IDS Forum

Re: Differences in RSAM page structure 32 and 64 b

Posted By: Madison Pruet
Date: Friday, 16 January 2015, at 1:32 p.m.

In Response To: Re: Differences in RSAM page structure 32 and 64 b (JACOB SALOMON)

The page header went through quite a few changes when we introduced
multiple page sizes, but is still contained in 24 bytes.

From: "JACOB SALOMON" <jakesalomon@yahoo.com>
To: ids@iiug.org
Date: 01/16/2015 09:39 AM
Subject: Re: Differences in RSAM page structure 32 and 64 b [34479]
Sent by: ids-bounces@iiug.org

After Marco posted his response last July, I allowed the issue to drop.=
24
bytes.
However, some recent events at my client revived my interest in this is=
sue.
In
the process, I got my hot little hands on an old (2007) Art Kagel
presentation
on Informix internal architecture. On page 30, he gives the following
structure for the page header as of release 10:
struct page_header
{
int PageNumber;
short ChunkNum;
short chksum;
short pagesize;
short NumSlots;
short Flags;
short FreeOffset;
short FreeBytes;
int NextPage;
int PrevPage;
};

There is one little problem with this: The structure is now 26 bytes lo=
ng,
that is: (7 * 2-bytes) + (3 * 4bytes). This is incompatible with Marco'=
s
assertion that the header is still 24 bytes.

This leads me to suspect a slight overreach in the above structure; tha=
t a
couple of the above "short" structure members are actually [unsigned] c=
har.
I
seek feedback on what is actually correct. But here's my reasoning:

Since the slot number has always been limited to 8 bits (unsigned) with=
a
max
value of 255, we don't need a 16-bit short to tell us the number of slo=
ts;
an
"unsigned character" (AKA uchar) would be sufficient.

And since the variety of page sizes is severely limited, it should be
sufficient to have a small (in the human sense) integer to tell us the =
page
is
2, or 4,8,10,12,14, or 16k. Or, perhaps, a number that multiplies the
default
page size. For example, in a 2k server, the number 5 in this spot would=

tell
us the page is 10k.

If my reasoning is correct (agreeably, an arguable proposition :-) that=

would
bring the page-header structure back to the original 24 bytes, as Marco=

explained all those months ago.

Anyone out there who can confirm or correct my reasoning and get me to =
the
correct layout?

BTW I am blindly assuming (praying?) that the structure remains unchang=
ed
through release 12.x. If not, we must prevail upon Art (promise to pay =
him
in
chocolate? :-) to update that presentation!

Thanks much for guidance!

-- Jacob S.

***********************************************************************=
********

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Messages In This Thread

• Differences in RSAM page structure 32 and 64 bit I
  JACOB SALOMON -- Tuesday, 29 July 2014, at 7:25 p.m.
  • Re: Differences in RSAM page structure 32 and 64 b
    Marco Greco -- Wednesday, 30 July 2014, at 4:10 a.m.
    • Re: Differences in RSAM page structure 32 and 64 b
      JACOB SALOMON -- Friday, 16 January 2015, at 10:38 a.m.
      • Re: Differences in RSAM page structure 32 and 64 b
        Madison Pruet -- Friday, 16 January 2015, at 1:32 p.m.
      • Re: Differences in RSAM page structure 32 and 64 b
        JACOB SALOMON -- Thursday, 25 January 2018, at 8:00 p.m.
        • Re: Differences in RSAM page structure 32 and 64 b
          JACOB SALOMON -- Wednesday, 7 February 2018, at 4:11 p.m.

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