Tuning Global Cache - Expert Oracle RAC Performance Diagnostics and Tuning - page 393

In this scenario, the following statistics are updated by Oracle. Considering this was a read-only block request,
only the cr (consistent read) statistics are updated.
gc cr blocks served: Number of consistent read blocks served. It indicates the total
number of blocks the current instance has sent to the requesting instances. In a multinode
RAC environment, multiple instances may request for the same block; the value displayed
by these statistics provides the total blocks served. In the example discussed previously,
these statistics would be incremented in the instance SSKY4 .
gc cr blocks received: When a consistent read version of the block is not found in the
local buffer cache of the requesting instance, an attempt is made to get the block from
the remote instance. This is the total number of consistent read blocks received by the
current instance from other instances. When blocks are not found in the local cache, the
block is requested from other instances. In the previous example, this statistics would
be incremented in the instance SSKY3 . Building a consistent read version of the block
may involve several operations, for example, reading the data or undo block from disk or
requesting a portion of the block from another instance. Due to this activity, occasionally
there could be delays in building the requested CR block. In these situations, an incomplete
copy of the block is sent. If a partial copy of the block is received, it may be rolled back when
the full version of the block is received at a later stage.
gc cr block receive time: The total time taken to receive the block from the holder of
the block. The activity is timed from the point when the request is made until it completes.
The matric is stored in centiseconds. The following formula helps determine the average
time per CR block in milliseconds.
10 * gc cr block receive time ÷ gc cr blocks received
gc cr block send time: When the block is requested by another instance, the value
represents the total time taken to send the blocks. The value is influenced by conditions
such as locks and time taken to downgrade the lock, and so forth.
gc cr block read wait time: Represents the time taken waiting to read the consistent
read block from buffer or into the buffer from disk.
gc current block send time: The value collected by this counter indicates the time
taken to send the “current” block requests that are held by the current instance to the
requesting instances in the cluster.
gc current block served: The data collected by this counter indicates the total number
of current blocks served by the current instance.
gc current block received: The data collected by this counter indicates the total
number of requests for modification of a current block received from other instances in the
cluster. Once the block is received, the counter is incremented.
gc current block receive time: This statistic indicates the time taken for receiving the
block from the current holding instance. It's the total time from the moment a request was
made by a session on the instance until the block is received. The unit of measurement
is centiseconds. The following formula gives the average roundtrip time for a request in
milliseconds:
10 × gc current block receive time ÷ gc current blocks received