akpm at linux-foundation.org
2015-Aug-21 21:41 UTC
[Ocfs2-devel] [patch 24/24] ocfs2: add feature document for online file check
From: Gang He <ghe at suse.com> Subject: ocfs2: add feature document for online file check This document describes the 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 at suse.com> Cc: Goldwyn Rodrigues <rgoldwyn at suse.de> Cc: Mark Fasheh <mfasheh at suse.com> Cc: Joel Becker <jlbec at evilplan.org> Signed-off-by: Andrew Morton <akpm at linux-foundation.org> --- Documentation/filesystems/ocfs2-online-filecheck.txt | 95 ++++++++++ 1 file changed, 95 insertions(+) diff -puN /dev/null Documentation/filesystems/ocfs2-online-filecheck.txt --- /dev/null +++ a/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. _