Well This are related to checkpoint and db writer process. Check if this are stuck somewhere. When you see this KO wait check also from v$session or v$session_wait for this two process. On Fri, Mar 12, 2010 at 9:12 AM, Rajesh Rao <Rajesh.Rao@xxxxxxxxxxxx> wrote: > > > I have an SR raised with Oracle support on this, but this list and your > blogs have been more helpful than Oracle support in the past with such > problems. Quoting verbatim from the SR I have raised with Oracle support: > > > > An 8-node RAC cluster 10.2.0.3 on Solaris 10. RAID-5. We have online > applications hitting the first 6 nodes and making DML changes to about 10 > tables. On the 7th node, upon kicking off a batch job, that invokes a select > with full and parallel hints for few of these tables in a single select > statement, we see that batch job waiting on an “enq:KO fast object > checkpoint”. We also see sessions on the first 6 nodes waiting on “gc cr > request” and “gc buffer busy” waits on calls that reference those tables. > > I understand that parallel scans need to do direct reads from disk and > hence the batch session would post the CKPT processes on all nodes to write > dirty buffers for those objects accessed to disk. The CKPT would post DBWR > to do the writes. The ODMSTAT data that we collected shows this heavy writes > happening on the datafiles that hold those objects. However, following this > write, the reads/writes as observed in odmstat suddenly drop . I guess that > this drop in i/o activity is because we see plenty of sessions are waiting > on the “gc” waits mentioned earlier, on calls that reference those tables. > After anywhere from 2 to 90 seconds, all the waits dissappear, the batch > continues to work and the application is back to normal. > > Question: What happens after a session initiates a fast object checkpoint > for a table in a RAC environment? Why do we see sessions that access those > some tables start waiting on “gc” related waits? Does Oracle lock up access > to all buffers until all the instances have completed the writes to disk? Is > there a way to ease the impact from it that we see? > > I understand fast object checkpoint can be disabled by setting > "_db_fast_obj_ckpt"=false". What is the impact to parallel scans with this > setting? Any other adverse impact from setting this parameter? > > I believe I am asking a general question that won’t need any > AWR’s/RDA’s/Systemdumps or hanganalyze traces to be uploaded. I am also not > soliciting advice on how to tune parallel queries or the preferred storage > for write intensive applications. If this needs to go to a specific group, > please direct it. > > > > Thanks > > Raj > > This communication is for informational purposes only. It is not intended > as an offer or solicitation for the purchase or sale of any financial > instrument or as an official confirmation of any transaction. All market > prices, data and other information are not warranted as to completeness or > accuracy and are subject to change without notice. Any comments or > statements made herein do not necessarily reflect those of JPMorgan Chase & > Co., its subsidiaries and affiliates. This transmission may contain > information that is privileged, confidential, legally privileged, and/or > exempt from disclosure under applicable law. If you are not the intended > recipient, you are hereby notified that any disclosure, copying, > distribution, or use of the information contained herein (including any > reliance thereon) is STRICTLY PROHIBITED. Although this transmission and any > attachments are believed to be free of any virus or other defect that might > affect any computer system into which it is received and opened, it is the > responsibility of the recipient to ensure that it is virus free and no > responsibility is accepted by JPMorgan Chase & Co., its subsidiaries and > affiliates, as applicable, for any loss or damage arising in any way from > its use. If you received this transmission in error, please immediately > contact the sender and destroy the material in its entirety, whether in > electronic or hard copy format. Thank you. Please refer to > http://www.jpmorgan.com/pages/disclosures for disclosures relating to > European legal entities. > -- Thanks & Regards, Taral Desai