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Ext3 filesystem modified while mounted read-only



I noticed that ext3 filesystems are modified if mounted read-only:

# mke2fs -j /dev/hdb1

[...]

# md5sum /dev/hdb1 > MD5SUM
# mount -t ext3 -o ro /dev/hdb1 /mnt/test1
# umount /mnt/test1
# md5sum -c MD5SUM 
md5sum: MD5 check failed for '/dev/hdb1'

On examination of a block-for-block copy of 'before' and 'after' images
with od and diff, I believe that the changes are the transaction number
in the journal being increased. I have also noticed that the filesystem
is not modified if mounted read-only via a loopback device.

Can anyone explain why it is necessary to increment transaction numbers
in a read-only mount? Or why this does not happen when a loopback device
is used? 

Thanks
Graham
-- 
If you have any trouble sounding condescending, find a Unix user to show you 
how it's done.
        -- Scott Adams




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