Try booting with the 'Fallback' kernel.

Type fallback or fallback-serial at the boot: prompt.

http://xenserver.org/component/easyblog/entry/xenserver-root-disk-maintenance.html?Itemid=179

Check disk space:

df -h | grep "/$" | head -n 1

Looking for files bigger than a given size (specify path and size):

find / -mount -type f -size +20M -exec ls -lh {} \; | awk '{ print $9 ": " $5 }'

You can pretty much ignore anythin in /var/run/sr-mount/ as it's not on the local filesystem.

Delete rotated log files:

find /var/log/ -type f -name *.gz -exec rm -f {} \;

Delete applied patches:

Works pool-wide if run on the pool master server.

for i in `xe patch-list --minimal|tr "," " "`;do xe patch-pool-clean uuid=$i;done

Delete 'orphaned' patches and backup patches:

Do not delete /var/patch/applied or its contents.

rm /var/patch/*-*-*-*-*

For true emergencies or for patch install problems, you can delete patch backups:

rm -rf /opt/xensource/patch-backup/*-*-*-*-*

On the Pool Masters, we mount an NFS backup share on /mnt/backup.

Unmount the backup share and delete any 'hidden' files:

umount /mnt/backup
rm -rf /mnt/backup/*
mount -a