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

[Michal Novotny <minovotn@redhat.com>]

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