Beyond what is and isn’t technically possible at the file system level there is 
always the problem that the user may have more data than can fit into the 
reduced disk.

I don’t want to take away useful functionality from folks if there are cases 
where it already works – mostly I just want to improve the user experience, and 
 to me the biggest problem here is the current failure mode where the user 
can’t tell if the request has been tried and failed, or just not happened at 
all for some other reason.

What if we introduced a new state of “Resize_failed” from which the only 
allowed operations are “resize_revert” and delete – so the user can at least 
get some feedback on the cases that can’t be supported ?

From: Aryeh Friedman [mailto:aryeh.fried...@gmail.com]
Sent: 13 June 2014 18:15
To: OpenStack Development Mailing List (not for usage questions)
Subject: Re: [openstack-dev] [nova] Do any hyperviors allow disk reduction as 
part of resize ?

Also ZFS needs to know what is on the guest for example bhyve (the only working 
hv for bsd currency [vbox kind of also works]) stores the backing store (unless 
bare metal) as single block file.   It is impossible to make that non-opaque to 
the outside world unless you can run commands on the instance.

On Fri, Jun 13, 2014 at 11:53 AM, Darren J Moffat 
<darren.mof...@oracle.com<mailto:darren.mof...@oracle.com>> wrote:


On 06/13/14 16:37, Daniel P. Berrange wrote:
The xenapi implementation only works on ext[234] filesystems. That rules
>out *BSD, Windows and Linux distributions that don't use ext[234]. RHEL7
>defaults to XFS for instance.
Presumably it'll have a hard time if the guest uses LVM for its image
or does luks encryption, or anything else that's more complex than just
a plain FS in a partition.

For example ZFS, which doesn't currently support device removal (except for 
mirror detach) or device size shrink (but does support device grow).  ZFS does 
support file system resize but file systems are "just" logical things within a 
storage pool (made up of 1 or more devices) so that has nothing to do with the 
block device size.

--
Darren J Moffat


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--
Aryeh M. Friedman, Lead Developer, http://www.PetiteCloud.org
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