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+ OCFS2 online file check
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+ -----------------------
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+
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+This document will describe OCFS2 online file check feature.
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+
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+Introduction
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+============
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+OCFS2 is often used in high-availaibility systems. However, OCFS2 usually
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+converts the filesystem to read-only when encounters an error. This may not be
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+necessary, since turning the filesystem read-only would affect other running
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+processes as well, decreasing availability.
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+Then, a mount option (errors=continue) is introduced, which would return the
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+-EIO errno to the calling process and terminate furhter processing so that the
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+filesystem is not corrupted further. The filesystem is not converted to
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+read-only, and the problematic file's inode number is reported in the kernel
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+log. The user can try to check/fix this file via online filecheck feature.
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+
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+Scope
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+=====
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+This effort is to check/fix small issues which may hinder day-to-day operations
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+of a cluster filesystem by turning the filesystem read-only. The scope of
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+checking/fixing is at the file level, initially for regular files and eventually
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+to all files (including system files) of the filesystem.
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+
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+In case of directory to file links is incorrect, the directory inode is
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+reported as erroneous.
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+
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+This feature is not suited for extravagant checks which involve dependency of
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+other components of the filesystem, such as but not limited to, checking if the
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+bits for file blocks in the allocation has been set. In case of such an error,
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+the offline fsck should/would be recommended.
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+
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+Finally, such an operation/feature should not be automated lest the filesystem
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+may end up with more damage than before the repair attempt. So, this has to
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+be performed using user interaction and consent.
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+
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+User interface
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+==============
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+When there are errors in the OCFS2 filesystem, they are usually accompanied
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+by the inode number which caused the error. This inode number would be the
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+input to check/fix the file.
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+
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+There is a sysfs directory for each OCFS2 file system mounting:
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+
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+ /sys/fs/ocfs2/<devname>/filecheck
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+
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+Here, <devname> indicates the name of OCFS2 volumn device which has been already
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+mounted. The file above would accept inode numbers. This could be used to
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+communicate with kernel space, tell which file(inode number) will be checked or
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+fixed. Currently, three operations are supported, which includes checking
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+inode, fixing inode and setting the size of result record history.
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+
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+1. If you want to know what error exactly happened to <inode> before fixing, do
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+
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+ # echo "<inode>" > /sys/fs/ocfs2/<devname>/filecheck/check
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+ # cat /sys/fs/ocfs2/<devname>/filecheck/check
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+
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+The output is like this:
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+ INO DONE ERROR
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+39502 1 GENERATION
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+
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+<INO> lists the inode numbers.
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+<DONE> indicates whether the operation has been finished.
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+<ERROR> says what kind of errors was found. For the detailed error numbers,
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+please refer to the file linux/fs/ocfs2/filecheck.h.
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+
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+2. If you determine to fix this inode, do
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+
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+ # echo "<inode>" > /sys/fs/ocfs2/<devname>/filecheck/fix
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+ # cat /sys/fs/ocfs2/<devname>/filecheck/fix
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+
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+The output is like this:
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+ INO DONE ERROR
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+39502 1 SUCCESS
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+
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+This time, the <ERROR> column indicates whether this fix is successful or not.
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+
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+3. The record cache is used to store the history of check/fix results. It's
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+defalut size is 10, and can be adjust between the range of 10 ~ 100. You can
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+adjust the size like this:
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+
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+ # echo "<size>" > /sys/fs/ocfs2/<devname>/filecheck/set
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+
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+Fixing stuff
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+============
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+On receivng the inode, the filesystem would read the inode and the
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+file metadata. In case of errors, the filesystem would fix the errors
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+and report the problems it fixed in the kernel log. As a precautionary measure,
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+the inode must first be checked for errors before performing a final fix.
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+
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+The inode and the result history will be maintained temporarily in a
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+small linked list buffer which would contain the last (N) inodes
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+fixed/checked, the detailed errors which were fixed/checked are printed in the
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+kernel log.
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Gang He