How can checksums (node41) be of real help?

How can checksums (node41) be of real help?

[Georgios Tsalikis]

I suppose they are great for discovering corrupted files, but is there 
any way we can use them to retrieve a good copy of corrupted files?

Re: How can checksums (node41) be of real help?

[Edward Shishkin]

On 05/24/2016 10:50 AM, Georgios Tsalikis wrote:

 > I suppose they are great for discovering corrupted files, but is 
there any way we can use them to retrieve a good copy of corrupted files?

Hello,

Currently if checksum verification failed, then the file system becomes 
readonly
with the suggestion (in kernel messages) to repair the partition with fsck.

Ofc, it is not convenient, but better than nothing. More efficient way 
is to support replicas, so
that every time when checksum verification failed, you upload respective 
replica. It is not difficult
to implement. Just allocate/release replicas simultaneously with 
conventional formatted blocks.
Pointers to replicas can be placed near the checksums (a new node42 
format is needed).
The patches 3.18.6-reiser4-add-node41-support.patch and 
reiser4progs-1.0.9-add-node41-support.patch
are a good example of how to add a new node format.

Thanks,
Edward.

Re: How can checksums (node41) be of real help?

[Edward Shishkin]

On 05/24/2016 12:01 PM, Edward Shishkin wrote:
> On 05/24/2016 10:50 AM, Georgios Tsalikis wrote:
>
> > I suppose they are great for discovering corrupted files, but is 
> there any way we can use them to retrieve a good copy of corrupted files?
>
> Hello,
>
> Currently if checksum verification failed, then the file system 
> becomes readonly
> with the suggestion (in kernel messages) to repair the partition with 
> fsck.
>
> Ofc, it is not convenient, but better than nothing. More efficient way 
> is to support replicas, so
> that every time when checksum verification failed, you upload 
> respective replica. It is not difficult
> to implement. Just allocate/release replicas simultaneously with 
> conventional formatted blocks.
> Pointers to replicas can be placed near the checksums (a new node42 
> format is needed).
> The patches 3.18.6-reiser4-add-node41-support.patch and 
> reiser4progs-1.0.9-add-node41-support.patch
> are a good example of how to add a new node format.


The cleanest option, however, would be to implement external mirrors in the
context of reiser4 logical volumes support, that I am currently working on.
So if you are patient, then wait for the announce of basic things (I 
think this
summer, but not sure).

Thanks,
Edward.