Re: RAID vs Redo log mirroring

Chuck Hamilton wrote in message <3587d571.5959750_at_news.dvol.com>...
>On 16 Jun 1998 15:48:25 GMT, "Robin Bishop" <rmb8_at_bton.ac.uk> wrote:
>
>>Given that the primary (only?) reason for mirrored redo logs is to guard
>>against media failure, can anyone suggest whether it is still necessary on
>>a system running RAID level 5?
>>
>>Is there a circumstance whereby mirroring would provide some security that
>>RAID would not?
>
>Accidental deletion or overwriting of a redo log.

Or corruption of the redo log file, whatever the cause.

>
>Be advised also that RAID-5 is inherently slow at doing writes which
>is all that occurs on redo logs (except during recovery). Each logical
>write requires 4 physical i/os (2 reads and 2 writes). Raid 1 is
>better for redo logs.
>
>For an OLTP systems I'd recommend RAID 0+1 for for database files over
>RAID-5 too.

If you have highly active redo, you might also want to consider using RAW devices for the redo logs, as Oracle suggest. Another thing to look at with RAID adapters is the cache (if fitted) settings: make sure it's set to 'write through', or a hardware failure can leave you with out-of-sync (read useless) redo-logs.

>--
>Chuck Hamilton
>chuckh_at_dvol.com
>
>If at first you don't succeed, sky diving isn't for you.
Received on Wed Jun 17 1998 - 09:17:23 CDT