Latches and Spinlocks - SQL Server 2012 Internals and Troubleshooting - page 190

Without Latching
This part of the example represents a world without latches. Assume that the row containing the
values (2,200) in Transaction 1 arrived at the page a fraction of a second before Transaction 2, when
the values (3,300) are written. Transaction 1 writes to Slot 1, as seen in Figure 7-9 and Figure 7-10.

Slot 1, Offset 0x6f, Length 15, DumpStyle BYTE

Record Type = PRIMARY_RECORD Record Attributes = NULL_BITMAP Record Size = 15

Memory Dump @0x000000001580A06F

0000000000000000: 10000c00 02000000 c8000000 020000 ........È......

FIGURE 7-9

HEADER INFORMATION m_slotCnt=1 m_freedata=111 HEADER

INFORMATION (1,100) (2,200)

FIGURE 7-10

The update has gone through, as you have a second row in the page in slot 1 with the hex values 02
and c8 (which are the values 2 and 200 , respectively). However, the page header is not yet updated.
They still appear as in Figure 7-7. m_freedata is still 111 , and the m_slotcnt value is still 1 .

Before the header information is written, Transaction 2 arrives and wants to write a row with its
values (3,300). Without a mechanism to stop it, Transaction 2 queries the m_freedata and m_
slotcnt values, and writes its data into Slot 1, as seen in Figure 7-11 and Figure 7-12.

Slot 1, Offset 0x6f, Length 15, DumpStyle BYTE

Record Type = PRIMARY_RECORD Record Attributes = NULL_BITMAP Record Size = 15

Memory Dump @0x0000000011FEA06F

0000000000000000: 10000c00 03000000 2c010000 020000 ........,......

FIGURE 7-11