Driver for emulating a tape device on top of a cd writer...

Driver for emulating a tape device on top of a cd writer...

[Stelian Pop]

Hi folks,

I've got this idea some time ago, thinking it would really be
a neat thing...

Basically, I would like to be able to use a cdwriter as a tape
device, with software like dump(8) or tar(1). With /dev/tcdw
as name (for example), I'd like to be able to do:
        mt -f /dev/tcdw rewind
        tar cvf /dev/tcdw /
        tar cvf /dev/tcdw /home
        mt -f /dev/tcdw rewind
        mt -f /dev/tcdw fsf 1
        tar xvf /dev/tcdw
        ...

As someone said to me, this seems to exist on some other UNIX
system (don't recall if it was AIX or something else)... But
I didn't find any work on Linux in this direction. 

I'll start to work on this, probably by looking at the cdrecord 
low level code and porting it into kernel space.

The only technical problem I see are simulating the tape filemarks
on the CD. I'm not sure how to do this, maybe there is some
session information which I could use as a filemark... 

What do you think about this project ? Would it be useful ? Feasible ?
Do you know about some similar work I could use ?

Thanks,

Stelian.
-- 
Stelian Pop <stelian.pop@alcove.fr>
Alcôve - http://www.alcove.fr
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Re: Driver for emulating a tape device on top of a cd writer...

[Jens Axboe]

On Mon, Dec 18 2000, Stelian Pop wrote:
> Hi folks,
> 
> I've got this idea some time ago, thinking it would really be
> a neat thing...
> 
> Basically, I would like to be able to use a cdwriter as a tape
> device, with software like dump(8) or tar(1). With /dev/tcdw
> as name (for example), I'd like to be able to do:
>         mt -f /dev/tcdw rewind
>         tar cvf /dev/tcdw /
>         tar cvf /dev/tcdw /home
>         mt -f /dev/tcdw rewind
>         mt -f /dev/tcdw fsf 1
>         tar xvf /dev/tcdw
>         ...
> 
> As someone said to me, this seems to exist on some other UNIX
> system (don't recall if it was AIX or something else)... But
> I didn't find any work on Linux in this direction. 

What you describe is actually one of the goals of the packet writing
driver. To do this reliably you need packet writing, I won't even
start to think about the headaches wihtout it...

> I'll start to work on this, probably by looking at the cdrecord 
> low level code and porting it into kernel space.

Oh god no! You can do all this from user space.

-- 
* Jens Axboe <axboe@suse.de>
* SuSE Labs
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Re: Driver for emulating a tape device on top of a cd writer...

[Stelian Pop]

On Mon, Dec 18, 2000 at 07:04:42PM +0100, Jens Axboe wrote:

> > Basically, I would like to be able to use a cdwriter as a tape
> > device, with software like dump(8) or tar(1). With /dev/tcdw
> > as name (for example), I'd like to be able to do:
> > [...]

> What you describe is actually one of the goals of the packet writing
> driver. To do this reliably you need packet writing, I won't even
> start to think about the headaches wihtout it...

Yes, I saw your patch for packet writing but:
- the CD written with packet writing software may not be readable
  on standard CD-ROM drives (and I want that, because almost 
  everybody has one).

- using packet writing you basically write _files_ on top of an
  UDF filesystem. Tar and dump (or afio, cpio etc) does not
  support that kind of access, they expect to be given a character
  device they can stream data to. (Of course, it is possible to
  add some additionnal level of indirection on top of the packet
  device and provide character based access to the UDF files, but
  IMHO _this_ would be overkill).

- data backups are expected to be fast. Writing data in DAO/TAO
  mode is much quicker than in packet mode.

- reliability is a question of implementation. cdrecord can
  be very reliable. If a user space application can provide this
  level of reliability, it should be even simpler to achieve it
  in kernel space (and I plan to use the BurnProof/etc extensions
  which will be present on all future cdwriters).

> > I'll start to work on this, probably by looking at the cdrecord 
> > low level code and porting it into kernel space.
> 
> Oh god no! You can do all this from user space.

Please pay attention to the fact that I was refering to the 'low level
code'. I don't intend to write a driver who can replace cdrecord. 
_This_ would be madness.

What I indend to do is just a 'small' driver, which supports only the
mmc drives. I expect the driver to be only some hundreds lines long.

Doing that from user space would mean propagating the data from
the user space application (dump or tar) to a character mode
driver, and back to a user space application (something like a hacked 
cdrecord), which will return in kernel space using sg interface...
It could be easier to write (even if I don't exactly feel confident
about hacking the cdrecord source :) ), but the reliability and
the performance would be far far away...

-- 
Stelian Pop <stelian.pop@alcove.fr>
| Alcôve - http://www.alcove.fr
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Re: Driver for emulating a tape device on top of a cd writer...

[Jens Axboe]

On Tue, Dec 19 2000, Stelian Pop wrote:
> > > Basically, I would like to be able to use a cdwriter as a tape
> > > device, with software like dump(8) or tar(1). With /dev/tcdw
> > > as name (for example), I'd like to be able to do:
> > > [...]
> 
> > What you describe is actually one of the goals of the packet writing
> > driver. To do this reliably you need packet writing, I won't even
> > start to think about the headaches wihtout it...
> 
> Yes, I saw your patch for packet writing but:
> - the CD written with packet writing software may not be readable
>   on standard CD-ROM drives (and I want that, because almost 
>   everybody has one).

On CD drives sold during the last two years or so, and of course
all DVD drives they are readable. But of course of you want 100%
coverage, it isn't good enough.

> - using packet writing you basically write _files_ on top of an
>   UDF filesystem. Tar and dump (or afio, cpio etc) does not
>   support that kind of access, they expect to be given a character
>   device they can stream data to. (Of course, it is possible to
>   add some additionnal level of indirection on top of the packet
>   device and provide character based access to the UDF files, but
>   IMHO _this_ would be overkill).

Why would you even want to use UDF for this? You want raw access
to the device. Packet writing or not, this is totally unrelated.

> - data backups are expected to be fast. Writing data in DAO/TAO
>   mode is much quicker than in packet mode.

No no no, not much quicker. Write large packets and it's just
as fast as dao/tao. 64Kb packets are a bit slower because of
run-in, run-out block over head, but using larger packets this
isn't the noticable. And packet writing has so many other
advantages...

> - reliability is a question of implementation. cdrecord can
>   be very reliable. If a user space application can provide this
>   level of reliability, it should be even simpler to achieve it
>   in kernel space (and I plan to use the BurnProof/etc extensions
>   which will be present on all future cdwriters).

Even simpler to achieve reliability in the kernel? I gather you
mean feeding-data reliability, and not stability.

> > > I'll start to work on this, probably by looking at the cdrecord 
> > > low level code and porting it into kernel space.
> > 
> > Oh god no! You can do all this from user space.
> 
> Please pay attention to the fact that I was refering to the 'low level
> code'. I don't intend to write a driver who can replace cdrecord. 
> _This_ would be madness.

Very much so

> What I indend to do is just a 'small' driver, which supports only the
> mmc drives. I expect the driver to be only some hundreds lines long.

A few hundred lines? *This* I look forward to seeing :)

> Doing that from user space would mean propagating the data from
> the user space application (dump or tar) to a character mode
> driver, and back to a user space application (something like a hacked 
> cdrecord), which will return in kernel space using sg interface...
> It could be easier to write (even if I don't exactly feel confident
> about hacking the cdrecord source :) ), but the reliability and
> the performance would be far far away...

Pipes and 100% user space based, then pass to sg? I don't see the
problem.

-- 
* Jens Axboe <axboe@suse.de>
* SuSE Labs
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