User Guide

SAP AG November 2002
Special Thread
A database instance [Page 132] has the following special threads [Page 151]:
• Coordinator [Page
17]
• Dev Threads [Page
17]
• Requester [Page
18]
• Temporary Dev Threads [Page
18]
• Timer [Page
18]
Coordinator
The coordinator is a special thread [Page 17]. The coordinator monitors all kernel threads in
the database instance [Page
132].
When the database instance is started, the coordinator is the first active thread. It coordinates
the starting processes of the other threads.
• If a thread fails while a UNIX operating system is running, the coordinator terminates all
other threads.
• If a thread fails while a Windows NT/Windows 2000 operating system is running, an
exception handler becomes responsible for terminating all the other threads in an orderly
way.
Dev Thread
Dev threads are special threads [Page 17]. Dev threads handle the read and write commands
that read and write tasks [Page
151] ask to have performed.
The number of dev threads is primarily dependent on the number of volumes [Page
153] in
the database instance [Page 132]. Under normal circumstances, two dev threads are
activated for every data volume [Page 130] and every log volume [Page 144]. Only one dev
thread is activated for writing the kernel trace, if it is enabled.
The dev thread dev0 plays a special role; dev0 coordinates and monitors the dev threads.
• For example, if a log volume fails in warm mode (bad volume), dev0 ensures that the
corresponding dev threads are terminated. The database instance is transferred to
OFFLINE mode [Page
145].
• If the database is enlarged while running by adding another data volume, dev0 ensures
that new dev threads are generated.
All the other dev<i> threads write data to or read data from the volumes.
User Manual: SAP DB 17