openzfs channel programs

ZFS Channel ProgramsProposed Project

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

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

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.

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)

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.

Sample Channel Program

zfs destroy -r

for snap in zfs.snapshots_of(target)

zfs.destroy(snap)

zfs.destroy(target)

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.