Re: Need help with redo log writer

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.

-- 
Andrew Mobbs - http://www.chiark.greenend.org.uk/~andrewm/