OpenZFS Channel programs
ZFS channel programs are proposed to address issues with ZFS administrative operations being done transactionally through synctasks. Channel programs would allow sending a stream of instructions to the kernel for atomic execution in syncing context. This would avoid long wait times between transactions and allow more efficient single-pass logic to be implemented in the kernel rather than splitting operations across transactions. Sample programs show how operations like recursive destroy could be implemented through a single channel program calling kernel intrinsics from within the syncing context. The plan is to refactor sync task code and add a Lua interpreter to the kernel to implement ZFS operations as Lua extensions executing the refactored sync tasks.
OpenZFS Channel programs
- 1. ZFS Channel Programs Proposed Project
- 2. Background: Administrative Ops & Synctasks ● ZFS administrative operations are done transactionally as part of “synctasks”. Examples: ● zfs create ● zfs snapshot ● zfs set ● ‘zfs’ command sends ioctl to kernel ● Kernel queues operation’s synctask for execution ● synctasks are processed at the end of each txg in syncing context (holds config lock for write) ● User must wait for txg to complete
- 3. What are the problems? ● The time between txgs can be 10+ seconds. a. When doing consecutive operations each one blocks for 10+ seconds. b. Example: Delphix stores VMs on ZFS and keeps metadata in ZFS properties, simple operations like “create VM” take 30+ seconds waiting for synctasks
- 4. What are the problems? ● Only transactionally consistent within synctask. a. Forces logic into user space that could be more efficiently/correctly implemented in the kernel b. Example: zfs destroy -r gathers the list of snapshots to destroy in open context from userland. Also destroy of snapshots and destroy of filesystem are separate transactions.
- 5. What are the problems? ● Encourages more and more complicated synctasks in the kernel for correct behavior ● zfs destroy -p ● zfs rollback -p ● zfs snapshot <snap>,<snap>,... ● zfs snapshot (unless property is set)
- 6. Solution: ZFS Channel Programs ● Send a stream of instructions into the kernel for execution by ZFS in syncing context. ● Being in syncing context guarantees atomicity, consistent state changes, single-TXG execution. ● Enables rapid development of new “sync tasks” from existing intrinsics.
- 7. Sample Channel Program zfs destroy -r for snap in zfs.snapshots_of(target) zfs.destroy(snap) zfs.destroy(target)
- 8. Plan of Action Refactor existing sync task code to unify shared execution paths. Add Lua interpreter to kernel, implement ZFS intrinsics (destroy, snapshot, etc) as extensions to the Lua language that call into refactored sync tasks. Add ZFS Lua iterators, e.g. children(fs), snapshots (fs), etc.