Re: [Xen-devel] [PATCH] Introduce dom0-min-space configuration option

[Michal Novotny <minovotn@xxxxxxxxxx>]

Ian,
what do you think about this?

Michal

On 07/14/2010 01:23 PM, Michal Novotny wrote:
> On 07/13/2010 08:02 PM, Ian Jackson wrote:
> > Michal Novotny writes ("[Xen-devel] [PATCH] Introduce dom0-min-space 
> > configuration option"):
> > > This is the patch to introduce configuration option called
> > > dom0-min-space since there were some issues with data inflation
> > > because of invalid input data stream for zlib decompression.
> > > The issue occured because of insufficient free space on the dom0 so
> > > this patch checks the free available space for /var/lib/xen
> > > and refuses to start up any guests when the space is below
> > > specified value. Setting up the value to 0 disables the check
> > > which preserves the behaviour before this patch applied and
> > > this is the default value for this option.
> > Thanks for the patch, but I'm not sure I entirely follow.
> >
> > What "issues with data inflation because of invalid input data stream
> > for zlib decompression" were there and how do they relate to lack of
> > space on /var/lib/xen ?
> Ian, the problem is when pygrub extracts the vmlinuz and initrd for PV 
> guests but there's insufficient space on the dom0, there's no error 
> message but the error is being raised from Xend itself, libxc to be 
> precise since there's the zStream inflation code but since the input 
> data are not valid (i.e. they're just partial, let's say only 50 KiB 
> was extracted to /var/lib/xen since after those 50 KiBs the dom0 had 
> no space available) the zStream (zlib decompression) fails with 
> Z_BUF_ERROR and then it outputs annoying and nothing saying message to 
> standalone users (non-developers) to doesn't do investigation on their 
> own. This patch would prevent going into those issues since there 
> would be always at least specified amount of free space available for 
> PV images to be extracted to /var/lib/xen .
> > Is this just for the situation where the dom0 filesystem hasn't enough
> > space to contain the uncompressed version of the domU kernel and
> > initrd which are to be loaded ?
> Well, this is for compressed version being extracted from the guest 
> image files. This is being saved to /var/lib/xen and if there's not 
> enough space to save it the deflation fails with -5 (Z_BUF_ERROR) 
> which says nothing to standalone users.
> > Your patch just raises an error.  Why does the situation in which the
> > patch is needed not already raise an appropriate error ?  Perhaps it
> > would be better to arrange that it does.
> Well, the reason is that there should be multiple reasons that could 
> make the decompression (gunzip) fail. Also, having the dom0 with no 
> space could cause some other (non-Xen) related errors so having this 
> option could prevent user running into those issues - mainly if the 
> user's saving images to the default location, i.e. 
> /var/lib/xen/images, he/she can get running out of free space on dom0 
> very easily.
> Michal
--
Michal Novotny<minovotn@xxxxxxxxxx>, RHCE
Virtualization Team (xen userspace), Red Hat


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