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

[Graham Cobb <g.btrfs@cobb.uk.net>]

On 09/09/2019 13:18, Qu Wenruo wrote:
> 
> 
> On 2019/9/9 下午7:25, zedlryqc@server53.web-hosting.com wrote:
>> 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.
> 
> In that case, you just need to replace that malfunctioning device other
> than fall back to SINGLE.

Actually, this case is the (only) one of the three that I think would be
very useful (backup is better handled by having a choice of userspace
tools to choose from - I use btrbk - and does anyone really care about
defrag any more?).

I did, recently, have a case where I had started to move my main data
disk to a raid1 setup but my new disk started reporting errors. I didn't
have a spare disk (and didn't have a spare SCSI slot for another disk
anyway). So, I wanted to stop using the new disk and revert to my former
(m=dup, d=single) setup as quickly as possible.

I spent time trying to find a way to do that balance without risking the
single copy of some of the data being stored on the failing disk between
starting the balance and completing the remove. That has two problems:
obviously having the single copy on the failing disk is bad news but,
also, it increases the time taken for the subsequent remove which has to
copy that data back to the remaining disk (where there used to be a
perfectly good copy which was subsequently thrown away during the balance).

In the end, I took the risk and the time of the two steps. In my case, I
had good backups, and actually most of my data was still in a single
profile on the old disk (because the errors starting happening before I
had done the balance to change the profile of all the old data from
single to raid1).

But a balance -dconvert=single-but-force-it-to-go-on-disk-1 would have
been useful. (Actually a "btrfs device mark-for-removal" command would
be better - allow a failing device to be retained for a while, and used
to provide data, but ignore it when looking to store data).

Graham