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

[Michal Novotny <minovotn@redhat.com>]

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@redhat.com>, RHCE
Virtualization Team (xen userspace), Red Hat