E2compr 0.4 User Manual – loop

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There’s no problem with mounting ext2 filesystems (whether or not theyuse ext2 compression) through the loop driver on anon-ext2-compressed file.

However, you can’t mount a loop-back device on an ext2-compressed file,like:

# chattr +c myfile# losetup /dev/loop0 myfile

If you want to do this (i.e. create a large block device from a smaller file orblock device) then you should look at DouBle, which was built for thisvery purpose. (See section Reference for DouBle.)

For those interested: The reason the loop driver won’t work on acompressed file is that the loop driver uses bmap, i.e. it asksthe filesystem “Which blocks hold this file?” and tries to access thatblock directly. Since the file data is stored in compressed format,there is no correct answer to that question. You’ll get a couple ofsyslog messages, like so:

EXT2-fs warning (device 03:02): ext2_bmap: compressed clusterloop: block 1 not presentend_request: I/O error, dev 07:00, sector 2

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