Subject: Exabyte tape marks (was Re: Maximum blocking factor)

From: sla at fast.ucsc.edu (Steve Allen)
Date: 4 Oct 91 22:39:32 GMT

In article <9110031830.AA05079 at fits.cx.nrao.edu> CUR%STARLINK.RUTHERFORD.AC.UK at VTVM2.CC.VT.EDU (Malcolm J. Currie) writes:
>                             More of a limitation on Exabyte is the
>maximum number of files caused by the huge tape marks (equivalent to
>about 2Mb).  You cannot get more than 320 files on a labelled cartridge
>or 950 on an unlabelled cartridge.
>I was intrigued by Richard Stover's findings.  Are there no inter-block
>gaps on Exabyte?
>Malcolm Currie                               Janet: CUR at UK.AC.RL.STAR
>Starlink                                     SPAN : RLVAD::CUR

Richard Stover has complete control over his Exabyte, including the
ability to write the short file marks (as opposed to the default
long file marks).  The short file marks take up only ~300 kbytes
and in addition to being short, they are faster to write.  This is extremely
important for some of the rapid-readout of large CCDs done at Lick.
All Exabyte hardware should be able to write these short marks;
the deficiency usually lies in the device driver software of the OS.

Richard Stover's data acquisition system actually puts a limit of
1600 FITS files per Exabyte tape to prevent having too much valuable
data on a single easy-to-lose-or-destroy tape.

_______________________________________________________________________________
Steve Allen          |                                |   sla at helios.ucsc.edu
UCO/Lick Observatory |     This space for rent.       | If the UC were opining,
Santa Cruz, CA 95064 |                                | it wouldn't tell me.

---

From: tody at noao.edu (Doug Tody)
Date: 5 Oct 91 07:58:55 GMT

>From article <21756 at darkstar.ucsc.edu>, by sla at fast.ucsc.edu (Steve Allen):
> and in addition to being short, they are faster to write.

The Exabyte can write two types of filemarks: short filemarks (about 0.5 Mb)
and long filemarks (2.2 Mb).  The difference is that the long filemark
includes a a gap allowing the filemark to be overwritten.  So long as one
only writes to a tape at EOT the difference is not important.

Whether or not one makes use of short filemarks depends upon the driver
used.  The Sun SCSI tape driver, at least to date, writes only long
filemarks.  Most people with Suns use the Sun st driver since it comes with
SunOS.   Better drivers can be found from third party vendors.  ApUNIX sells
such a driver, although we have not tried it here (we do use their DAT
driver).  Features such as short filemarks for Exabyte or fast file
seeks for DAT may not be available with a poor driver.

If I recall correctly DAT filemarks are 130 Kb or so...

These drives employ considerable data buffering internally (246 Kb in the
case of Exabyte) which is why the block size has little effect on the
sustained transfer rate.
-- 
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)