![]() If the transaction group contains a synctask, the synctask runs last and must complete and return its results before the next transaction group can run. If a transaction group contains an administrative operation, the step of actually running the operation is known as a synctask. In order to provide filesystem consistency, transaction groups are numbered sequentially, are always committed in sequential order, and there is only one open transaction group at any given time. These operations could be disk operations, such as writes, or they might be administrative operations, such as zfs list or zfs destroy. To understand the difference, let’s do a quick overview of OpenZFS transaction groups and synctasks.Ī transaction group-also known as a TXG-is a collection of queued ZFS operations. ![]() For example, a Channel Program guarantees that the zfs list and zfs destroy commands both see the same ZFS pool state, whereas running these commands from a script leaves the chance that another ZFS operation outside of the script has modified the pool state before the next command in the script is run. Running Channel Programs within the kernel gives the added advantage that Channel Programs guarantee consistency with concurrent ZFS modifications. In contrast, a Channel Program is evaluated within the kernel (and kernel operations are faster than userland operations). The first reason for this is that the operations are issued by the zfs userland command. However, you may have noticed that it takes some time for all of the snapshot operations to complete, especially when iterating over a large amount of snapshots. If you’re using snapshots, you have probably already written a script that does this for you. You might be wondering, why not just write a script using my favorite scripting language for zfs batch operations? That brings us to the next question: Why Should I Use Channel Programs?Ĭonsider this common scenario: as part of your OpenZFS snapshot management, you need to periodically traverse the list of snapshots and destroy snapshots that meet certain criteria. ![]() By default, a Channel Program will stop if it runs longer than 10 million Lua instructions or uses more than 10MB of memory. Channel Programs may only be run with root privileges. As an additional security measure, FreeBSD executes Channel Programs in a sandboxed environment and enforces memory and time limits to prevent poorly-written or malicious Lua scripts from consuming all the memory in the kernel or causing an operation to block forever.Either all of the tasks in the channel program will succeed, or if any one of them fails, all of the work will not be committed, and it will be as if the commands had not run at all. Channel Programs are executed atomically, meaning they are not affected by other processes and you don’t have to stop any running applications before running the Channel Program.Channel Programs use the Lua programming language to make ZFS kernel calls which combine ZFS operations and provide iteration and error handling.(You can learn more about these commands in Basics of ZFS Snapshot Management ). Channel Programs are used to program a batch of ZFS administrative operations, such as a combination of the zfs snapshot, list, and destroy commands.Let’s take a closer look at that definition: Channel programs may only be run with root privileges. A library of ZFS calls is made available to channel program scripts. The entire script is executed atomically, with no other administrative operations taking effect concurrently. Zfs-program(8) defines it this way: The ZFS channel program interface allows ZFS administrative operations to be run programmatically via a Lua script. Today’s article answers some common questions regarding ZFS Channel Programs and provides some resources for learning how to create your own Channel Programs. ZFS Channel Programs-essentially, a way to batch multiple ZFS operations together in a single, atomic operation-are a new OpenZFS feature available in FreeBSD 12.0 and newer.
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