Re: e2fsck -c to check for bad blocks

On Wed, 13 Feb 2008 11:12:52 -0500
Haines Brown <brownh@hartford-hwp.com> wrote:

> I gather one can use the e2fsck with -c option to have it call
> /sbin/badblocks to report bad blocks on an unmounted partition.

This option causes e2fsck to use badblocks(8) program to do a read-only scan of the device in order to find any bad blocks. If any bad blocks are found, they are added to the bad block inode to prevent them from being allocated to a file or directory. If this option is specified twice, then the bad block scan will be done using a non-destructive read-write test.

>
> 1. Although the -c option causes fsck to use badblocks to identify any
> bad blocks present, does e2fsck then proceed to use this information to
> fix corruption as usual? That is, how does
>
> # e2fsck -cy /dev/sda1
>
> differ from simply:
>
> # e2fsck -y /dev/sda1
>

Yes, it does fix it. "they are added to the bad block inode"

> 2. If badblocks is non-destructive, why does the targeted filesystem
> have to be unmounted?

Does it have to be? What does -f say? fsck always complains if the fs is mounted.

Note that in general it is not safe to run e2fsck on mounted filesystems. The only exception is if the -n option is specified, and -c, -l, or -L options are not specified. However, even if it is safe to do so, the results printed by e2fsck are not valid if the filesystem is mounted. If e2fsck asks whether or not you should check a filesystem which is mounted, the only correct answer is ‘‘no’’. Only experts who really know what they are doing should consider answering this question in any other way.

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Badblocks may be non-destructive, but adding the found blocks to the inode may be bad. You don't know what fsck will do other than running badblocks.

>
> 3. While e2fsck is run on an unmounted file system, the man page says,
> "If this [-c] option is specified twice, then the bad block scan will be
> done using a non-destructive read-write test." Does this "specified
> twice" simply mean "-cc"? If the test is non-destructive, can it be run
> on a mounted filesystem? I assume not, but wanted to be sure.

correct.

>
> 4. Both badblocks and e2fsck -c can identify bad blocks as part of a
> check of hard disk viability. Is the difference only that while
> badblocks just reports bad blocks, e2fsck -c actually goes ahead and
> tries to fix them?

correct.

>
> --
>
> Haines Brown, KB1GRM
>

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