Database Reference
In-Depth Information
•
Scalability
: Instead of buying larger and larger machines to handle an increasing workload
(known as
vertical scaling
), RAC allows you to add resources in the form of more machines
in the cluster (known as
horizontal scaling
). Instead of trading your 4 CPU machine in for
one that can grow to 8 or 16 CPUs, RAC gives you the option of adding another relatively
inexpensive 4 CPU machine (or more than one).
The following processes are unique to a RAC environment. You will not see them otherwise.
•
Lock monitor
(
LMON
)
process
: The
LMON
process monitors all instances in a cluster to detect
the failure of an instance. It then facilitates the recovery of the global locks held by the failed
instance. It is also responsible for reconfiguring locks and other resources when instances
leave or are added to the cluster (as they fail and come back online, or as new instances are
added to the cluster in real time).
•
Lock manager daemon
(
LMD0
)
process
: The
LMD
process handles lock manager service requests
for the global cache service (keeping the block buffers consistent between instances). It
works primarily as a broker sending requests for resources to a queue that is handled by the
LMSn
processes. The
LMD
handles global deadlock detection/resolution and monitors for lock
timeouts in the global environment. Also, starting with Oracle 12
c
, there may be
LDDn
slave
processes spawned by the
LMD0
process to assist with the workload.
•
Lock manager server
(
LMSn
)
process
: As noted earlier, in a RAC environment, each instance
of Oracle is running on a different machine in a cluster, and they all access, in a read-write
fashion, the same exact set of database files. To achieve this, the SGA block buffer caches must
be kept consistent with respect to each other. This is one of the main goals of the
LMSn
process.
In earlier releases of Oracle Parallel Server (OPS) this was accomplished via a
ping
. That is, if a
node in the cluster needed a read-consistent view of a block that was locked in exclusive mode
by another node, the exchange of data was done via a disk flush (the block was pinged). This
was a very expensive operation just to read data. Now, with the
LMSn
, this exchange is done via
very fast cache-to-cache exchange over the clusters' high-speed connection. You may have up
to ten
LMSn
processes per instance.
•
Lock
(
LCK0
)
process
: This process is very similar in functionality to the
LMD
process described
earlier, but it handles requests for all global resources other than database block buffers.
The following are common background processes seen with most single instance or RAC instances:
•
Process Spawner
(
PSP0
)
Process
: This process is responsible for spawning (starting/creating)
the various background processes. It is the process that creates new processes/threads for the
Oracle Instance. It does most of its work during instance startup.
•
Virtual Keeper of Time
(
VKTM
)
Process
: Implements a consistent, fine-grained clock for the
Oracle instance. It is responsible for providing both wall clock time (human readable) as well
as an extremely high resolution timer (not necessarily built using wall clock time, more of a
ticker that increments for very small units of time) used to measure durations and intervals.
•
Virtual Sch(K)eduler for Resource Manager
(
VKRM
)
Process
: Scheduler for the resource
manager. Manages CPU scheduling and managed processes with active resource plans.
•
Space Management Coordinator
(
SMCO
)
Process
: This process is part of the manageability
infrastructure. It coordinates the proactive space management features of the database such
as the processes that discover space that could be reclaimed and the processes that perform
the reclamation.
