IO stats in glance

[Jason Preble]

What is the difference between Physical IO and Logical IO reported in glance?

[Jeff Schussele]

Hi Jason,

Physical involves a read/writes all the way from/to the disk.
Logical reads/writes come out of the buffer cache.

Rgds,
Jeff

[Sridhar Bhaskarla]

Hi,

Physical IO -

To/From buffer cache from/to Disk if buffer cache is used

To/From application from/to Disk if buffer cache is not used

Logical IO

To/From Application from/to buffer cache.

-Sri

[A. Clay Stephenson · Accepted solution]

Physical IO's are actual, honest-to-God go to the disk IO operations whereas Logical IO's access the buffer cache.

Consider the read of blocks 1-5 of myfile. The first time myfile is accessed a physical read must be done; the data is copied to the UNIX buffer cache; and the process is supplied with the data. Subsequent reads of those same blocks need only read (logical) from the buffer cache. Writes are handled in a similar manner (logical to cache) and then written to disk (physical) when system load permits or when the buffer cache must be flushed.