Mailing List Archive: A though on multi-session cds

A though on multi-session cds

Oct 7, 1999, 6:59 AM

Post #1 of 6 (499 views)

A thought on multi-session cds. I've been considering using CD-Writers
as a way to create snapshots of certain types of critical data by
writing a new track to the CD periodically (essentially creating a set
of generation datasets on the CD). One of the down sides of this
is that there isn't a particularily easy way (that I've located) to access
the data tracks beyond track 01 -- ie, mount mounts track 01, but there
isn't a way to mount a different track that also has an isofs on it.
This leaves you with having to use something like readxa to dump a track
and then use the loop device to mount the dump image.
A potentially useful improvement would be to use a mount option
(such as track=#) to be able to specify which track on a cd should be
mounted (defaulting to track 01 for backwards compatibility sake).
Any thoughts?
...Eric
Eric Kasten
kasten@nscl.msu.edu
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Re: A though on multi-session cds [ In reply to ]

ingram at symsys

Oct 7, 1999, 12:24 PM

Post #2 of 6 (494 views)

Permalink

On Thu, 7 Oct 1999, Steve Dodd wrote:
> On Thu, Oct 07, 1999 at 09:59:54AM -0400, Eric Kasten wrote:
>
> > A thought on multi-session cds. I've been considering using CD-Writers
> > as a way to create snapshots of certain types of critical data by
> > writing a new track to the CD periodically (essentially creating a set
> > of generation datasets on the CD). One of the down sides of this
> > is that there isn't a particularily easy way (that I've located) to access
> > the data tracks beyond track 01 -- ie, mount mounts track 01, but there
> > isn't a way to mount a different track that also has an isofs on it.
>
> isofs in 2.3.x (for suitable values of x, certainly >=15) has a
> session= parameter that you should be able to pass in via mount.
>
> What would be even more interesting is for cdrom block devices to make
> multiple sessions appear as partitions -- assuming people don't ever put
> _real_ partition tables on CD-ROMs, of course.
I once used a multisession CD-ROM (don't recall the brand) whose authoring
software and device driver allowed for incremental backups. The driver
would present the multisession CD-ROM as one filesystem where you only saw
the most recent version of a given file. It was pretty slick.
- Greg
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Re: A though on multi-session cds [ In reply to ]

kasten at nscl

Oct 7, 1999, 12:55 PM

Post #3 of 6 (494 views)

Permalink

> On Thu, 7 Oct 1999, Steve Dodd wrote:
>
> > On Thu, Oct 07, 1999 at 09:59:54AM -0400, Eric Kasten wrote:
> >
> > > A thought on multi-session cds. I've been considering using CD-Writers
> > > as a way to create snapshots of certain types of critical data by
> > > writing a new track to the CD periodically (essentially creating a set
> > > of generation datasets on the CD). One of the down sides of this
> > > is that there isn't a particularily easy way (that I've located) to access
> > > the data tracks beyond track 01 -- ie, mount mounts track 01, but there
> > > isn't a way to mount a different track that also has an isofs on it.
> >
> > isofs in 2.3.x (for suitable values of x, certainly >=15) has a
> > session= parameter that you should be able to pass in via mount.
> >
> > What would be even more interesting is for cdrom block devices to make
> > multiple sessions appear as partitions -- assuming people don't ever put
> > _real_ partition tables on CD-ROMs, of course.
>
> I once used a multisession CD-ROM (don't recall the brand) whose authoring
> software and device driver allowed for incremental backups. The driver
> would present the multisession CD-ROM as one filesystem where you only saw
> the most recent version of a given file. It was pretty slick.
That would be a lot like having a CD-ROM filesystem that acted like an
inherited filesystem. Thus, you could do a backup on track 1 and then
incrementally write only the files that had changed to subsequent tracks.
When you mounted the CD-ROM you'd see only the latest versions of the
files that were backed up. I remember their being an IFS in the kernel
sometime ago, but it disappeared (I think (?)).
...Eric
Eric Kasten
kasten@nscl.msu.edu
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Re: A though on multi-session cds [ In reply to ]

hpa at transmeta

Oct 7, 1999, 2:05 PM

Post #4 of 6 (497 views)

Permalink

Followup to: <Pine.LNX.4.04.9910071416590.25867-100000@maestro.symsys.com>
By author: Greg Ingram <ingram@symsys.com>
In newsgroup: linux.dev.kernel
> >
> > isofs in 2.3.x (for suitable values of x, certainly >=15) has a
> > session= parameter that you should be able to pass in via mount.
> >
> > What would be even more interesting is for cdrom block devices to make
> > multiple sessions appear as partitions -- assuming people don't ever put
> > _real_ partition tables on CD-ROMs, of course.
>
> I once used a multisession CD-ROM (don't recall the brand) whose authoring
> software and device driver allowed for incremental backups. The driver
> would present the multisession CD-ROM as one filesystem where you only saw
> the most recent version of a given file. It was pretty slick.
>
mkisofs can do this. Note that the proper default behaviour for
mounting a multisession CD-ROM is to mount the *latest* session.
-hpa
--
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
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Re: A though on multi-session cds [ In reply to ]

dirk at loth

Oct 9, 1999, 12:17 PM

Post #5 of 6 (496 views)

Permalink

On Thu, Oct 07, 1999 at 03:55:02PM -0400, Eric Kasten wrote:
[Someone wrote:]
> > I once used a multisession CD-ROM (don't recall the brand) whose authoring
> > software and device driver allowed for incremental backups. The driver
> > would present the multisession CD-ROM as one filesystem where you only saw
> > the most recent version of a given file. It was pretty slick.
> That would be a lot like having a CD-ROM filesystem that acted like an
> inherited filesystem. Thus, you could do a backup on track 1 and then
> incrementally write only the files that had changed to subsequent tracks.
> When you mounted the CD-ROM you'd see only the latest versions of the
> files that were backed up. I remember their being an IFS in the kernel
> sometime ago, but it disappeared (I think (?)).
I don't know about isofs internals, but I thought you could do this without
any special support on the reader? When laying down the new session I believe
directory entries can be written which refer to files in previous sessions,
if the file is unchanged. If it is changed, the file is written to the new
session and the new directory entry points to that. If it doesn't exist, no
directory entry is written. Or are the directories from all the sessions
overlaid, in which case whiteouts would be needed?
--
Horses are forbidden to eat fire hydrants in Marshalltown, Iowa.
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Re: A though on multi-session cds [ In reply to ]

axboe at image

Oct 10, 1999, 8:27 AM

Post #6 of 6 (494 views)

Permalink

On Thu, Oct 07 1999, Steve Dodd wrote:
> What would be even more interesting is for cdrom block devices to make
> multiple sessions appear as partitions -- assuming people don't ever put
> _real_ partition tables on CD-ROMs, of course.
I did that for 2.3, it is in current kernels. Only works on
ATAPI devices, though, since SCSI CD-ROM's use minors to
differentiate between devices. So you can do
mount /dev/hdc3 /mnt/cdrom
and get session 3 mounted.
--
* Jens Axboe <axboe@image.dk>
* Linux CD-ROM Maintainer
* http://www.kernel.dk
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