09.09.2019 20:11, webmas...@zedlx.com пишет: ... >> >> Forgot to mention this part. >> >> If your primary objective is to migrate your data to another device >> online (mounted, without unmount any of the fs). > > This is not the primary objective. The primary objective is to produce a > full, online, easy-to-use, robust backup. But let's say we need to do > migration... >> >> Then I could say, you can still add a new device, then remove the old >> device to do that. > > If the source filesystem already uses RAID1, then, yes, you could do it,
You could do it with any profile. > but it would be too slow, it would need a lot of user intervention, so > many commands typed, so many ways to do it wrong, to make a mistake. > It requires exactly two commands - one to add new device, another to remove old device. > Too cumbersome. Too wastefull of time and resources. > Do you mean your imaginary full backup will not read full filesystem? Otherwise how can it take less time and resources? >> That would be even more efficient than LVM (not thin provisioned one), >> as we only move used space. > > In fact, you can do this kind of full-online-backup with the help of > mdadm RAID, or some other RAID solution. It can already be done, no need > to add 'btrfs backup'. > > But, again, to cumbersome, too inflexible, too many problems, and, the > user would have to setup a downgraded mdadm RAID in front and run with a > degraded mdadm RAID all the time (since btrfs RAID would be actually > protecting the data). > >> If your objective is to create a full copy as backup, then I'd say my >> new patchset of btrfs-image data dump may be your best choice. > > It should be mountable. It should be performed online. Never heard of > btrfs-image, i need the docs to see whether this btrfs-image is good > enough. > >> The only down side is, you need to at least mount the source fs to RO >> mode. > > No. That's not really an online backup. Not good enough. > >> The true on-line backup is not that easy, especially any write can screw >> up your backup process, so it must be done unmounted. > > Nope, I disagree. > > First, there is the RAID1-alike solution, which is easy to perform (just > send all new writes to both source and destination). It's the same thing > that mdadm RAID1 would do (like I mentioned a few paragraphs above). > But, this solution may have a performance concern, when the destination > drive is too slow. > > Fortunately, with btrfs, an online backup is easier tha usual. To > produce a frozen snapshot of the entire filesystem, just create a > read-only snapshot of every subvolume (this is not 100% consistent, I > know, but it is good enough). > > But I'm just repeating myself, I already wrote this in the first email. > > So, in conclusion I disagree that true on-line backup is not easy. > >> Even btrfs send handles this by forcing the source subvolume to be RO, >> so I can't find an easy solution to address that. > > This is a digression, but I would say that you first make a temporary RO > snapshot of the source subvolume, then use 'btrfs send' on the temporary > snapshot, then delete the temporary snapshot. > > Oh, my. > >
