Subject: Blocking factor

From: forveill at gag.observg.grenet.fr (Thierry Forveille)
Date: Tue, 8 Oct 1991 08:33:01 GMT

>>From article <9110031830.AA05079 at fits.cx.nrao.edu>, by CUR%STARLINK.RUTHERFORD.AC.UK at VTVM2.CC.VT.EDU (Malcolm J. Currie):
>> In the IRAF 2.9.1 patch of Sept. 6 there is an `enhancement' that makes
>> the maximum FITS blocking factor 22 rather than 10.  In my humble
>> opinion this is a mistake.  I think that the importance of
>> transportability far outweighs the efficiency gain.  Users should be
>> protected from shooting themselves (or the recipients of the
>> non-standard tape) in the foot.
>
>The program is capable of the higher blocking factor but will issue a stern
>warning message that the tape is non standard and may be unreadable
>elsewhere if the user attempts to do this.  Obviously, any user producing a
>tape to be read elsewhere should produce a standard FITS tape.  FITS is not
>always used to produce data to be read elsewhere.
>
>> On the question of increasing the maximum blocking factor for fixed-
>> block sequential media, I'm not convinced that we need blocksizes
>> greater than 28800 bytes for Exabyte...
>
>The blocking factor has little effect on tape usage for either Exabyte or
>DAT since the drives are intelligent and will reblock the data and store
>it on the tape in fixed size blocks.  The only wasted storage is that
>required to pad the last physical device record to accomodate the logical
>block size specified by the client.  In principle a larger block size
>could aid streaming but in tests I have performed the block size has had
>little effect on the sustaiied transfer rate.
>
>The larger block size is useful mainly for 6250 bpi reel tape tape and
>cartridge tape.  6250 bpi tapes require more storage for a smaller blocking
>factor.  In the case of cartridge tapes this doesn't matter but the larger
>block can aid streaming (this is best dealt with at a lower level than FITS
>however).

Even with stern warning messages, this alteration of the maximum blocking 
factor remains a violation of the FITS standard. If the IRAF group wants 
to stick to their "enhancement", they should simultaneously set SIMPLE
to F, and shouldn't call the result a FITS file. 

A standard is a standard! When the IRAF group vetoed hierarchical keywords
a few years ago, the radio community kept complying to the unmodified
FITS agreement, even though most of these tapes will never enter a non-radio
package. Even though many FITS tapes aren't meant to be read elsewhere in 
principle, some of them do turn out to. Even worse, some tapes will not be 
read elsewhere, but they will be read later, and some of them will survive 
IRAF.

The fact that this modification is only justified by a marginal capacity 
enhancement on an already obsolescent medium makes it even harder to 
justify. Who cares about 20% on a 6250 bpi reel when DATs and Exabyte
drives are now cheaper than any tape drive?

	Thierry Forveille
        Observatoire de Grenoble
        forveill at frgag51.bitnet