Re: [Xen-users] propagating blktap partition changes

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Thu, Feb 05, 2009 at 03:35:18PM +0100, Maarten Vanraes wrote:
> Op Thursday 05 February 2009 15:27:55 schreef Pasi Kärkkäinen:
> > On Thu, Feb 05, 2009 at 01:59:57PM +0100, Maarten Vanraes wrote:
> > > Op Thursday 05 February 2009 13:05:01 schreef Pasi Kärkkäinen:
> > > > On Thu, Feb 05, 2009 at 11:52:56AM +0100, Maarten Vanraes wrote:
> > > > > Hi,
> > > > >
> > > > > I have 2 questions:
> > > > >
> > > > >  1. I was not able to let the guest know that the partition size was
> > > > > changed. 2. I was not able to use block-attach with the same
> > > > > backendfile to different machines (eg: guest + dom0)
> > > > >
> > > > > I've read that block-reconfigure could've been used for this, except
> > > > > that it wasn't designed to work for filebased vbd on PVs, only for
> > > > > switching CDROMs in HVM
> > > > >
> > > > > When you use a local disk on a machine, you can use fdisk to change
> > > > > partitions and sizes, it then rereads the partition table to reflect
> > > > > those changes.
> > > > >
> > > > > Since i'm not using a disk, but directly partitions, isn't it
> > > > > possible to just give a signal to the guest that it should recheck
> > > > > the partition? can be from the guest itself.
> > > > >
> > > > > Some kind of workaround?
> > > >
> > > > Sorry, this is not possible.
> > > >
> > > > There's a a feature request about it for Xen 3.4.
> > > >
> > > > Dunno if it will be implemented.
> > > >
> > > > -- Pasi
> > >
> > > Yes, i read your thread, i was hoping for some kind of workaround, some
> > > kind of "thing" i can execute on the guest, that would make the kernel
> > > recheck it's partition size or something.
> >
> > The problem is not about rereading the partition table; this can be done
> > using for example "blockdev --rereadpt".
> >
> > The problem is to get the disk device (/dev/xvd*) resized.. ie. getting the
> > kernel to see the DISK has been resized.
> >
> > After the disk has been resized, partition table can be resized after that.
> >
> > Disk resizing on the fly is not supported today with Xen.
> 
> Well I understand that it is inside the kernel, but, i'm not using disks, i'm 
> using partitions directly:
> 
> e.g.:
> 
> 'tap:aio:/opt/data/file.img, sda1, w'
> 
> I can make the partition file larger on dom0 no problem, i could even resize 
> the partition live on dom0 no problem.
> 
> But (i guess the kernel) doesn't see the partition as larger, even with 
> forced 
> operations, i still get read errors at the end.
> 
> I'm not even sure the blktap device detected this change in size on dom0.
> 
> But if i'm using directly partitions, maybe there is some code that i can 
> execute that does kernel calls or something to refresh it's device. ie: a 
> workaround.
> 
> I mean, hotplug seems to be able to do it just fine. ie: memory is detected 
> as 
> larger, i imagine there is such code for block devices as well, no? So i 
> figured there should be something i can do in the guest (maybe even on kernel 
> level) that could force the re-detection of the device or something.
> 
> Of course if blktap can't even pass on the information to the guest, this 
> would not work at all...
> 

Like said, there's no way for Xen (or dom0) to tell domU that disk or partition 
has been resized.
It is not yet implemented.

Solution: Use LVM in domU and xm block-attach new disk/partition to domU, 
create PV to
it and add that PV to existing VG.

-- Pasi

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