Linux on System z under z/VM may offer better performance, improved security, and easier maintenance through exploitation of Linux eXecute-In-Place (XIP) filesystems saved, Discontiguous Saved Segments (DCSSs), and a Named Saved System (NSS).
DCSS, XIP Defined
A DCSS is a z/VM construct that saves an image of memory in a virtual machine in a system-managed file. These images can later be accessed by virtual guests, typically read-only, and shared by multiple guests. DCSS access is high performance, as z/VM manages the actual data in its paging subsystem.
With z/VM Version 5 Release 4 or later and Novell SUSE Linux Enterprise Server 11 (SLES 11), users can save a copy of a filesystem as a read-only image in a DCSS. Normally, Linux executables must be loaded into memory for execution; however, when such files are located in an XIP filesystem, they can be executed in place. When multiple Linux guests mount an XIP filesystem, high-use pages are more likely to remain resident in main storage, reducing guest paging; the read-only pages are shared among the guests, reducing overall system paging.
Saving data common to Linux guests in XIP filesystems also improves security because the guests are more “locked down”: The data can’t be changed by accident—not even by a malicious Linux user. Maintenance is also simplified because the single “golden image” in XIP can be centrally maintained. Because there’s only a single copy of the data no matter how many guests are using it, only that one copy needs to be updated, rather than one for each guest.
In z/VM releases prior to V5.4, DCSSs had to reside in the first 2GB of guest memory and couldn’t be larger than 2GB. In current z/VM releases, DCSSs are still limited to 2GB each, but can now reside above 2GB, and multiple DCSSs can be concatenated. With corresponding support in the Linux DCSS block device driver, dcssblk, you can now create larger DCSS-backed filesystems. SLES 11 now fully supports DCSS technology.
Beyond XIP filesystems, read-write DCSSs can be used for fast Linux swap space. The dcssblk device driver provides disk-like access to these saved segments.
A whitepaper titled “Sharing and Maintaining SLES 11 Linux Under z/VM Using DCSSs and an NSS,” which details how to use DCSSs for XIP and swap space, is available at www.vm.ibm.com/linux/dcss/. This Web page provides links to the paper in Adobe Acrobat PDF format and to an associated tar file containing Linux scripts and z/VM Rexx programs. A detailed performance analysis of z/VM V5.4 DCSS technology titled “DCSS Above 2GB” is available at www.vm.ibm.com/perf/reports/zvm/html/540dcss.html.
Virtual Machine Scenario
The whitepaper previously cited describes a scenario using four virtual machines for cloning Linux images: