[Ocfs2-devel] [RFC] Online File(system) check

[Goldwyn Rodrigues]

On popular demand, here is an RFC. If you think there is a better
way to communicate with the kernel module for the check, please
let me know.


Intro
-----
OCFS2 is often used in high-availaibility systems. However, ocfs2
converts the filesystem to read-only at the drop of the hat. This
may not be necessary, since turning the filesystem read-only would
affect other running processes as well, decreasing availability.

This attempt is to add errors=continue, which would return the EIO
to the calling process and terminate furhter processing so that
the filesystem is not corrupted further. However, the filesystem
is not converted to read-only.

Scope
-----
This effort is to fix small issues which may hinder day-today operations
of a cluster filesystem by turning the filesystem read-only. The scope of
fixing is at the file level, initially for regular files and eventually
to all files (including system files) of the filesystem.

In case of directory to file links is incorrect, the directory inode
is reported as erroneous.

This feature is not suited for extravagant checks which involve dependency of
other components of the filesystem, such as but not limited to, checking if the bits for file blocks in the allocation has been set. In case of such an error,
the offline fsck should/would be recommended.

Finally, such an operation/feature should not be automated lest the filesystem
may end up with more damage than before the repair attempt. So, this has to
be performed using user interaction and consent.


Communication
-------------
When there are errors in the ocfs2 filesystem, they are usually accompanied
by the inode number which caused the error. This inode number would be the
input to fixing the file.

One of these options could be considered:

A file in the sys filesytem which would accept inode numbers. This
could be used to communication back what has to be fixed or is fixed.
You could write:
  # echo "CHECK <inode>" > /sys/fs/ocfs2/filecheck
  or
  # echo "FIX <inode>" > /sys/fs/ocfs2/filecheck


Fixing stuff
------------

On receivng the inode, the filesystem would read the inode and the
file metadata. In case of errors, the filesystem would fix the errors
and report the problems it fixed. As a precautionary measure, the
inode must first be checked for errors before performing a final fix.

The inode and the fix history will be maintained temporarily in a
small linked list buffer which would contain the last (N) inodes
fixed/checked, alongwith the logs of what errors were reported/fixed.


Comments/Criticism welcome.