code a
> spring clean and do away with the grow/srink control messages
> altogether. At the same time it would be sensible to add support for
> multiple blk rings, and possibly the ability to 'tunnel' certain ioctls
> e.g. eject CD.
>
> Ian
>
>
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