LJ Archive

Letters to the Editor


Issue #11, March 1995

Readers sound off.

Hardware vs. Software

In “An introduction to block device drivers” (Issue 9, January) you write:

“... you will not be able to mount the tape, even though it contains the same information as the disk”.

I hope you know that the half-inch tape drives have all the features allowing half-inch tapes, written in a standard way, to be mounted read-only and (if the tape quality permits) to use the tape as a full-featured random-access read/write media after formatting.

I believe the sentence I've quoted has been written to simplify things and not because of lack of knowledge.

Also, I have another question close to the topic. Why are there no raw disk devices in Linux, unlike, for example, BSD and SVR3 (I don't know for sure about SVR4)? Is it because the buffering is implemented in a more correct way so that one is able to do fsck on a block device and then mount it immediately or because of something else?Sincerely,Leo Broukhis leo@zycad.com

LJ Responds:

I was perhaps not explicit enough. My statement was not intended to say anything about the hardware capabilities. The Linux kernel does not have block device drivers for any form of tapes and, therefore, is unable to mount them as filesystems, even if the hardware supports it.

The reason that there are no raw devices is that the VFS is implemented such that an open of a block device, while it does buffer, is immediate. See block_read() and block_write() and notice that they aren't used for reading and writing blocks on mounted devices, but only opened devices.

rm Your Way to Fun and Disaster!

I found the remove article interesting (in January 1995). The remove alias needs to be more general. It only handles one file: you need to do something like:

alias rm='mv $* ~/.rm'

Actually, it's better to use the -b (backup) option in GNU fileutils and do:

alias rm='mv -b $* ~/.mv'

and default to numbered backups. Then, if you remove foo.c in two different directories, you'll get entries like:

foo.c      foo.c.~1~

so you can recover each one, with some work.

I also find the /bin/rm -r ~/.rm in your .profile incredibly bad.

You often find you want to check something after you log out, so if you log in, you automatically erase your backup right away. It is much better to use cron.

Also, I haven't used it, but I want to try the MIT Athena delete/undelete system (posted to comp.source.misc, v17). It may be worth a try. You should also review this.Marty Leisner leisner@sdsp.mc.xerox.com

LJ Responds:

The MIT Athena delete/undelete system sounds interesting; we hope to review it. Mark answers another letter about that column at the end of his column for this month.

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