[Stgt-devel] Thoughts on vtl setup

[Ming Zhang]

On Thu, 2007-04-05 at 21:32 +0200, Albert Pauw wrote:
> Here are a few thoughts I wanted to share regarding the virtual tape
> library, just shoot at it if you think otherwise. Hope you don't find
> this too nosy Mark (Harvey) as you started the idea.
> 
> General - Best would be to have the changer and the tapedrives on one
> target, the changer can be LUN 0, the drive(s) LUN 1 and onwards.

in real world, tape and changer most time does not use 1 target. though
some of them are. so ideally is to have both.

of course, for simplicity purpose, i agree with u. but since it is
emulation, it should looks like real stuff.

> 
> Configuration - Would be best to keep this simple. The changer can be
> a standard one, implementing all necessary functions. For the drives
> you define the type once, and how many drives you want. If you use
> symbolic names (as to numeric) for the drive types, then this can be
> easily expanded. E.g. LTO3, DDS4, etc. for the drive type. If you need
> three different type of drives, then you just add one of each type.
> Since these types are predefined you don't have to set other
> parameters for these drives (like size etc). Lastly you define the
> directory where the tapeslots are.

problem is different real changer will not always perform the same and
some have features others do not have. so no one emulation fit all
solution.

ideal way is the code implement as many feature sets as possible. and
specific model can enable/disable or override the generic
implementation.


> 
> Tapes - Well, virtual tapes. I guess the filename can be the barcode,
> the extension the tape type, e.g. A00001.LTO2. However, when these
> files are accidentally renamed you loose the barcode and type. So
> every tape should have a header containing the barcode, and tape type.
> The barcode and tapetype should be in clear text (e.g. 4 characters
> LTO2). Another addition could be a two-byte field, containing
> bitfields, like this:
> 
> 0000 0000 0000 RCCC

yes. need a header or extra meta file to describe the tapes.

> 
> were
> CCC -> compression 0-7, where 0 is no compression, 1 is LZ, etc. for
> different types of compression, if implemented. We can start without
> compression.

> 
> R -> Readonly bit, even if the file was readonly and set to readwrite,
> it would protect the data. Don't know if this setting can be done in
> the SCSI set.

virtual tape drive can read this and disable write. 

> 
> Tape handling - Before starting stgt vtl and point it to the tapeslot
> directory you create tape slot subdirectories like this, where you
> define at startup the directory /var/tape as the slot directory:
> 
> /var/tape/slot-01
> ...
> /var/tape/slot-24
> 
> In the /var/tape directory there would be symbolic links like this
> 
> /var/tape/drive-1 -> /var/slot-01/A00001.LTO3
> 
> if a tape is loaded into drive-1, when a drive is empty (startup
> setting) it does not have a symbolic link.
> 
> Note that by creating these directories by hand you can define how
> many slots you have, but also leave existing "tapes" in the slots
> between restarts of the vtl stgt service. And stopping the service
> should delete the symbolic drive links, so the drives "eject" the
> virtual tape and start up "empty" next time.

this sounds interesting. but what is the action to "insert" a tape to
drive? then program has to capture the symlink event.


> 
> Well, just a thought.
> 
> Albert
> 
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