[Stgt-devel] Fwd: Implementation of vtl

[Mark Harvey]

Oops. Forgot to 'reply all'...

Forwarding to stgt mailing list - just in case I've got my facts wrong.

Cheers
Mark

---------- Forwarded message ----------
From: Mark Harvey <markh794 at gmail.com>
Date: Thu, May 8, 2008 at 7:32 AM
Subject: Re: [Stgt-devel] Implementation of vtl
To: aftab azmi <max_aftab at yahoo.com>


I'm not quite sure of your meaning of "couldn't find any value for
 filemark" so please excuse me, if I'm talking to the converted...

 A 'filemark' is a special bit pattern written to tape which is not
 actually data. Just a 'mark' so the tape drive firmware can find
 (position to) it. Many backup programs (VERITAS NetBackup, Legato
 NetWorker as larger examples) use filemarks as delimiters between
 segments of data. Especially in the older days when tape drives did
 not support the FAST BLOCK LOCATE cmd.

 The ability to be able to detect multiple filemarks will be critical
 for correct tape drive emulation.

 Re: Fixed block:
 How do you compute the block size ?

 From my quick review of the code, I did not see any reference to the
 MODE SELECT/MODE SENSE commands and the BLOCK LENGTH field. i.e. No
 way for a backup program to set / query the block size. (except the
 READ BLOCK LIMITS).

 Re: Block under/over run detection.
 This is only a requirement for variable block READ - so fixed block
we are safe.

 Cheers
 Mark





 On Thu, May 8, 2008 at 1:26 AM, aftab azmi <max_aftab at yahoo.com> wrote:
 > Actually i couldnt find any value for file mark so i choose ascii
code 28(file separator) as filemark.i guess it will not be present in
any user file so will not produce false positive...
 >
 >  I have assummed fixed block write..i forgot to include check for
the FIXED bit in read and write command..
 >
 >  For variable block write i think we have to use your
structure...for underrun we can compare the blk_size in blk_header
against the MINIMUM BLOCK LENGTH LIMIT field in READ BLOCK LIMITS data
and for overrun we can compare blk_size and MAXIMUM BLOCK LENGTH
LIMIT.
 >
 >
 >
 >  ----- Original Message ----
 >  From: Mark Harvey <markh794 at gmail.com>
 >  To: aftab azmi <max_aftab at yahoo.com>
 >  Cc: stgt-devel at lists.berlios.de; fujita.tomonori at lab.ntt.co.jp
 >  Sent: Wednesday, 7 May, 2008 1:52:45 PM
 >  Subject: Re: [Stgt-devel] Implementation of vtl
 >
 >  Hello aftab azmi,
 >
 >  First off, thanks... You beat me to the punch :) It might save me
some work....
 >
 >  While I can't speak authoritatively, I'm sure this will be rejected
 >  first off due the the code base not following the Linux kernel coding
 >  style... You can find the coding style document in the Linux kernel
 >  source under the Documentation dir.
 >
 >  I've passed it thru the scripts/checkpatch.pl script and attached are
 >  the coding style errors: (Lots of tab/space issues)
 >
 >  2nd. The 'detection' of a filemark appears a little week. i.e. first
 >  byte at a 512 byte boundary to contain '28'. This could produce many
 >  false positives.
 >
 >  3rd. I see no way to detect variable block underrun/overrun detection
 >  (the main sticking point with my code base)..
 >
 >  4th. You assume variable block writes. This code would fail if the
 >  user set the tape block size to 'fixed block'.
 >
 >  Point 1 is easily resolved with a little effort.
 >  Do you have any ideas on the solution to points 2 & 3 (just not coded yet) ?
 >  Point 4 is an easy code fix.
 >
 >  Keep up the good work.
 >
 >  p.s. To make it much easier for people on this list to review the
 >  code, please refer to the Linux kernel 'submitting patches' document.
 >  You will receive much faster code reviews...
 >
 >  Regards
 >  Mark Harvey
 >
 >  On Tue, May 6, 2008 at 5:24 PM, aftab azmi <max_aftab at yahoo.com> wrote:
 >  > Hiii everyone...
 >  >     Attached herewith  is our implementation  of  Virtual  Tape
Library....I have implemented all the mandatory commands mentioned in
ssc-r42  except  erase command.....
 >  >  For this to run u have  to change the default-bst in iscsid.c  to "ssc"..
 >  >  We can use tar command to read/write  backup..
 >  >  Please do send in your comments and suggestions for further devlopment.
 >  >
 >  >
 >  >
 >  >
 >  >
 >  >
 >  >       Bollywood, fun, friendship, sports and more. You name it,
we have it on http://in.promos.yahoo.com/groups/bestofyahoo/
 >  > _______________________________________________
 >  >  Stgt-devel mailing list
 >  >  Stgt-devel at lists.berlios.de
 >  >  https://lists.berlios.de/mailman/listinfo/stgt-devel
 >  >
 >  >
 >
 >
 >
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 >
 >