Re: Measuring the impact of Redo Log change

From: Deepak Sharma <>
Date: Tue, 9 Sep 2008 08:43:55 -0700 (PDT)
Message-ID: <>

Hi Dave,

Using your idea, we did see a drop in the "seconds to archive" - almost half the prior values.

We also see a drop in 'log file sequential read' wait event based on AWR data. I think that the 'log file sequential read' waits are when the Arc process is trying to read the Redo logs and creating an archived file on the disk.

I also wanted to measure the throughput of the redo logs itself, basically - are we now writing faster to the redo logs.

Anyone:  Should we be looking at 'log file parallel write' and 'log file single write' waits, or something in addition to these?


----- Original Message ----
From: "Roberts, David (GSD - UK)" <>
Sent: Tuesday, September 9, 2008 3:47:03 AM
Subject: RE: Measuring the impact of Redo Log change

While the following SQL isnt going
to be able to tell you anything about the performance of the application you are
running, it may give an indication of increased stability, and depending on the
size of your control file may give you data from both before and after the change
for you to compare.
One of the problems I have encountered is when
redo logs are all on a single disk device, there will be contention between the
archive log process and the log writer, which will be seen in an increasing
amount of time taken to archive logs (Im assuming youre in
archive log mode) when the system is under duress.
SELECT recid,
next_time, 'YYYY-MON-DD HH24:MI:SS' ),
(completion_time - next_time) * ( 24 * 60 * 60 ) "Seconds to archive"
  FROM v$archived_log
 ORDER BY recid
If the peek or average time to archive
logs has dropped, it would be possible to argue that you have increased the
theoretical throughput!
Obviously if you had the opportunity,
placing redo log groups on alternating physical devices would obviously be a
better way to reduce this form of contention!
David Roberts
Logica- Releasing your
Logica UK Limited
Registered in Englandand Wales(registered number 947968)
Registered office: Stephenson House, 75 Hampstead Road, LondonNW1 2PL, United Kingdom
-----Original Message-----
From:[] On Behalf Of Deepak Sharma
Sent: 09 September 200805:33
Subject: Measuring the impact of
Redo Log change
We recently (today) moved the redo logs from RAID5
to RAID1+0.

The obvious reason of doing this is to make things better in terms of
performance, as RAID1+0 is better suited for read/writes. Someone pls correct
me if that's not true.

What are different ways to measure the impact of this change? 

The platform is AIX, DB is and DB size is 55TB, and 2TB redo is
generated each day. There are plans underway to reduce the redo generation
using direct path etc. (so let's not get diverted by that). 

We can try measuring the wait event stats using AWR, session stats etc., but
what exactly to look for?  'log file switch completion', 'log file sync',
are some things that come to mind - what else?

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Received on Tue Sep 09 2008 - 10:43:55 CDT

Original text of this message