online blockdev-backup, a clarification (was: Summary on new backup interfaces in QEMU)

On Mar 15, 2022, at 12:57 PM, Vladimir Sementsov-Ogievskiy <v.sementsov-og(a)ya.ru&gt; wrote: Hi all! Here I want to summarize new interfaces and use cases for backup in QEMU. TODO for me: convert this into good rst documentation in docs/.

OK, let's begin. First, note that drive-backup qmp command is deprecated. Next, some terminology: push backup: the whole process is inside QEMU process, also may be called "internal backup" pull backup: QEMU only exports a kind of snapshot (for example by NBD), and third party software reads this export and stores it somehow, also called "external backup" copy-before-write operations: We usually do backup of active disk, guest is running and may write to the disk during the process of backup. When guest wants to rewrite data region which is not backed up yet, we must stop this guest write, and copy original data to somewhere before continuing guest write. That's a copy-before-write operation. image-fleecing: the technique that allows to export a "snapshotted" state of the active disk with help of copy-before-write operations. We create a temporary image - target for copy-before-write operations, and provide an interface to the user to read the "snapshotted" state. And for read, we do read from temporary image the data which is already changed in original active disk, and we read unchanged data directly from active disk. The temporary image itself is also called "reverse delta" or "reversed delta". == Simple push backup == Just use blockdev-backup, nothing new here. I just note some technical details, that are relatively new: 1. First, backup job inserts copy-before-write filter above source disk, to do copy-before-write operation. 2. Created copy-before-write filter shares internal block-copy state with backup job, so they work in collaboration, to not copy same things twice.