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Re: [Xen-devel] About Copy-on-Write in Xen

To: Bin Ren <br260@xxxxxxxxx>
Subject: Re: [Xen-devel] About Copy-on-Write in Xen
From: Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>
Date: Fri, 16 Jan 2004 16:36:38 +0000
Cc: stevegt@xxxxxxxxxxxxx, Devel Xen <xen-devel@xxxxxxxxxxxxxxxxxxxxx>, Ian.Pratt@xxxxxxxxxxxx
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> In Xen, VBDs are implemented as 'extents'. One 'extent' is a group of
> contiguous sectors of the real disks. VBDs can grow by adding extra
> extents from the underlying real disks. Obviously, VBDs work at a
> much lower level.
> Therefore, you can say, you have created a VBD of 4G but actually
> starts with only 500M. Whenever the free space drops below a
> threshold, extra extents are automatically added. This function is
> already present in 1.2 and 1.3

Erm, no.

virtual disks may be extended or contracted, but this is not
currently automatic: the vd tools must be used to extend the
virtual disk, and then typically the guest will need to resize
the file system on the virtual disk (I believe ext3 supports
on-line resizing, but not automatically).

> BTW, this idea is quite similar to LVM.

Yes, just like LVM i.e. manual.

Right now, the best way to get 'statistical multiplexing' of disk
space is to give everyone a directory tree in domain 0 and export
it to other domains via local NFS (169.254.1.x).  This doesn't
currently get you CoW, but Russ is working on a CoW capable
user-space NFS server.

We'll then be able to bakeoff Russ and Bin's approaches.


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