Large discrepancy between 'log file parallel write' and 'db file parallel write' times

Oracle 11.2.0.2 EE on Exadata X2-2 quarter rack with extra cells (7 cells in total).

Under heavy I/O load (i.e. when the cells are saturated) we are seeing a very large difference between the average latencies of 'log file parallel write' and 'db file parallel write' waits.

From a 30 minute AWR report in which each cell was performing its theoretical maximum 1500 IOPS we observe the following write latencies:

DBFPW = 263ms
LFPW = 2ms

when the I/O subsystem is not pushed to its limits the values for both wait events are in the order of single digit milliseconds.

So my question is why do the DBWR write times suffer so much during I/O bottlenecks when the LGWR write times do not..?

The LGWR writes will be significantly smaller and are sequential as opposed to the DBWR writes which are random writes, but are those differences enough to justify the large gap, or is it possible that we are losing 'db file parallel write' time to some other processing..?

and are the LGWR writes genuinely sequential..?  because ultimately the redo logs are on the same physical disks as the datafiles because both the RECO and DATA diskgroups are carved from the same cell disks.

Matt --
http://www.freelists.org/webpage/oracle-l Received on Mon Dec 31 2012 - 02:51:02 CET