Overflow block (NK-ISAM)

Overflow blocks are extensions of data blocks. They are created if records longer than the usable length of a data block are written. This occurs, for example, if the user uses records with the same keys and, when defining the record and block lengths, forgets that NK-ISAM internally appends an 8-byte time stamp to such records. Overflow blocks also result when converting K-ISAM files into NK-ISAM files if the maximum record length was utilized in the K-ISAM file. The data which “overflows” is always placed at the beginning of an overflow block, and the end of this block contains a pointer to the associated data block. The data area of the overflow block must not contain any part of the ISAM key (primary key), of the flags or of a secondary key.

Maximum record length / Usable block length

No overflow blocks will be created if the following rules are observed:

• the maximum permissible record length is determined by the BLKSIZE or BUFFER-LENGTH specification: n * 2048, where 1 <= n <= 16 (for BLKSIZE=(STD,n) or BUFFER-LENGTH=*STD(SIZE=n))

• the usable block size is the block size minus the space needed for management information and the time stamp

• as ISAM places a 4-byte record length field in front of each record, the usable block size for records with RECFORM=F or RECORD-FORMAT=*FIXED is reduced by a further 4 bytes

This results in the following formula for the usable block size:

RECSIZE <= n * 2032 - 16 - T - F

n	blocking factor in BLKSIZE = (STD,n), where 1 <= n <= 16 or BUFFER-LENGTH=*STD(SIZE=n)
T	time stamp T = 8 for files with duplicate keys T = 0 for files without duplicate keys
F	record format F = 0 bei RECFORM = V or RECORD-FORMAT=*VARIABLE F = 4 bei RECFORM = F or RECORD-FORMAT=*FIXED

As each data block contains at least one record, a further 16 bytes for the management information must be taken into account when defining the record length. The table below shows the relationship between the maximum permissible record length and the usable block size for NK-ISAM files without overflow blocks for ISAM files with standard attributes (record format V, no duplicate keys).

Blocking factor n (BLKSIZE=(STD,n))	Maximum record length = BLKSIZE (in Byte)	Usable block length (in Byte)
1	2048	2016
2	4096	4048
3	6144	6080
4	8192	8112
5	10240	10144
6	12288	12176
7	14336	14208
8	16384	16240
9	18432	18272
10	20480	20304
11	22528	22336
12	24576	24368
13	26624	26400
14	28672	28432
15	30720	30464
16	32768	32496

Table 49: Usable block size for NK-ISAM files

Maximum key position and length

As neither the ISAM key nor the flags may extend into an overflow block, there are certain maximum values for key position (KEYPOS), key length (KEYLEN), value flag length (VALLEN) and logical flag length (LOGLEN). These values depend on the blocking factor and the record format. The permissible maximum values can be calculated from the usable block size (see table 49) with the aid of the following formulas:

ISAM key length and position:

KEYPOS <= SL - F - KEYLEN - VALLEN - LOGLEN + 1

KEYLEN <= SL - F - KEYPOS - VALLEN - LOGLEN + 1

KEYPOS	key position
KEYLEN	Key length
VALLEN	value flag (length)
LOGLEN	logical flag (length)
SL	Usable block length
F	contingent on the record format: F = 0 for RECFORM=V F = 4 for RECFORM=F

Example

File attributes:

BLKSIZE = (STD,1) KEYLEN = 12

The values for KEYPOS are as follows:

RECFORM = V => KEYPOS <= 2016 - 12 + 1 = 2005
RECFORM = F => KEYPOS <= 2016 - 4 - 12 + 1 = 2001