Re: [PATCH v4 00/13] Add support for other checksums

[David Sterba <dsterba@suse.cz>]

On Mon, Jun 03, 2019 at 09:56:06PM +0200, waxhead wrote:
> Johannes Thumshirn wrote:
> > This patchset add support for adding new checksum types in BTRFS.
> > 
> > Currently BTRFS only supports CRC32C as data and metadata checksum, which is
> > good if you only want to detect errors due to data corruption in hardware.
> > 
> > But CRC32C isn't able cover other use-cases like de-duplication or
> > cryptographically save data integrity guarantees.
> > 
> > The following properties made SHA-256 interesting for these use-cases:
> > - Still considered cryptographically sound
> > - Reasonably well understood by the security industry
> > - Result fits into the 32Byte/256Bit we have for the checksum in the on-disk
> >    format
> > - Small enough collision space to make it feasible for data de-duplication
> > - Fast enough to calculate and offloadable to crypto hardware via the kernel's
> >    crypto_shash framework.
> > 
> > The patchset also provides mechanisms for plumbing in different hash
> > algorithms relatively easy.
> > 
> 
> Howdy , being just a regular user I am in fact a bit concerned about 
> what happens to my delicious (it's butter after all) filesystems if I 
> happen to move disks between servers. Let's say server A has a 
> filesystem that support checksum type_1 and type_2 while server B only 
> supports type_1.
> 
> If the filesystem only has checksum of type_2 stored I would assume that 
> server B won't be able to read the data.
> 
> Ignoring checksums will kind of make BTRFS pointless, but I think this 
> is a good reason to consider adding a 'ignore-checksum' mount option - 
> at least it could make the data readable (RO) in a pinch.

That's a good idea. The availability of checksum modules is
unpredictable so we should provide some way to access the filesystem.

> ....actually since you could always fall back to the original crc32c 
> then perhaps RO might not even be needed at all ?!

The checksum type is per-filesystem, so write support cannot be enabled.
Theoretically, switching checksum should be possible with scrub that
will switch that on-the fly, but this needs to be tought out due the
intermediate state.