On Wed, Nov 25, 2015 at 12:01:49AM +0100, Christoph Anton Mitterer wrote: > On Tue, 2015-11-24 at 22:33 +0000, Hugo Mills wrote: > > whereas a read-only mount of a journalling FS _must_ modify the disk > > data after an unclean shitdown, in order to be useful (because the FS > > isn't consistent without the journal replay). > I've always considered that rather a bug,... or at least a very > annoying handling in ext* > If I specify "read-only" than nothing should ever be written. > If that's not possible because of an unclean shutdown and a journal > that needs to be replayed, the mount should (without any further > special option) rather fail then mount it pseudo-read-only.
At one point, I _think_, btrfs did replay the log tree unconditionally, even on a RO mount, but it doesn't any more. There was certainly some discussion on the point. It's actually quite handy sometimes -- if you have a corrupt log tree, you can check it by mounting RO (when it works) and RW (when it fails because the log tree is broken), and the do btrfs-zero-log to clear it. For the record, this is about the only good use for btrfs-zero-log. It doesn't magically fix anything else. (Yes, this is another futile attempt at killing the persistent "btrfs-zero-log fixes everything" meme that's been doing the rounds for the last few years). Hugo. -- Hugo Mills | Never underestimate the bandwidth of a Volvo filled hugo@... carfax.org.uk | with backup tapes. http://carfax.org.uk/ | PGP: E2AB1DE4 |
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