Re: RedHat 7.4 Release Notes: "Btrfs has been deprecated" - wut?

[Christoph Anton Mitterer <calestyo@scientia.net>]

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Qu Wenruo wrote:
>Although Btrfs can disable data CoW, nodatacow also disables data 
>checksum, which is another main feature for btrfs.

Then decoupling of the two should probably decoupled and support for
notdatacow+checksumming be implemented?!

I'm not an expert, but I wouldn't see why this shouldn't be possible
(especially since metadata is AFAIC anyway *always* CoWed +
checksummed).


Nearly a year ago I had some off-list mails exchanged with CM and AFAIU
he said it would technically be possible...


What's the worst thing that can happen?! IMO, that noCoWed data would
have been correctly written on a crash, but not the checksum, thereby
the (bad) checksum would invalidate the actually good data.
How likely is that compared to the other way round? I'd guess not so
much.
And even if, it's IMO still better to have then false positives (which
the higher application layers should take care of anyway) than to not
notice silent data corruption at all.


Of course checksuming would possibly impact performance, but anyway
could still use nodatacow+nochecksum (or any other fs) if he focuses
more on performance than data integrity.
But all those who focus on integrity would get that, even in the
nodatacow case.


IIRC, CM brought as an argument, that some people rather get the bad
data than nothing at all (respectively EIO)... but for those btrfs is
probably anyway a bad choice (at least in the normal non-nodatacow
case),... also any application should properly deal with EIO... and
last but not least, one could still provide a special tool that, after
crash (with possibly non-matching data/csum) allows a user to find such
cases and decide what to do,... so a user/admin who rather takes the
bad data an tries for forensical recovery could be given a tool like
btrfs csum --recompute-invalid-csums (or some better name), in which
either all (or just some paths) csums are re-written in case they don't
match.


Cheers,
Chris.

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