Messages in this thread | | | From | (Eric W. Biederman) | Date | Sun, 17 Feb 2013 17:25:43 -0800 | Subject | Re: [PATCH RFC 10/12] userns: Convert xfs to use kuid/kgid/kprojid where appropriate |
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Dave Chinner <david@fromorbit.com> writes:
> On Wed, Feb 13, 2013 at 10:13:16AM -0800, Eric W. Biederman wrote: > >> The crazy thing is that is that xfs appears to >> directly write their incore inode structure into their journal. > > Off topic, but it's actually a very sane thing to do. It's called > logical object logging, as opposed to physical logging like ext3/4 > and ocfs2 use. XFS uses a combination of logical logging > (superblock, dquots, inodes) and physical logging (via buffers).
Not putting your structures in disk-endian before putting them on-disk seems silly. As far as I can tell if you switch endianness of the machine accessing your xfs filesystem and have to do a log recover it won't work because a lot of the log entries will appear corrupted.
It also seems silly to require your in-memory structure to be binary compatibile with your log when you immediately copy that structure to another buffer when it comes time to queue a version of it to put into the log.
The fact that you sometimes need to allocate memory and make a copy so you can stuff your data into the logvec whose only purpose is to then copy the data a second time seems silly and wasteful.
Logical logging itself seems reasonable. I just find the implementation in xfs odd.
It looks like with a few little changes xfs could retain backwards compatibility with today, remove extra memory copies, and completely decouple the format of the in-core structures with the format of the on-disk structures. Allowing scary comments to be removed.
Eric
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