Trigger xen_vbd_resize

Trigger xen_vbd_resize

[Steve Prochniak]

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I can see that there is support for dynamically resizing of virtual disks in blockback, but how do I trigger backend_changed->xen_blkif_schedule->xen_vbd_resize() when I use something like dd to expand my file backed storage?

 

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Re: Trigger xen_vbd_resize

[Pasi Kärkkäinen]

On Thu, Aug 15, 2013 at 09:03:11AM -0700, Steve Prochniak wrote:
>    I can see that there is support for dynamically resizing of virtual disks
>    in blockback, but how do I trigger
>    backend_changed->xen_blkif_schedule->xen_vbd_resize() when I use something
>    like dd to expand my file backed storage?
> 

Not sure.. online resizing has only been used with LVM volumes in dom0 so far..

-- Pasi

Re: Trigger xen_vbd_resize

[Konrad Rzeszutek Wilk]

On Thu, Aug 15, 2013 at 09:03:11AM -0700, Steve Prochniak wrote:
> I can see that there is support for dynamically resizing of virtual disks in blockback, but how do I trigger backend_changed->xen_blkif_schedule->xen_vbd_resize() when I use something like dd to expand my file backed storage?

That would imply you are using the loop back device. If you look in the
blkback driver you will see that it checks on every request that:

#define vbd_sz(_v)      ((_v)->bdev->bd_part ? \
                         (_v)->bdev->bd_part->nr_sects : \
                          get_capacity((_v)->bdev->bd_disk))

And if you follow get_capacity, you get to:

static inline sector_t get_capacity(struct gendisk *disk)
{
        return disk->part0.nr_sects;
}

Looking in the loop device.. well, I will let you figure this out -
hint: figure_loop_size


> 
>  

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> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

Re: Trigger xen_vbd_resize

[Konrad Rzeszutek Wilk]

On Fri, Aug 16, 2013 at 07:23:53AM -0700, Steve Prochniak wrote:
> The piece that I'm missing comes before all that.  

Adding xen-devel back so that other interested parties can benefit from
this knowledge.

> 
> In dom0, I use dd to expand my disk.img file.  I see that in xen_blkif_schedule we check to see if the disk size changed using vbd_sz as you mentioned.  But how do we get from doing the dd to executing that code path in blkback?
> 
> I see that backend_changed in xenbus will indirectly call xen_blkif_schedule, but nothing about doing a dd in dom0 is going to trigger a callback to backend_changed.  What's the proper way for me to signal this?
> 
> All of the examples of people using dynamically resizing disks that I can find are using LVM, but we are using a simple file.
> 
> disk = ['file:/OVS/OVM_OL6U4_X86_PVM_2GB/System.img,xvda,w',
>         'file:/OVS/OVM_OL6U4_X86_PVM_2GB/Disk.img,xvdb,w']
> 

If you look in the loop.c file you will see this:

        case LOOP_SET_CAPACITY:                                                 
                err = -EPERM;                                                   
                if ((mode & FMODE_WRITE) || capable(CAP_SYS_ADMIN))             
                        err = loop_set_capacity(lo, bdev);                      
                break;                                            

which ends up calling figure_loop_size which calls 
        set_capacity(lo->lo_disk, x);                                           


In blkback there is xen_blkif_schedule which is a thread running
all the time servicing the guest fronted for its I/Os which does:

	     while (!kthread_should_stop()) {                                        
                if (try_to_freeze())                                            
                        continue;                                               
                if (unlikely(vbd->size != vbd_sz(vbd)))                         
                        xen_vbd_resize(blkif);                

Every time there is an request it will trigger the check to see if it
needs to resize the disk.

Doing a google search for LOOP_SET_CAPACITY tells me that losetup might
have this implemented, and sure enough  in the manpage of losetup you will see:

     Resize loop device:

            losetup -c loopdev



Hopefully this helps.
> Steve 
> 
> -----Original Message-----
> From: Konrad Rzeszutek Wilk 
> Sent: Thursday, August 15, 2013 4:18 PM
> To: Steve Prochniak
> Cc: xen-devel@lists.xensource.com
> Subject: Re: [Xen-devel] Trigger xen_vbd_resize
> 
> On Thu, Aug 15, 2013 at 09:03:11AM -0700, Steve Prochniak wrote:
> > I can see that there is support for dynamically resizing of virtual disks in blockback, but how do I trigger backend_changed->xen_blkif_schedule->xen_vbd_resize() when I use something like dd to expand my file backed storage?
> 
> That would imply you are using the loop back device. If you look in the
> blkback driver you will see that it checks on every request that:
> 
> #define vbd_sz(_v)      ((_v)->bdev->bd_part ? \
>                          (_v)->bdev->bd_part->nr_sects : \
>                           get_capacity((_v)->bdev->bd_disk))
> 
> And if you follow get_capacity, you get to:
> 
> static inline sector_t get_capacity(struct gendisk *disk)
> {
>         return disk->part0.nr_sects;
> }
> 
> Looking in the loop device.. well, I will let you figure this out -
> hint: figure_loop_size
> 
> 
> > 
> >  
> 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@lists.xen.org
> > http://lists.xen.org/xen-devel
> 
> 
> _______________________________________________
> Xen-devel mailing list
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> http://lists.xen.org/xen-devel