RE: VARLEN field... what good are they?

I think I got it figured out.

I actually have to place the length of the field in the first two bytes.
What a hassle!  Well, it dropped my file from 1meg to 100k.  I guess it's
worth it... :)

Bradley V. Stone
Taylor Corporation - OASIS Programmer/Analyst   
bvstone@taylorcorp.com


> -----Original Message-----
> From: Simon Coulter [SMTP:shc@flybynight.com.au]
> Sent: Saturday, May 15, 1999 2:27 AM
> To:   MIDRANGE-L@midrange.com
> Subject:      Re: VARLEN field... what good are they?
> 
> OE
> Hello Brad,
> 
> Exactly how did you specify the VARLEN keyword?  You should specify an
> allocated length for the 
> normally expected amount of data and let the unusual excess spill over
> into the overflow area.  
> It is usually bad practice to let VARLEN default to zero.
> 
> VARLEN fields on the AS/400 are implemented as a fixed length in the
> record (which may be zero) 
> and an overflow area to contain the excess.  The advantage is reduced
> total size of the file at 
> the expense of some performance.  I am surprised that your file actually
> grew although the 
> recompile of the PF may have caused different disk allocations to occur.
> 
> There is a certain amount of overhead involved with VARLEN fields:
> 
> 1/ The 2-byte binary length of data.  This is part of the record format
> and exists for each 
> VARLEN field.
> 
> 2/ The information linking the field to the data in the shadow area.  This
> takes 25 bytes of 
> data in the overflow area and exists for each overflowed field.
> 
> 3/ The overflow area is managed sequentially and if an update to the
> VARLEN field increases its 
> current overflow allocation an additional allocation is made with
> additional linkage 
> information.
> 
> It is really good practice to reorganise VARLEN files because the overflow
> area is rearranged 
> to minimize the number of linkage sections.
> 
> I would suggest that you have a lot of data in the overflow area probably
> due to a poorly 
> chosen allocation value thus may records now have at least 27 bytes more
> data than they did 
> previously.
> 
> Regards,
> Simon Coulter.
> 
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> //--- forwarded letter
> -------------------------------------------------------
> > X-Mailer: Internet Mail Service (5.5.2448.0)
> > Date: Fri, 14 May 99 12:59:13 -0500
> > From: "Stone, Brad V (TC)" <bvstone@taylorcorp.com>
> > To: "'MIDRANGE-L@midrange.com'" <MIDRANGE-L@midrange.com>
> > Reply-To: MIDRANGE-L@midrange.com
> > Subject: VARLEN field... what good are they?
> 
> > 
> > I was playing around with the VARLEN keyfield today.  I have a field in
> a
> > file that can be a max of 31000 bytes.  I thought "hey, the VARLEN field
> > will save some space."
> > 
> > This field contains text, and the length is varying.  No telling what
> will
> > go in there.
> > 
> > Anyhow, I changed the DDS.  The file size actually grew about 3k after
> the
> > change.  
> > 
> > So, my question is, what good does the VARLEN keyword do in a case where
> I
> > have a field that could be anywhere from 1byte to 31k?  Is this just not
> the
> > best place for it?
> > 
> > Bradley V. Stone
> > Taylor Corporation - OASIS Programmer/Analyst       
> > bvstone@taylorcorp.com
> > 
> > 
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