(3) Differencing hard disk image: the differencing VHD image relies on a master or parent image and is only the difference from that image; a parent image could be associated with multiple differencing images .
This storage is split into primary and secondary volumes; the primary volume is where the guest VHD image files are stored and, therefore, where the customer's sensitive and personal data is held.
This posed a very interesting query, the virtual machines were all Linux operating systems with Ext4 formatted partitions but these partitions were encapsulated within a virtual hard drive (VHD).
In a cloud environment, if a malicious insider could gain access to the primary storage where the VHD files are stored and take copies of these images, they would, effectively, steal the servers in their entirety.
However, looking at the directory listing, it is not clear which VHD belongs to the target host.
Using the cloud interface and examining the volume further shows a volume id of 32098385-7aa9-45cd-9caa9f5dbd140fa1; comparing this volume ID in Figure 16 to the VHD files in Figure 15 does not show a match, suggesting that this information is hidden from even the privileged cloud user.
The CloudStack database structure is not too complicated and, by gaining access to the database command line interface, the following MySQL commands soon identified which tables were required to find the VHD image associated with the target host.
Armed with the database and table structure, a further and more specific MySQL query was run on the database to map the UUID value from Figure 16 to the actual VHD file listed in Figure 15.
With the target VHD image identified in Figure 18, listed in the path column, the next stage is to identify what kind of implementation was selected when the VHD image was deployed.
Because the VHD image is a file, a number of unsupported utilities have been developed that allow these files to be viewed and even updated without the involvement of a hypervisor.
With test results indicating as much as a 25 percent performance improvement after defragmentation, the VHD Save tests show quite clearly the effect of writing a very large file to a fragmented hard drive.
Automated background defragmentation results in a major reduction in fragmentation even with an active VHD. Regular use of the background defragmenter will continue to minimize fragmentation.