Date:	Mon, 20 Jul 2015 12:59:29 +0800
From:	Gang He <ghe@...e.com>
To:	mfasheh@...e.com, ghe@...e.com, rgoldwyn@...e.de
Cc:	linux-kernel@...r.kernel.org, ocfs2-devel@....oracle.com,
	akpm@...ux-foundation.org
Subject: [PATCH] ocfs2: add feature document for online file check

This document will describe OCFS2 online file check feature.
OCFS2 is often used in high-availaibility systems. However, OCFS2 usually
converts the filesystem to read-only on errors. 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) was introduced,
which would return the EIO 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.

Signed-off-by: Gang He <ghe@...e.com>
---
 .../filesystems/ocfs2-online-filecheck.txt         | 95 ++++++++++++++++++++++
 1 file changed, 95 insertions(+)
 create mode 100644 Documentation/filesystems/ocfs2-online-filecheck.txt

diff --git a/Documentation/filesystems/ocfs2-online-filecheck.txt b/Documentation/filesystems/ocfs2-online-filecheck.txt
new file mode 100644
index 0000000..d319237
--- /dev/null
+++ b/Documentation/filesystems/ocfs2-online-filecheck.txt
@@ -0,0 +1,95 @@
+		    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 on errors. 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) was introduced,
+which would return the EIO 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 file 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 "CHECK <inode>" > /sys/fs/ocfs2/<devname>/filecheck
+  # cat /sys/fs/ocfs2/<devname>/filecheck
+
+The output is like this:
+  INO		TYPE		DONE		ERROR
+39502		0		1		GENERATION
+
+<INO> lists the inode numbers.
+<TYPE> is what kind of operation you've done, 0 for inode check,1 for inode fix.
+<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 "FIX <inode>" > /sys/fs/ocfs2/<devname>/filecheck
+  # cat /sys/fs/ocfs2/<devname>/filecheck
+
+The output is like this:
+  INO		TYPE		DONE		ERROR
+39502		1		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 result. Its
+defalut size is 10, and can be adjust between the range of 10 ~ 100. You can
+adjust the size like this:
+
+  # echo "SET <size>" > /sys/fs/ocfs2/<devname>/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 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.
-- 
2.1.2

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