Re: tapefs
Michael Wilhelm (michael@mydot.com)
Wed, 16 Sep 1998 07:22:35 -0500
>
> A tape file-system by any name is just a program (read user-mode)
> that puts directories for all the files on the tape SOMEWHERE.
>
> You can preallocate some blocks at the beginning for a directory
> that contains directory information plus the starting offset tape
> block, or you can put directory information between the files
> themselves. Both methods have been used by Digital. You can make
> a RMS file-system on a tape and then "mount" it, or you can
> mount a raw tape (called "foreign" in DEC lingo). Note that
> `tar` creates a "file-system" on the tape so you can restore
> individual files, etc.
>
> The problem is that a tape is a sequential device. You can't extend
> a file, you can't delete a file (although you can pretend to by
> marking its directory entry deleted).
>
> The result is a write-once file-system with abysmal performance.
> In the "olden" days, we had reel-to-reel tapes that could "seek".
> These tapes could rapidly find "file-marks" so you could get some
> random-access. The modern tape drives don't "remember" where they
> are, so to count file-marks, you have to rewind and start from
> the beginning. They are just not designed for random access.
I would have to disagree with this... a lot of high end SCSI drives
like dat, or dlt drives can randomly acesss the tape. you can seek
to any point on the drive.. I have a scsi dat that that way I can write
a tar file then type in mt tell and it will tell me it position take the tape
out place it back in type in mt seek number (number is the number
i got from mt tell) and it will seek to that point with in less than 5-10
sec. so I still think that a tape fs can be writen with some farly quick
response time..
> Nevertheless, a user-mode program can be written to process file-
> system data, buffer it to improve performance, and write dirty
> buffers to a tape. You can even run such a program as a daemon
> and "mount" it so, to a user, it looks like an ext2 file-system.
>
> It becomes basically a RAM disk, with overflow , and everything
> necessary to restore it, written to the tape.
>
> However, before you write such a thing, experiment with a simple
> program that opens/writes/rewinds/reads your tape. The performance
> you get is positively awful. I've done this to stream raw data
> to a SCSI Tape, once it gets going, it's okay. However, a rewind
> to read takes ...er well, maybe you can compile the kernel while
> you are waiting.
>
> FYI, the source-code for `mt` contains everything you need to know
> about accessing these beasties.
>
> Cheers,
> Dick Johnson
> ***** FILE SYSTEM WAS MODIFIED *****
> Penguin : Linux version 2.1.118 on an i586 machine (66.15 BogoMips).
> Warning : It's hard to remain at the trailing edge of technology.
------------
Michael W.
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