Chris Mason wrote on 2015/11/30 11:48 -0500:
On Sat, Nov 28, 2015 at 01:46:34PM +0000, Hugo Mills wrote:
We've just had someone on IRC with a problem mounting their FS. The
main problem is that they've got a corrupt log tree. That isn't the
subject of this email, though.
The issue I'd like to raise is that even with -oro as a point
option, the FS is trying to replay the log tree. The dmesg output from
mount -oro is at the end of the email.
Now, my memory, experience and understanding is that the FS
doesn't, and shouldn't replay the log tree on a RO mount, because the
FS should still be consistent even without the reply, and
RO-means-actually-RO is possible and desirable. (Compared to a
journalling FS, where journal replay is required for a consistent,
usable FS).
So, this looks to me like a regression that's come in somewhere.
(Just for completeness, the system in question usually runs 4.2.5,
but the live CD the OP is using is 4.2.3).
We do need to replay the log tree, even on readonly mounts. Otherwise
files created and fsunk before crashing may not even exist.
We'll bail out of the log replay on readonly media, but otherwise the
replay always happens.
-chris
Or disable log_tree (making fsync as slow as sync).
And there will be no log replay, making RO mount real RO.
I think we can add it to kernel btrfs documentation.
Or, in my wildest dream, introduce a per-inode tree to record file
extents/dir items.
Then fsync will only need to sync the inode file extent/dir item
tree.(and its direct parent maybe)
And better random read/write performance.
Although that's just my dream....
But I'm a little curious about why btrfs choose to pack dir items and
file extents into the same subvolume tree at design time.
Unlike most of other file systems(ext4 for example).
Is it just designed for simplicity?
Thanks,
Qu
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