UNDERSTANDING BUFFER BUSY WAITS AND REMEDIES
Hi ,
Buffer busy wait comes in top 5 wait events.Below is the detail of dealing with them.
The Buffer Busy Waits Oracle metric occur when an Oracle session needs to access a block in the buffer cache, but cannot because the buffer copy of the data block is locked. This buffer busy wait condition can happen for either of the following reasons:
The block is being read into the buffer by another session, so the waiting session must wait for the block read to complete.
Another session has the buffer block locked in a mode that is incompatible with the waiting session's request.
Because buffer busy waits are due to contention between particular blocks, there's nothing you can do until you know which blocks are in conflict and why the conflicts are occurring. Tuning therefore involves identifying and eliminating the cause of the block contention.
One of the most confounding problems with Oracle is the resolution of buffer busy wait events. Buffer busy waits are common in an I/O-bound Oracle system, as evidenced by any system with read (sequential/scattered) waits in the top-five waits in the Oracle STATSPACK report, like this:
Top 5 Timed Events
% Total
Event Waits Time (s) Ela Time
--------------------------- ------------ ----------- -----------
db file sequential read 2,598 7,146 48.54
db file scattered read 25,519 3,246 22.04
library cache load lock 673 1,363 9.26
CPU time 2,154 934 7.83
log file parallel write 19,157 837 5.68
The main way to reduce buffer busy waits is to reduce the total I/O on the system. This can be done by tuning the SQL to access rows with fewer block reads (i.e., by adding indexes). Even if we have a huge db_cache_size, we may still see buffer busy waits, and increasing the buffer size won't help.
The resolution of a "buffer busy wait" events is one of the most confounding problems with Oracle. In an I/O-bound Oracle system, buffer busy waits are common, as evidenced by any system with read (sequential/scattered) waits in the top-five waits.
Reducing buffer busy waits reduces the total I/O on the system. This can be accomplished by tuning the SQL to access rows with fewer block reads by adding indexes, adjusting the database writer or adding freelists to tables and indexes. Even if there is a huge db_cache_size , the DBA may still see buffer busy waits and, in this case, increasing the buffer size will not help.
The most common remedies for high buffer busy waits include database writer (DBWR) contention tuning, adding freelists (or ASSM), and adding missing indexes
Best regards,
Rafi.
salam rafi this is shameer basha.u r responce is good to all.I would like to know about patches.
ReplyDeleteWalakum Salam Shameer,
ReplyDeleteThanks for the appreciation.
Database patches are of various kinds like,But in broad there are 2 types:
1)CPU(Critical Patch Update):
Critical Patch Update, quarterly delivered by Oracle to fix security issues.
2)PSU(Patch Set Update):
PatchSet Update, also quarterly delivered by Oracle,it includes CPU and a bunch of other one-off patches.
PSUs have been introduced for the first time on Oct-2009,We can use them, and stop to apply CPUs. PSUs, as CPUs, are cumulatives, no need to apply all of them, just apply the last one should be enough. One-off patch is single fix, to fix a particular issue.
We need to have metalink(My Oracle Support)Account to download this patch and applying instructions will be there in readme file of Patch.
Check my Label:Database Upgrade for Appying patch to upgrade the Database.
Hope it helps.
Best regards,
Rafi.