ocfs2: add feature document for online file check

		    OCFS2 online file check

		    -----------------------

This document will describe OCFS2 online file check feature.

Introduction

============

OCFS2 is often used in high-availaibility systems. However, OCFS2 usually

converts the filesystem to read-only when encounters an error. This may not be

necessary, since turning the filesystem read-only would affect other running

processes as well, decreasing availability.

Then, a mount option (errors=continue) is introduced, which would return the

-EIO errno to the calling process and terminate furhter processing so that the

filesystem is not corrupted further. The filesystem is not converted to

read-only, and the problematic file's inode number is reported in the kernel

log. The user can try to check/fix this file via online filecheck feature.

Scope

=====

This effort is to check/fix small issues which may hinder day-to-day operations

of a cluster filesystem by turning the filesystem read-only. The scope of

checking/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.

User interface

==============

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 check/fix the file.

There is a sysfs directory for each OCFS2 file system mounting:

  /sys/fs/ocfs2/<devname>/filecheck

Here, <devname> indicates the name of OCFS2 volumn device which has been already

mounted. The file above would accept inode numbers. This could be used to

communicate with kernel space, tell which file(inode number) will be checked or

fixed. Currently, three operations are supported, which includes checking

inode, fixing inode and setting the size of result record history.

1. If you want to know what error exactly happened to <inode> before fixing, do

  # echo "<inode>" > /sys/fs/ocfs2/<devname>/filecheck/check

  # cat /sys/fs/ocfs2/<devname>/filecheck/check

The output is like this:

  INO		DONE	ERROR

39502		1	GENERATION

<INO> lists the inode numbers.

<DONE> indicates whether the operation has been finished.

<ERROR> says what kind of errors was found. For the detailed error numbers,

please refer to the file linux/fs/ocfs2/filecheck.h.

2. If you determine to fix this inode, do

  # echo "<inode>" > /sys/fs/ocfs2/<devname>/filecheck/fix

  # cat /sys/fs/ocfs2/<devname>/filecheck/fix

The output is like this:

  INO		DONE	ERROR

39502		1	SUCCESS

This time, the <ERROR> column indicates whether this fix is successful or not.

3. The record cache is used to store the history of check/fix results. It's

defalut size is 10, and can be adjust between the range of 10 ~ 100. You can

adjust the size like this:

  # echo "<size>" > /sys/fs/ocfs2/<devname>/filecheck/set

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 in the kernel log. As a precautionary measure,

the inode must first be checked for errors before performing a final fix.

The inode and the result history will be maintained temporarily in a

small linked list buffer which would contain the last (N) inodes

fixed/checked, the detailed errors which were fixed/checked are printed in the

kernel log.