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Re: Need help with redo log writer

From: Flyfish <d_at_deja.com>
Date: Thu, 17 Jan 2002 14:28:14 -0500
Message-ID: <MPG.16b0efd75e2bf6cf98968a@News.CIS.DFN.DE>


In article <DKF*lvCep_at_news.chiark.greenend.org.uk>, andrewm_at_chiark.greenend.org.uk says...
> Flyfish <d_at_deja.com> wrote:
> >I have a DSS system on a HP9000 (K 460) running HPUX 11.0, Oracle
> >8.1.6.0.
> >
> >Redo log write time is "high" per Spotlight on Oracle monitoring.
> >Averages between 60-120+ ms.
> >
> >Redo logs are on a striped logical (3 physical drives, 10K rpm scsi-2)
> >volume with low disk I/O according to sar (7-10% utilized)
> >
> >The redo buffer is set to 1536K.
> >
> >The database is not in archivelog mode.
> >
> >What can I do to reduce the redo log write time? Would increasing or
> >decreasing the size of the redo log buffer have any impact?
>
> Firstly, is "log sync wait" or "log parallel write" a major wait event
> on your system? If not, it's probably not important to tune.
>
> When you say "striped", do you mean RAID-0 or RAID-5 or something
> else? For low to moderate redo rates single disks are fine for redo.
> RAID-5 should never be used for redo.
>
> What does sar report the average service time as? This will indicate
> if the disks are a problem or something else.
>
> If the disks aren't the bottleneck, it might be LGWR is being kept off
> the CPU by other processes. Is the CPU load very high?
>
> Is your redo buffer that large because you need it? 1.5MB log buffer
> means that LGWR won't start writing in the background until 512kB
> uncommitted redo is in the buffer, so if your application is producing
> smaller transactions than this, it's likely that the redo for the whole
> transaction needs to be written on a commit. Try a smaller log buffer.
>
>

The wait is log parallel write.

The striping is in fact, Raid 5, unfortunately not subject to change as the system owner 'requires' raid 5 for reliability.

Average service time is low, CPU time is all over the place but it's never pegged over 75-80% for any amount of time at all, usually running 30-50% of total CPU.

The size of the redo buffer was set before I got involved, it's not clear to me why it's so big. I will try to set it smaller.

thanks

dave Received on Thu Jan 17 2002 - 13:28:14 CST

Original text of this message

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