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Re: Redo log & RAID

From: Michail Brzitwa <michail_at_brzitwa.de>
Date: 7 Apr 2001 16:15:49 GMT
Message-ID: <9anefl$n59$1@ichabod.han.de>

Norman Dunbar <ndunbar_at_lynxfinancialsystems.co.uk> wrote:
> I haven't tried this yet (I feel a test coming on) but what happens if
> HW mirroring is in use and someone (say the trainee DBA) decided to
> delete one of the mirrored redo files ?

Provided you have write access to the directory containing the redo logs you can remove the logs. The current redo log is opened by LGWR thus this redo log will not be deleted until oracle closes it. When oracle tries to switch to the next log armageddon starts. In a correct installation (unixoid os) no one (even from the dba group) has write access to that directory except the main user. Thus it's up to the oracle user herself to call for trouble. Any dba using file tools (cp, mv, ln, rm etc.) in an oracle db data directory while the db is up shows suicidal tendencies anyway imho.

HW mirroring is transparent to all applications so oracle won't notice it. Contrary to the opinions here I think that both kinds of mirroring have their uses. As an analogy look at network firewall levels: there can be packet and application level firewalls; one doesn't care for intrusive high level contents, the other doesn't care for illegally constructed packets. The first one may guard against ip spoofing, the second against e.g. "I Love You" viruses. Both have their advantages and shortcomings yet the combination may prove safer than any single filter.

Now both types of mirroring (HW and SW) have their advantages and disadvantages as well. HW mirroring doesn't guard against user/dba errors, SW mirroring won't handle all cases HW mirroring can. For instance if you mirror redo logs and the disk containing the (unmirrored) system tablespace dies, oracle will stop working. When using a HW mirror the OS will be signalled of the disk death; the operator then can split up the mirror, hotswap a new disk and merge it into the mirror again without any application noticing it (well, disk performance will suck when the mirrors will be re-synchronized). I think there's an overall safety gain in using both types of mirroring.

-- 
Michail Brzitwa           <michail_at_brzitwa.de>            +49-511-343215
Received on Sat Apr 07 2001 - 11:15:49 CDT

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