Difference between buffered disk reads and cached reads?

pinga123 · 07-13-2011, 05:20 AM

I was analyzing the Disk read using hdparm utility.

This is what i got as a result.

Code:

# hdparm -t /dev/sda

/dev/sda:
 Timing buffered disk reads:  108 MB in  3.04 seconds =  35.51 MB/sec

Code:

# hdparm -T /dev/sda

/dev/sda:
 Timing cached reads:   3496 MB in  1.99 seconds = 1756.56 MB/sec

I m not sure what does Cache read and buffered disk read mean.

Please share your hdparm output for comparison purpose.

tronayne · 07-13-2011, 11:40 AM

OK, here's what the manual page for hdparam sas:

Code:

       -t     Perform timings of device reads for benchmark and comparison purposes.   For
              meaningful results, this operation should be repeated 2-3 times on an other‐
              wise inactive system (no other active processes) with at least a  couple  of
              megabytes  of  free  memory.  This displays the speed of reading through the
              buffer cache to the disk without any prior caching of data.   This  measure‐
              ment  is  an  indication  of  how fast the drive can sustain sequential data
              reads under Linux, without any filesystem overhead.  To ensure accurate mea‐
              surements, the buffer cache is flushed during the processing of -t using the
              BLKFLSBUF ioctl.

and

Code:

       -T     Perform timings of cache reads for benchmark and comparison  purposes.   For
              meaningful results, this operation should be repeated 2-3 times on an other‐
              wise inactive system (no other active processes) with at least a  couple  of
              megabytes  of free memory.  This displays the speed of reading directly from
              the Linux buffer cache without disk access.  This measurement is essentially
              an  indication  of the throughput of the processor, cache, and memory of the
              system under test.

And you can take a look at http://linux.about.com/od/lsa_guide/a/gdelsa44.htm for some additional discussion.

Hope this helps some.