1. XenFS Sharing data in a virtualised environment
Mark Williamson
University of Cambridge

2. Overview Introduction to Xen Why share filesystems in a VMM?
Limitations of existing methods
XenFS will save the world!
Status and further work

3. Xen Paravirtualised hypervisor for x86
Domain 0
(privileged)
Unprivileged
domain
Unprivileged
domain
Device drivers
Virtual devices
Virtual devices
Xen
Paravirtualised hypervisor for x86
Run device drivers in privileged virtual machines
Use inter-VM communication for virtual devices
Terminology:
Domain == running VM
Domain 0 == Privileged VM
Virtualise through dom0 for unprivileged machines' devices

4. Virtual devices under Xen
Domain 0
Guest
Requests
Shared
ringbuffer
Backend
Frontend
Reponses
Event
notifications
Event
notifications
Ringbuffer in shared memory
Transfer data using DMA / transient shared mappings / page transfers, depending on the context
Xen

5. Filesystem sharing under Xen
Good for administration, efficiency, etc.
Main options:
Share at block level (inflexible but fast)
Share via network filesystem (inefficient)
Want (at least) the performance of block-level sharing but NFS-style flexibility

6. XenFS: the concept Virtualisation-aware shared filesystem
Use shared memory directly as a data transport:
Filesystem best placed to optimise data transfers
Maximise performance
Eliminate duplicated IO and in-memory data
May perform better than existing virtual IO
Page sharing, page flipping, events
NFS is our rolemodel, usage-wise
Block-device is our target performance

7. XenFS: planned features
Export directory hierachies from server
Multiple modes of operation:
FS-level Copy on Write
(Reasonably) coherent writable sharing
interdomain shared memory (mmap shared files)
Unify IO caching across the whole host

8. XenFS: Read mechanism Minimal overheads
Page
Client
Server
Page IO request
Sharing reference
Fetch Data
Shared page
Shared page
Shared page
Page IO Request
Page
Minimal overheads
Share identical memory directly
Eliminate unnecessary disk IO
No modification to core OS page-cache
Sharing reference
Client 2

9. XenFS: status Read-only prototype with page transfer / sharing
Rewriting for this week's Xen control interface :-)
Page frame reclamation: designed, no code yet
Coherent writing mechanisms: but be efficient
Live migration: soft state in the server
Benchmarking: XenFS vs NFS vs Block device

10. Summary Sharing data is important in a virtualised system
Filesystem sharing is an opportunity to improve on block IO
Virtualising the filesystem gives flexibility & performance

11. Questions?

12. XenFS: Undoing sharing
Problem:
Guests may try to write shared frames:
Modify private copy of data
Reallocated page cache frames
Solution:
Mechanism to reclaim memory from server
New kind of fault: “Copy To Write”
Flip side of the coin: how to undo the sharing established by a read

13. XenFS: “Copy To Write” Faults
Private copy
Copy data
Guest attempting to transition a shared page to writable
Fault handler runs
Shared page
Client
Server
Reclaim request
Empty page
Ack
CTW Fault from hypervisor
Page
Rewrite page tables to point to private copy – transparent to the rest of kernel