Re: Redo and Archive Log Question

From: Chuck <ccarson_at_echeeba.com>
Date: Fri, 07 Mar 2003 12:10:00 -0800
Message-ID: <3E68FC98.3020506@echeeba.com>

Richard Foote wrote:
> Hi Chuck
>
> Comments embedded.
>
> "Chuck" <ccarson_at_echeeba.com> wrote in message
> news:3E6846E0.7040708_at_echeeba.com...
>

>>We are trying to add some more redundancy to a very busy OLTP database
>>and I have this question.
>>
>>I currently have 4 dedicated logging disks as follows:
>>
>>disk 1 = control01.ctl, redo1a.log
>>disk 2 = control02.ctl, redo2a.log
>>disk 3 = control03.ctl, redo1b.log
>>disk 4 = redo2b.log
>>disk 5 = archive_dest_1
>>
>>These 5 'disks' are all RAID 0+1 volume groups on an IBM SAN. (18Gb 10k
>>rpm) I want to add another logging disk which will be on another SAN
>>Switch/RAID Controller, in order to protect against failures in our
>>Brocade Switch or IBM FastT controller. (We have two independent SAN's)
>>
>>disk 6 = redo3.log, archive_dest_2
>>
>>Disk 6 is a larger, slower disk (70 Gb 10k rpm) that will also have more
>>I/O contention than disks 1 - 5. (due to redo and archive on the same

>
> disk)
>

>>
>>My questions are:
>>
>>1) If I have 3 copies of each online log file (as configured above) but
>>one of the copies is on a slower disk (disk 6), will the slower disk be
>>the 'weakest link' so to speak. OR, can the ARCH process start archiving
>>  the first full online log written within the group even though the
>>third member may be lagging behind due to poor I/O?

>
>
> Yes, you will have a "weak link" requiring the ARCH process to wait.
>
>

>>2) Same thing as question 1, but applied to the archive process.

>
>
> If I understand your question, it will depend on whether you're destinations
> are mandatory or optional.
>
>

>>3) Can I have a third group with only one member even though the first
>>two log groups have two each?

>
>
> Yes you can, but I would question the logic in doing so. Don't forget
> Murphy's Law. The group with just the one member will be the one that will
> cause you grief.
>
>

>>I hope this makes sense, it has been a long day.
>>

>
>
> Has it ever.
>
> I would recommend using the new disk 6 as *just* the second archive dest and
> use your disks 1-4 as the disks for your new redo log members, keeping the
> online log members as you currently have such that no two members are on the
> same disk and such that the lgwr and arch processes do not contend at the
> same time (eg. group 3 on disk 1 and 3, group 4 on disk 2 and 4 and so on).
> Just keep to the same pattern and add as many new groups as you require to
> avoid archive/checkpoint delay issues.
>
> This way, all groups have 2 members on separate disks, there is no
> contention by lgwr/arch trying to write to the same disk and no contention
> between lgwr writing and arch reading from the same disk (providing the
> archiving is completed in sufficient time).
>
> Make sense ?
>
> Good luck (and good night :)
>
> Richard
>
>

Yes, that makes sense. I made a mistake before, I meant I wanted to add a third log member to the two log groups, not a third log group I want one of the archive destinations to be optional. What impact does that have if I/O is poor or the second arch dest runs out of space?

Thx,
CC

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