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SubjectRe: is there a COW inside the kernel ?
hello !

thanks to all for providing that information.

i think i will take a closer look on device-mapper, but i'm unsure if it`s
perfectly suited.

can i only use devices, not files for the cow?
what about merging a cow-dev/file back to the r/o-dev/file ?
cowloop can do this, and because it can use files, i don`t need to provide a
fixed amount of diskspace for the cow. you have direct feedback about how
big you cow grows....
i'm unsure about dm, but i will dig into the details. (wanted to learn more
about dm anyway)

any chance of cowloop being merged into mainline ?
loop.c is 1343 lines of code, cowloop.c is around 1000 lines more so it`s
"reasonable" small....and it has a _really_ nice user manual.


btw - mountlo looks really "freaky" - i like uml very much, but i think i
need each bit of performance.
will take a look at mountlo just for personal interest, though.

----- Original Message -----
From: "Kevin Corry" <>
To: <>
Cc: "roland" <>
Sent: Friday, March 03, 2006 3:28 PM
Subject: Re: is there a COW inside the kernel ?

> On Fri March 3 2006 2:29 am, roland wrote:
>> hello !
>> is there an equivalent of something like
>> cowloop ( ) or md based
>> cow
>> device ( ),
>> 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

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