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

zedlryqc
2019-09-09T11:25:25Z

Quoting Qu Wenruo <quwenruo.btrfs@gmx.com>:
>> 1) Full online backup (or copy, whatever you want to call it)
>> btrfs backup <filesystem name> <partition name> [-f]
>> - backups a btrfs filesystem given by <filesystem name> to a partition
>> <partition name> (with all subvolumes).
>
> Why not just btrfs send?
>
> Or you want to keep the whole subvolume structures/layout?

Yes, I want to keep the whole subvolume structures/layout. I want to
keep everything. Usually, when I want to backup a partition, I want to
keep everything, and I suppose most other people have a similar idea.

> I'd say current send/receive is more flex.

Um, 'flexibility' has nothing to do with it. Send/receive is a
completely different use case.
So, each one has some benefits and some drawbacks, but 'send/receive'
cannot replace 'full online backup'

Here is where send/receive is lacking:
- too complicated to do if many subvolumes are involved
- may require recursive subvolume enumeration in order to emulate
'full online backup'
- may require extra storage space
- is not mountable, not easy to browse the backup contents
- not easy to recover just a few selected files from backup
There's probably more things where send/receive is lacking, but I
think I have given sufficient number of important differences which
show that send/receive cannot successfully replace the functionality
of 'full online backup'.

> And you also needs to understand btrfs also integrates volume
> management, thus it's not just <partition name>, you also needs RAID
> level and things like that.

This is a minor point. So, please, let's not get into too many
irrelevant details here.

There can be a sensible default to 'single data, DUP metadata', and a
way for a user to override this default, but that feature is
not-so-important. If the user wants to change the RAID level, he can
easily do it later by mounting the backup.

>
> All can be done already by send/receive, although at subvolume level.

Yeah, maybe I should manually type it all for all subvolumes, one by
one. Also must be carefull to do it in the correct order if I want it
not to consume extra space.
And the backup is not mountable.

This proposal (workaround) of yours appears to me as too complicated,
too error prone,
missing important features.

But, I just thought, you can actually emulate 'full online backup'
with this send/receive. Here is how.
You do a script which does the following:
- makes a temporary snapshot of every subvolume
- use 'btrfs send' to send all the temporary snapshots, on-the-fly
(maybe via pipe), in the correct order, to a proces running a 'brtfs
receive', which should then immediately write it all to the
destination partition. All the buffers can stay in-memory.
- when all the snapshots are received and written to destination, fix
subvol IDs
- delete temporary snapshots from source
Of course, this script should then be a part of standard btrfs tools.

> Please check if send/receive is suitable for your use case.

No. Absolutely not.

>> 2) Sensible defrag
>> The defrag is currently a joke.

>> How to do it:
>> - The extents must not be unshared, but just shuffled a bit. Unsharing
>> the extents is, in most situations, not tolerable.

> I definitely see cases unsharing extents makes sense, so at least we
> should let user to determine what they want.

Maybe there are such cases, but I would say that a vast majority of
users (99,99%) in a vast majority of cases (99,99%) don't want the
defrag operation to reduce free disk space.

> What's wrong with current file based defrag?
> If you want to defrag a subvolume, just iterate through all files.

I repeat: The defrag should not decrease free space. That's the
'normal' expectation.

>> - I think it would be wrong to use a general deduplication algorithm for
>> this. Instead, the information about the shared extents should be
>> analyzed given the starting state of the filesystem, and than the
>> algorithm should produce an optimal solution based on the currently
>> shared extents.
>
> Please be more specific, like giving an example for it.

Let's say that there is a file FFF with extents e11, e12, e13, e22,
e23, e33, e34
- in subvolA the file FFF consists of e11, e12, e13
- in subvolB the file FFF consists of e11, e22, e23
- in subvolC the file FFF consists of e11, e22, e33, e34

After defrag, where 'selected subvolume' is subvolC, the extents are
ordered on disk as follows:

e11,e22,e33,e34 - e23 - e12,e13

In the list above, the comma denotes neighbouring extents, the dash
indicates that there can be a possible gap.
As you can see in the list, the file FFF is fully defragmented in
subvolC, since its extents are occupying neighbouring disk sectors.

>> 3) Downgrade to 'single' or 'DUP' (also, general easy way to switch
>> between RAID levels)
>> Currently, as much as I gather, user has to do a "btrfs balance start
>> -dconvert=single -mconvert=single
>> ", than delete a drive, which is a bit ridiculous sequence of operations.

> That's a shortcut for chunk profile change.
> My first idea of this is, it could cause more problem than benefit.
> (It only benefits profile downgrade, thus only makes sense for
> RAID1->SINGLE, DUP->SINGLE, and RAID10->RAID0, nothing else)

Those listed cases are exactly the ones I judge to be most important.
Three important cases.

> I still prefer the safer allocate-new-chunk way to convert chunks, even
> at a cost of extra IO.

I don't mind whether it allocates new chunks or not. It is better, in
my opinion, if new chunks are not allocated, but both ways are
essentially OK.

What I am complaining about is that at one point in time, after
issuing the command:
btrfs balance start -dconvert=single -mconvert=single
and before issuing the 'btrfs delete', the system could be in a too
fragile state, with extents unnecesarily spread out over two drives,
which is both a completely unnecessary operation, and it also seems to
me that it could be dangerous in some situations involving potentially
malfunctioning drives.

Please reconsider.