Re: Feature requests: online backup - defrag - change RAID level

Andrei Borzenkov
2019-09-10T17:39:51Z

09.09.2019 20:11, webmaster@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.
>
>