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Re: Which oracle server ?

From: Niall Litchfield <>
Date: Mon, 13 Dec 2004 06:42:37 -0000
Message-ID: <41bd39db$0$19164$>

"DA Morgan" <> wrote in message news:1102914028.50143_at_yasure...
> Howard J. Rogers wrote:
>>> You don't want to use raid 5.
>> He might do.
>> HJR
> Source:
> Quote:
> "Move the redo logs to faster disks or a faster I/O subsystem (for
> example, switch from RAID 5 to RAID 1)."

as a way to address log file sync waits.

btw Amazingy the advice in this segment of the docs doesn't mention looking at whether the application is over committing or not.

> Source:
> Quote:
> "Avoiding the use of RAID 5 for redo logs."

This is a wonderful piece of advice actually, I'll point folk at it in my bog later. What does it say

  1. determine that your io bottleneck really is an io bottleneck.
  2. determine where it lies
  3. *if* it lies with redo take appropriate action.

which is a little different from your quote

> Source:
> Quote:
> "Striping data across a number of disks is one example of a redundant
> array of inexpensive disks (RAID). There are several different types of
> RAID, also referred to as RAID levels, ranging from high performance to
> high reliability. The three most common RAID levels in Oracle Database
> installations are RAID-0, RAID-1, and RAID-5. Descriptions of each RAID
> level follow Table 7-2, which shows each level's read and write penalties.
> RAID Level Read Penalty Write Penalty
> 0 (Disk Striping) 1:1 1:1
> 1 (Disk Mirroring) 1:1 2:1
> 0 + 1 1:1 2:1
> 5 (Distributed Data Gathering) 1:1 4:1"
> .. and ..
> "The write penalty of 4:1 results from 2 reads and 2 writes during parity
> calculation."

It does rather miss out the cost factor a bit though. RAID10 is better, but it isn't cheaper (or before folk jump on me as much more expensive as raid vendors say).

> Source:
> Quote:
> "RAID 0+1: Striping and Mirroring
> This level combines the technologies of RAID 0 and RAID 1. It is widely
> used because it provides good reliability and better read and write
> performance than RAID 1."
> and
> "RAID 5 striping is similar to striping in RAID 0 ... while write
> performance can suffer. This configuration might not be ideal for
> write-intensive applications."
> You are, of course, correct as always and I have obviously
> misunderstood. Thank you, in advance, for correcting both me and Oracle.

The Oracle quotations you provide address specific problems (or the benefits and costs of different raid configurations) - they do not amount to a *do not use RAID5* directive from Oracle. They amount (in my view anyway) to a list of the advantages and disadvantages of various IO platforms and the strategies to deal with specific problems. Howard used the word *may* and I think that is exactly correct RAID 5 may be appropriate or it may not.

Niall Litchfield
Oracle DBA
Received on Mon Dec 13 2004 - 00:42:37 CST

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