Re: [Ocfs2-devel] [PATCH v2 0/4] Add online file check feature

[Gang He]

Hello Srini,

There is a doc about ocfs2 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 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. So,the filesystem is
not converted to read-only, and the problematic file's inode number is reported
in the kernel log so that 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 details, 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 fix history will be maintained temporarily in a
small linked list buffer which would contain the last (N) inodes
fixed/checked, along with the logs of what errors were reported/fixed.

Thanks
Gang


>>> 
> Hi Gang,
> 
> thank you for implementing this. I would like to understand this better 
> on where and how it helps ... would you mind sharing couple 
> examples(real scenarios).
> 
> Thanks,
> --Srini
> 
> 
> On 10/27/2015 11:25 PM, Gang He wrote:
>> 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/devname/filecheck
>> or
>> $# echo "FIX <inode>" > /sys/fs/ocfs2/devname/filecheck
>>
>> Compare with first version, I use strncasecmp instead of double strncmp
>> functions. Second, update the source file contribution vendor.
>>
>> Gang He (4):
>>    ocfs2: export ocfs2_kset for online file check
>>    ocfs2: sysfile interfaces for online file check
>>    ocfs2: create/remove sysfile for online file check
>>    ocfs2: check/fix inode block for online file check
>>
>>   fs/ocfs2/Makefile      |   3 +-
>>   fs/ocfs2/filecheck.c   | 566 
> +++++++++++++++++++++++++++++++++++++++++++++++++
>>   fs/ocfs2/filecheck.h   |  48 +++++
>>   fs/ocfs2/inode.c       | 196 ++++++++++++++++-
>>   fs/ocfs2/inode.h       |   3 +
>>   fs/ocfs2/ocfs2_trace.h |   2 +
>>   fs/ocfs2/stackglue.c   |   3 +-
>>   fs/ocfs2/stackglue.h   |   2 +
>>   fs/ocfs2/super.c       |   5 +
>>   9 files changed, 820 insertions(+), 8 deletions(-)
>>   create mode 100644 fs/ocfs2/filecheck.c
>>   create mode 100644 fs/ocfs2/filecheck.h
>>
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/