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Re: Differences in RSAM page structure 32 and 64 b

Posted By: JACOB SALOMON
Date: Wednesday, 7 February 2018, at 4:11 p.m.

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

Hi. Jake the obsessed still here and still at it.

It takes a long time between my posts because I don't always have the opportunity to boot my home PC to Lunix. But here is the next installment of my investigations.

First, I must confess that I have an error in my previous post: I had directly counted the slots but counted 2 slots as one. There were actually 122 slots, not 62. And that 7A in first full word of line 1 is exactly 122 back in decimal.

My next point of attack was to determine where in the header it tells us what size page this is. Toward this end I created DBspaces of 2,3,6,8,10,12,14,and 16K pages. Notice after the 7A you see the number 38. For each page size I found a different number, according to the following table. Unfortunately, since I can't post fix-width fonts I have to beg your indulgence:

// Page size codes
// Code k/pg Binary
// 00 = 2k 0000 0000
// 08 = 4k 0000 1000
// 10 = 6k 0001 0000
// 18 = 8K 0001 1000
// 20 = 10k 0010 0000
// 28 = 12K 0010 1000
// 30 = 14k 0011 0000
// 38 = 16K 0011 1000

At first I could not get the pattern, then I looked at the bit-field values. Taking the bit-field binary[2-4] I see the binary codes that flag the page size:
0: 2K
1: 4K
2: 6K
3: 8K
4: 10K
5: 12K
6: 14K
7: 16K

If the other bits have any potential meanings, I'd love to hear about it. But for that one byte I can set up a structure:
typedef struct
{
unsigned int idk_1 : 2; /* First <I Don't Know>: 2 bits */
unsigned int pg_sz_flag : 3; /* Page Size flag: 3 bits */
unsigned int ifk_2 : 3; /* Second <I Don't Know>: 3 bits */
} page_size_flag_t;

So far I have managed to reverse engineer this much of the RSAM-64 page header:

typedef struct page_header
{

long page_id; /* 4-byte signed page number within the chunk */

short chunk_num; /* 2 byte signed chunk num within the server */

short idontknow_1; /* 2 bytes. Checksum?? */

unsigned char nslots; /* 1 byte: Slot count in this page */

page_size_flag_t pgsize_flags; /* 1 byte: As per above typedef */

unsigned short pg_type; /* 2 bytes: Flags representing page type */

short free_ptr; /* 2 bytes: Offset to 1st free byte in page */

short free_cnt; /* 2 bytes: # bytes in page not in a row */

long next; /* 4 bytes: For index pages - -> next page */

long prev; /* 4 Bytes: For index pages - -> prev page */
} page_header_t;

Now *that* adds up to 24 bytes!

I have not been able to quite resolve the free_cnt field. For example, here is the "states" lookup table in an 8K page:
0000/0000||640F0000 1000D3EF 34180100 8C03A01B |d~~~~~~~4~~~~~~~|
0016/0010||00000000 00000000 414B416C 61736B61 |~~~~~~~~AKAlaska|
0032/0020||20202020 20202020 20484948 61776169 | HIHawai|
0048/0030||69202020 20202020 20204341 43616C69 |i CACali|

Yes, this is page 0F64 (3940) in chunk[16]. That's as measured in 2K pages; it is actually 985 as measured by 8K pages.

There are 34(hex) entries, for 52 states (+ DC and Puerto Rico. Hey, where's Guam? ;-) Where there are no entries the data is all zeros. And I can count 7056 null bytes. How did I count that?

0896/0380||72746F20 5269636F 20202020 00000000 |rto Rico ~~~~|
0912/0390||00000000 00000000 00000000 00000000 |~~~~~~~~~~~~~~~~|
------------------- Skipped <440> null lines -------------------
7968/1F20||00000000 00000000 00000000 7B031100 |~~~~~~~~~~~~{~~~|

The free pointer in the header is 038C (= 908(10), which matches exactly the offset of the first 00000000 you see above.

The 00 bytes cover 441 completely null lines, plus 16 more null bytes. Total:

Yet, the free count reads 1BA0, which is 7072. This discrepancy is a mild one compared to some others I have run into.

WOW!! Did I write all that? Who in blazes is gonna read all that?

All that said, however, I think I have enough information to start that update to pgdump.

Thanks to a few folks who replied privately, so I presume they to not wish to be publicly named.

One man's obsession is another man's idiocy!

-- Jacob S.

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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