VirtFS—A paravirtual File System pass-through in KVM/QEMU
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One Line Summary
VirtFS is a paravirtual file system which exports the host file systems to the guest on 9P2000.L protocol through VirtIO transport.
Virtualization systems have historically focused on providing the illusion of access to underlying hardware devices (either by emulating the physical device interface or through a paravirtualization API). What has been largely ignored within the mainstream virtualization community is the opportunity for raising the level of interface from the physical device layer to higher-level system and even application services. One of the more compelling arguments for virtualizing system services is a higher degree of information available to hypervisor about what the guest system or the guest system’s user) is trying to do.
File systems are a particularly good target as a paravirtual systems service. One of the principle problems with virtualized storage in the form of virtual disks is that data on the disk can not be concurrently accessed by multiple guests and it is very in-efficient with multiple levels of indirection and cache duplication. Traditional distributed file system (such as NFS or CIFS) over a virtualized network device to access storage is also not very good solution as these protocols are not designed for virtualization and has various security and double caching issues.
There are various uses and perhaps the most straightforward application of a paravirtualized file system is to replace the virtual disk as the root file system. Such an approach would allow host-based stackable file systems to be used for rapid cloning, management, and update.
The development is progressing at brisk pace and the feature is already in the mainline (QEMU 0.13 and mainline kernel). We will take this opportunity to briefly update the community with our progress and discuss and take community view/help on resolving some of the hot problems we are facing like interfaces missing/needed for user-level fileservers, 9P fid management issues, Linux ACL (Posix and Rich) and issues around cache sharing between host and the guest.
KVM, Virtualization, file system, QEMU, virtfs, filesystems
Venkateswararao Jujjuri (JV) works in IBM’s Linux Technology Center as Sr. Software Engineer and is currently leading an effort to develop the VirtFS filesystem pass-through over KVM and QEMU using 9p. Prior to this assignment JV worked on various Linux and Unix technologies like Real-Time, Distributed File Systems (SANFS), Clusters(HACMP) and storage virtaulization (SANVC).