Linux – What does the errors=continue mount option mean

This is an attempt to summarize from the chat troubleshooting session.

The setup turns out to be physical disk -> mdraid raid1 -> LVM. So there are several layers to work through. The old setup was (due to unfortunate prior recovery efforts) not available.

However, the NAS gui had been used to create another volume on a different disk, and thankfully the GUI created the new volume exactly the same way. So it was possible to discover the setup from the new disk:

• mdadm -E new-disk provided the offset to the start of the data, under the mdraid layer (2048 sectors).
• dmsetup table provided the start block of the logical volume (relative to the start of the physical volume) (1152 sectors)
• There is a magic number (0x53ef) in the third sector of an ext4 volume. Using dd and xxd, we verified that the magic number is present at that offset on the disk we're trying to recover data from.

Armed with the start sector of the ext4 filesystem, you can use a read-only loop device to recover the data:

# losetup /dev/loop0 -o $((512*(1152+2048))) -r /dev/sda1
# mount -text4 -o ro /dev/loop0 /mnt

And then copy it off.

Don't get misled by the fact that only writeback mentions internal filesystem integrity.
With ext3, whether you use journal, ordered or writeback, file system metadata is always journalled and that means internal file system integrity.

The data modes offer a way of control over how ordinary data is written to the file system.
In writeback mode, metadata changes are first recorded in the journal and a commit block is written. After the journal has been updated, metadata and data write-outs may proceed. data=writeback can be a severe security risk: if the system crashes while appending to a file, after the metadata has been committed (and additional data blocks allocated), but before the data has been written (data blocks overwritten with new data), then after journal recovery that file may contain blocks filled with data from previously deleted files – from any user¹.

So, if data integrity is your main concern and speed is not important, data=journal is the way to go.