Re: Purposely using btrfs RAID1 in degraded mode ?

[Alphazo]

I'm a former bup user but I switched to borgbackup
https://borgbackup.readthedocs.org/en/stable/ which is a more active
fork of Attic and that solves two issues I had with bup: increasing
time required to perform the incremental backup on large dataset with
only few modifications and more importantly the impossibility to prune
older backups. Also borgbackup natively supports encryption (AES256)
and authentication (HMAC-SHA256).

For offline long term backups I also used to work with hashdeep to
perform and store a hash of all the files and recently started playing
with FIM https://evrignaud.github.io/fim/ which is similar but with a
git backend for storing history. Don't get fooled by fim being a java
application. It easily outperformed hashdeep on large datasets.

Alphazo

On Thu, Jan 7, 2016 at 1:57 PM, Psalle  wrote:
> 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  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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>>>
>>>
>
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Re: Purposely using btrfs RAID1 in degraded mode ?

[Alphazo]

Thanks Chris for the warning. I agree that mounting both drives
separately in degraded r/w will lead to very funky results when trying
to scrub them when put together.

On Mon, Jan 4, 2016 at 6:41 PM, Chris Murphy  wrote:
> On Mon, Jan 4, 2016 at 10:00 AM, Alphazo  wrote:
>
>> 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?
>
> As long as you realize you're testing a sort of edge case, but an
> important one (it should work, that's the point of rw degraded mounts
> being possible), then I think it's fine.
>
> The warning though is, you need to designate a specific drive for the
> rw,degraded mounts. If you were to separately rw,degraded mount the
> two drives, the fs will become irreparably corrupt if they are
> rejoined. And you'll probably lose everything on the volume. The other
> thing is that to "resync" you have to manually initiate a scrub, it's
> not going to resync automatically, and it has to read everything on
> both drives to compare and fix what's missing. There is no equivalent
> to a write intent bitmap on Btrfs like with mdadm (the information
> ostensibly could be inferred from btrfs generation metadata similar to
> how incremental snapshot send/receive works) but that work isn't done.
>
>
>
>
> --
> Chris Murphy
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Re: Purposely using btrfs RAID1 in degraded mode ?

[Psalle]

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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Re: Purposely using btrfs RAID1 in degraded mode ?

[Chris Murphy]

On Mon, Jan 4, 2016 at 10:00 AM, Alphazo  wrote:

> 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?

As long as you realize you're testing a sort of edge case, but an
important one (it should work, that's the point of rw degraded mounts
being possible), then I think it's fine.

The warning though is, you need to designate a specific drive for the
rw,degraded mounts. If you were to separately rw,degraded mount the
two drives, the fs will become irreparably corrupt if they are
rejoined. And you'll probably lose everything on the volume. The other
thing is that to "resync" you have to manually initiate a scrub, it's
not going to resync automatically, and it has to read everything on
both drives to compare and fix what's missing. There is no equivalent
to a write intent bitmap on Btrfs like with mdadm (the information
ostensibly could be inferred from btrfs generation metadata similar to
how incremental snapshot send/receive works) but that work isn't done.




-- 
Chris Murphy
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