Re: Purposely using btrfs RAID1 in degraded mode ?

[Psalle <psalleetsile@gmail.com>]

On 06/01/16 13:34, Alphazo wrote:
> Thanks Psalle. This is the kind of feedback I was looking for. I do
> realize that using a filesystem in a degraded mode is not the wisest
> thing to do. While I looked at git-annex I'm not sure it can help to
> solve bit-rot detection. Now I noticed that my current backup solution
> borg-backup also has a checksum verification feature so I can at least
> detect errors. In addition it provides incremental deduplicated backup
> so it should get me covered if I discover that something went wrong.

Is that bup? I see it isn't, I guess they're similar. That is 
interesting too.

Git (or hg or similar) helps with bit rot because 'git fsck' will check 
the hashes of the objects in the repository. If you detected a problem 
you could re-clone from the good copy (assuming you have two drives with 
the same repository in each one). Admittedly, it's a purely manual 
method but is better than being unable to detect problems at all. 
git-annex is a layer on top of git that automates things to some extent 
and is tailored to large files, although the learning curve is not 
shallow in my experience.

-Psalle.

>
> alphazo
>
> On Tue, Jan 5, 2016 at 5:34 PM, Psalle <psalleetsile@gmail.com> wrote:
>> Hello Alphazo,
>>
>> I am a mere btrfs user, but given the discussions I regularly see here about
>> difficulties with degraded filesystems I wouldn't rely on this (yet?) as a
>> regular work strategy, even if it's supposed to work.
>>
>> If you're familiar with git, perhaps git-annex could be an alternative.
>>
>> -Psalle.
>>
>>
>> On 04/01/16 18:00, Alphazo wrote:
>>> Hello,
>>>
>>> My picture library today lies on an external hard drive that I sync on
>>> a regular basis with a couple of servers and other external drives.
>>> I'm interested by the on-the-fly checksum brought by btrfs and would
>>> like to get your opinion on the following unusual use case that I have
>>> tested:
>>> - Create a btrfs with the two drives with RAID1
>>> - When at home I can work with the two drives connected so I can enjoy
>>> the self-healing feature if a bit goes mad so I only backup perfect
>>> copies to my backup servers.
>>> - When not at home I only bring one external drive and manually mount
>>> it in degraded mode so I can continue working on my pictures while
>>> still having checksum error detection (but not correction).
>>> - When coming back home I can plug-back the seconde drive and initiate
>>> a scrub or balance to get the second drive duplicated.
>>>
>>> I have tested the above use case with a couple of USB flash drive and
>>> even used btrfs over dm-crypt partitions and it seemed to work fine
>>> but I wanted to get some advices from the community if this is really
>>> a bad practice that should not be used on the long run. Is there any
>>> limitation/risk to read/write to/from a degraded filesystem knowing it
>>> will be re-synced later?
>>>
>>> Thanks
>>> alphazo
>>>
>>> PS: I have also investigated the RAID1 on a single drive with two
>>> partitions but I cannot afford the half capacity resulting from that
>>> approach.
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>>