Subject: Re: Maximum blocking factor

From: tody at noao.edu (Doug Tody)
Date: 4 Oct 91 21:37:17 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).

Yes, the huge (2 Mb) file mark associated with current Exabyte drives is a
serious problem if you write many small FITS files.  If you are going to
write many small files you are probably better off with DAT, which has a
small file mark and which can position to any file very quickly.
-- 
Doug Tody, National Optical Astronomy Observatories, Tucson AZ, 602-325-9217
UUCP: {arizona,decvax,ncar}!noao!tody  or  uunet!noao.edu!tody 
Internet: tody at noao.edu             SPAN/HEPNET: NOAO::TODY (NOAO=5355)