Abstract
The traditional virtual machine (VM) building and and deployment process is centered around the virtual machine hard disk image. The packages comprising the VM operating system are carefully selected, hard disk images are built for a variety of different hypervisors, and images have to be distributed and decompressed in order to instantiate a virtual machine. Within the HEP community, the CernVM File System (CernVM-FS) has been established in order to decouple the distribution from the experiment software from the building and distribution of the VM hard disk images. We show how to get rid of such pre-built hard disk images altogether. Due to the high requirements on POSIX compliance imposed by HEP application software, CernVM-FS can also be used to host and boot a Linux operating system. This allows the use of a tiny bootable CD image that comprises only a Linux kernel while the rest of the operating system is provided on demand by CernVM-FS. This approach speeds up the initial instantiation time and reduces virtual machine image sizes by an order of magnitude. Furthermore, security updates can be distributed instantaneously through CernVM-FS. By leveraging the fact that CernVM-FS is a versioning file system, a historic analysis environment can be easily re-spawned by selecting the corresponding CernVM-FS file system snapshot.
Highlights
Virtual machines provide a uniform and portable environment for high energy physics applications [1]
The scratch space is used as a persistent writable overlay for local changes to the root file system and as a cache for the CernVM-FS client that loads the operating system
(iii) To avoid silent and unwanted updates of the virtual machine operating system, μCernVM exploits the fact that CernVM-FS is a versioning file system
Summary
Virtual machines provide a uniform and portable environment for high energy physics applications [1]. The result is a tiny virtual machine image only consisting of a Linux kernel and the CernVM-FS client. The purpose of the steering script is to create the virtual machine’s root file system stack that is constructed by unifying the CernVM-FS mount point with the writable scratch space.
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