Re: When many disks are involved in a physical database layout

Comments embedded.

Hth,

Sybrand Bakker, Oracle DBA

"Dino Hsu" <dino1_at_ms1.hinet.net> wrote in message news:gu22ht04frm33j5989rqt0kvssn6u2om3e_at_4ax.com...
> Dear all,
>
> In Ch4 Physical Database Layouts of Kevin Loney's book, an ideal
> 22-disk model is presented as:
>
> Disk Contents
> 1 Oracle software
> 2 SYSTEM tablespace
> 3 RBS tablespace
> 4 DATA tablespace
> 5 INDEXES tablespace
> 6 TEMP tablespace
> 7 TOOLS tablespace
> 8 Online Redo log 1
> 9 Online Redo log 2
> 10 Online Redo log 3
> 11 Control file 1
> 12 Control file 2
> 13 Control file 3
> 14 Application software
> 15 RBS_2
> 16 DATA_2
> 17 INDEXES_2
> 18 TEMP_[USER]
> 19 TOOLS_I
> 20 USERS
> 21 Archived redo log destination disk
> 22 Export dump file desination disk
>
> He then tries in successive iterations to reduce the number of disks
> to 17-disk, 15-disk, 12-disk, 9-disk and 7-disk solutions.
>
> The goals for the disk layout, he defines, are as follows:
> 1.The database must be recoverable.
> 2.The online redo log files must be mirrored via the system or the
> database.
> 3.The database file I/O weights must be estimated.
> 4.Contention between DBWR, LGWR, and ARCH must be minimized.
> 5.Contention between disks for DBWR must be minimized.
> 6.The performance goals of the system must be defined.
> 7.The disk hardware options must be known.
> 8.The disk mirroring architecture must be known.
> 9.Disks must be dedicated to the database.
>
> I think the author is trying to cover all kinds of situations, if
> there are less than 7 disks involved, more compromises have to be
> made, and this model would become less useful. Unfortunately, the
> current databases we have all reside on only one disk; control files,
> data files, online redo log files,... everything lives on the same
> disk.

So if the disk dies, your database is toast. You are running in archivelog and making frequent backups? Many sites have NT databases on one single disk and no proper backup procedure in place.

It seems (I might be wrong) that on Windows NT there seldom are
> more than 5 disks, and when there are, they could be combined into one
> by RAID. From the practical point of view, I have questions:
> 1.Do you usually use 7 or more 'dedicated' disks for an Oracle
> database?

If I can, I will. Four disks is my absolute minimum and I realise Loney's minimum is 5. If I have only one disk, I will warn customer as to the impact his config has

> 2.Do you prefer the disks to be RAID'ed or not?

Online redolog files shouldn't be on a RAID-5 disk, and their should be minimum 4 RAID-5 disks to overcome the write penalty associated with RAID.

> 3.Do you DBA's get involvied in the matter of server purchase so that
> hardware spec. are good for the physical layout?

I work as a consultant, and when I come in, the disaster usually already happened.

> 4.The issue of file sizes is not really taken into account in the
> decisions about which tablespaces should be placed on which disks by
> the author. He focuses on reducing and balancing the contentions most
> of the time. This might imply there are a lot of wasted disk space on
> disks where data are small, and because disks must be dedicated to the
> database (rule 9), they will not be used to accomodate other files.

Disk is cheap nowadays, and databases use to grow.

>
> Thanks, Dino
>
Received on Sun May 27 2001 - 15:59:38 CDT