Mirror Oracle-Redologfiles on Oracle-Level even when on Raid-1 or EMC-Systems ?

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From: Harald Wakonig <wakonig_at_compuserve.com>
Date: Sun, 09 Jan 2000 18:24:01 +0100
Message-ID: <3878C431.F7D60B66@compuserve.com>

Hi,

Within another thread on comp.databases.oracle.server ws1_at_netcom.com wrote:

WS> Even if place on RAID-1 arrays, the redo logs should 
WS>still be mirrored at the WS> Oracle level.  
WS> If the log writer process determines that it does not
WS> know whether the contents of a particular redo log are valid, 
WS> it will mark that redo log as "STALE" in the v$log table.  
WS> If this redo log is the only copy, then it cannot be archived, 
WS> and will cause a database halt.

How often does this really happen, that the log writer marks a redo log as "STALE" ?
What is the typical reason that this happens ?
Could such a redolog-file marked as "STALE" be used during an instance recovery ?

The problem of an additionally mirroring on Oracle-Level are the COSTS... Please don't argument that you can buy a 4-GB disk for a PC for a few dollars...

The following Oracle-related questions are not only valid for EMC2-storage arrays, but are valid for ALL (expensive) state-of-the-art storage arrays from other vendors:

Facts:
* We use EMC2 storage array , Raid-1 for ALL disks (each disk is
mirrored in the local EMC2 storage-array)

We use BCV's (3rd mirror, which can be separated after short database-shutdown , allowing a cold-backup to tape although the database is again running on the other 2 mirrors)
Additionally we use the SRDF-Option of EMC2 (Symmetrix Remote Data Facility), thus mirroring every write-call to a second EMC2-storage array in a second data center (some miles away...), where it is again mirrored within that EMC-box.
Currently I have 4 Redo-groups with 100-MB redo file size .

For performance reasons its suggested to keep the redolog-files on one reserved disk...

Our EMC2 storage array contains only disks with size of 18-GB (No of the Oracle-DBA's - was asked by the department which bought the storage array)

The consequences are: for 4x100 = 400 MB

2 x 18-GB disks (Raid-1 mirrored) in the local EMC-box 1 x 18 GB disks (local 3rd mirror, "BCV")

2 x 18 GB disks in the remote EMC-box in the second data center 1 x 18 GB disk "BCV" (third mirror) in the remote EMC-box

6 x 18 GB = 108 GB disk space for the redolog files

If I will do an additional mirroring at Oracle-Level, I just would need another 6 disks (108 GB disk space)...

Some tuning experts suggest to stripe Oracle-Redolog-Files for faster write rate ... that would again double the required disk space ....

But even if I would have only one local Raid-1 mirror I would need 2x18 GB = 36 GB disk space for the redolog-files (still not striping...), and the complete net data volume is about 50 GB ...

If you know the costs of disk space in such state-of-the-art storage arrays you know that those costs are some factors beyond the price of an 4-GB disk at a PC discounter... (and the costs the Oracle department or the project gets internally charged from the UNIX department operating the disks contain additional dollars for the administration, usually a certain amount per Gigabyte...

As many Oracle customers operate databases on expensive storage arrays, how is this topic typically solved ?

Idea 1: Do no mirroring for the Disks containing Oracle-Redolog-Files

Problem: The UNIX department operating the EMC-box will make HIGH additional internal charges to the Oracle department because of additional work ..... (Our internal ISO-9000 - checklist have mandatory mirroring, we would need to add additionally exemptions for the case that mirroring is done by an application, this additional rules must pass the QA-department ...)

Idea 2: Put Oracle-Redolog-Files on local disks, which are cheaper..

No chance if you want to mirror everything to a second data center ...
No chance if you use BCV's or any other "third mirror" for fast cold-backups

Idea 3a: Buy some 4-GB spindles instead of 18-GB spindles

Are such configurations (18- and 4-GB spindles supported by the storage array vendors ?
How many Oracle DBA's are asked by the UNIX department BEFORE the contract with the storage array vendor is signed ?

Idea 3b: Buy an additional Storage Array ONLY for all Oracle-redolog-files of all your databases

That's of course only for sites with many databases ...

We have currently 3 EMC-Arrays which are mirrored with SRDF to a second Data center (6 EMC-Boxes)

For easy operating I could imagine, that we by an additional set (1 Box for each data center), but only with small (4 GB spindles), and the default settings, that in this system NO local mirroring is activated, only the SRDF mirroring into the second data center.

The costs for the 4th system can be realistic, because there is always a need in more disk space, and by moving all redologs to the new system the same space becomes available on the existing 3 storage arrays.

BUT each UNIX server will need a SECOND host adapter, because it will be connected now to two different EMC boxes (on most UNIX Servers we use only ONE host adapter, on some we use two). And that are definitely additional costs.

Idea 4: Share the redolog disks with other databases
As we operate many databases using the same EMC2 box it might be possible, but: - the databases reside on different UNIX servers, and each UNIX server has one or two dedicated EMC-host-adapters (Channel adapters). I don't know if it is possible, to have on one physical EMC-spindel (which is linked to exactly one EMC-Disk-Adapter (DA)) more file systems, which are dedicated to UNIX servers using DIFFERENT EMC-Channel-Adapters...
This will not improve the performance .....
Idea 5 : Put other Oracle-Files with LOW usage on that disk
What would you suggest ?
Oracle-Binaries ?
temporary tablespace (I have usually less then 5 disk sorts an hour .., I just need the temporary tablespace during "analyze" ..
- But that are only 3 GB... in that case I would have 3,4 GB used out of 18 GB
Some projects use huge staging areas (keeping several old database-exports....) - If the export is run, than there is usually no redo-activity, and its possible to live with the "read-export-file" and "write redolog" in case of the very seldom imports ...

Thank you for your comments or hints to further information (white papers...)

Harald Received on Sun Jan 09 2000 - 11:24:01 CST