Raid configuration for database

[Trent Shipley]

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A critical component in the product is an Oracle database.  The database is
heavily biased toward OLTP.  It has to execute transactions very fast, and
it can never go down.  No customer is large enough to effectively use
tape-backup.  For the most part they cannot be relied upon to replace hard
drives on a regular schedule.

We have had terrible experiences with RAID-5.  It is slow and has a
distressing tendency to die catastrophically.

We have been using RAID-1.

We are currently building what for us is a large system.  The Oracle
database is getting 7 drives.  If we follow tradition there will be three
logical drives, each with two physical drives mirrored to each other.  Drive
number 7 is hot backup.

Lets forget the hot backup for this problem.  (Also ignore the fact that we
should use RAID-1+0.)

With the default RAID-1 configuration we have 6 drives: scd0, 1, 2, 3, 4,
and 5.  We pair them getting logical drives mirror01, 23, and 45.

Alternatively we could partition each drive.  If we use every possible
combination, ignoring order (that is mirror01 == mirror10), and never
mapping a drive to itself, then each drive has 6 partitions.  Furthermore,
each partition is shared by two disks. There are 15 combinations.

01 | 12 | 23 | 34 | 45
02 | 13 | 24 | 35
03 | 14 | 25
04 | 15 |
05 |

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Assume that the simple RAID-1 of 01 | 23 | 56 is the null hypothesis.

1) Is the 15 partition model more or less reliable than the null case?

2) Is the 15 partition model generally faster or slower than the null case?

3) More specifically, is the 15 partition model likely to generate more or
less disk contention when used by a database than the null case?




Trent Shipley

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