Re: is there a COW inside the kernel ?

[Kevin Corry <kevcorry@us.ibm.com>]

On Fri March 3 2006 2:29 am, roland wrote:
> hello !
>
> is there an equivalent of something like
>
> cowloop ( http://www.atconsultancy.nl/cowloop/total.html ) or md based cow
> device ( http://www.cl.cam.ac.uk/users/br260/doc/report.pdf ),
>
> i.e. a feature called "Copy On Write Blockdevice" inside the current or the
> near-future mainline kernel (besides UserModeLinux Arch)?

Device-Mapper has a snapshot module, which is used by LVM and EVMS. You can 
also use dmsetup if you want lower-level access than provided by the volume 
managers. To do the equivalent of the cowloop driver that you linked to 
above, you could do something like this:

Say you have a read-only block-device (say a cd-rom) at /dev/hdc. And you have 
a small disk partition, /dev/hdb1, that you want to use for your "COW file". 
Run:

cow_size=`blockdev --getsize /dev/hdc`
chunk_size=64   # Size of each copied-on-write chunk, in 512 byte sectors
cow_name="my_cow_dev"
echo "0 $cow_size snapshot /dev/hdc /dev/hdb1 p $chunk_size" | \
   dmsetup create $cow_name

This will give you a device called /dev/mapper/$cow_name. Presuming /dev/hdc 
has a filesystem on it, you can mount /dev/mapper/$cow_name and get a 
read-write version of the filesystem on /dev/hdc, where updates to the 
filesystem will be stored on /dev/hdb1. The size of /dev/hdb1 can be 
significantly smaller than /dev/hdc, depending on the amount of writes you 
expect to happen on /dev/mapper/$cow_name. While this device is active, don't 
try to mount /dev/hdc read-write (assuming that's possible), or it will 
corrupt the view of /dev/mapper/$cow_name. If you need read-write access to 
both devices simultaneously, you'll probably just want to use LVM or EVMS and 
create snapshot volumes, since manually activating that kind of setup with 
dmsetup is incredibly tricky.

Use "dmsetup remove $cow_name" to deactivate the device.

> i would find this useful for several purpose, but i don`t want to patch my
> system with 3rd party drivers or "non-standard" stuff -  or even recompile
> the kernel.

This should work with any recent 2.6 kernel. You'll also need to have the 
device-mapper package installed, which should be available with any recent 
Linux distro.

-- 
Kevin Corry
kevcorry@us.ibm.com
http://www.ibm.com/linux/
http://evms.sourceforge.net/
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