Document ID: 38281.1
Subject: RAID and Oracle - 20 Common Questions and Answers
Last Modified: 03 Jun 96
Author: JFARRING
RAID and Oracle - 20 Common Questions and Answers
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1. What is RAID?
RAID is an acronym for Redundant Array of Inexpensive (or
Independent) Disks. Physically, it consists of an enclosure
containing a number of disk drives, connected to each other and to
one or more computers by a fast interconnect. Six levels of RAID
are defined: RAID-0 simply consists of a number of standard UNIX
disk volumes, and RAID-1 is a mirrored set of two or more physical
disk volumes. The only other widely-used level is RAID-5, which is
the subject of this article. Higher RAID levels exist but there is
no generally-accepted standard for features included.
2. What are its main features?
The main feature of RAID-5 is prevention of data loss. Depending on
the application, performance may be better or worse.
3. How is data loss prevented?
If a disk is lost because of a head crash, for example, the
contents of that disk can be reconstituted using the information
stored on other disks in the array. In RAID-5, redundancy is
provided by error-correcting codes (ECCs), with parity information
(to check on data integrity) stored with the data and thus striped
across several physical disks. (The intervening RAID levels between
1 and 5 work in a similar way, but with differences in the way the
ECCs are stored.)
4. What are the performance implications of using RAID-5?
The basic principle of RAID-5 is that files are not stored on a
single disk, but are divided into chunks, which are stored on a
number of different disk drives. This means that the effective disk
spindle speed is increased, which makes reads faster. However, the
involvment of more disks, and the more complex nature of a write
operation, means that writes will be slower. So applications where
the majority of transactions are reads are likely to give better
response times, whereas write-intensive applications may show worse
performance.
5. How does RAID-5 differ from RAID-1?
RAID-1 (mirroring) is a strategy which aims to prevent downtime due
to loss of a disk, but whereas RAID-5 in effect divides a file
into chunks and places each on a separate disk, RAID-1 maintains a
copy of the contents of a disk on another disk, referred to a
mirrored disk. Writes to a mirrored disk may be a little slower as
more than one physical disk is involved, but reads should be faster
as there is a choice of disks (and hence head positions) to seek to
the require location.
5. How do I decide between RAID-5 and RAID-1?
RAID-1 is indicated for systems where complete redundancy of data
is considered essential and disk space is not an issue. RAID-1 may
not be practicable if disk space is not plentiful. On a system
where uptime must be maximised, Oracle recommends mirroring at
least the control files, and preferably also the redo log files.
RAID-5 is indicated in situations where avoiding downtime due to
disk problems is important, or when better read performance is
needed and mirroring is not in use.
6. Do all drives used for RAID-5 have to be identical?
Most UNIX systems allow a failed disk to be replaced with one of
the same size or larger. This is highly implementation-specific, so
the vendor should be consulted.
7. Is RAID-5 enough to provide full fault-tolerance?
No. A truly fault-tolerant system will need to have a separate
power supply for each disk, to allow for swapping of one disk
without having to power down the others in the array. A fully
fault-tolerant system has to be purpose-designed.
8. What is hot swapping?
This refers to the ability to replace a failed drive without having
to power down the whole disk array, and is now considered an
essential feature of RAID-5. An extension of this is to have a hot
standby disk, which eliminates the time taken to swap a replacement
disk in - it is already present in the disk array, but not used
unless there is a problem.
9. What is a logical drive, and how does it relate to a physical drive?
A logical drive is a virtual disk constructed from one or (usually)
more than one physical disks. It is the RAID-5 equivalent of a UNIX
logical volume; the latter is a software device, whereas RAID-5 uses
additional hardware.
10. What are the disadvantages of RAID-5?
The need to tune an application via placement of 'hot' (i.e.
heavily accessed) files on different disks is reduced by using
RAID-5. However, if this is still desired, it is less easy to
accomplish, as the file has already been divided up and distributed
across disk drives. There is an additional consideration for
Oracle, in that if a database file needs recovery, several physical
disks will be involved in the case of a striped file, whereas only
one would be involved in the case of a normal file. This is a
side-effect of the capability of RAID-5 to withstand the loss of a
single disk.
11. What variables can affect the performance of a RAID-5 device?
The major ones are: access speed of constituent disks; capacity of
internal and external buses; number of buses; size of caches;
number of caches; and nature of the algorithms used for
determining how reads and writes are done.
12. What types of file are suitable for placement on RAID-5 devices?
Placement of data files on RAID-5 devices is likely to give the
best performance benefits, as these are usually accessed randomly.
More benefit will be seen in situations where reads predominate
over writes. Rollback segments and redo logs are accessed
sequentially (usually for writes) and therefore are not suitable
candidates for being placed on a RAID-5 device. Also, datafiles
belonging to temporary tablespaces are not suitable for placement
on a RAID-5 device.
13. What about using multiple DBWRs as an alternative to RAID-5?
Using at least as many Database Writer processes (DBWR) as you
have database disks will maximise synchronous write capability,
by avoiding one disk having to wait for a DBWR process which is
busy writing to another disk. However, this is not an alternative
to RAID-5, because it improves write efficiency, and RAID-5
usually results in writes being slower.
14. What about other strategies?
Two strategies that can be used as alternatives to RAID-5, or in
addition to it, are Asynchronous I/O (aio) and List I/O (listio).
15. What is Asynchronous I/O?
Asynchronous I/O is a means by which a process can proceed with
the next operation without having to wait for a write to complete.
For example, after starting a write operation, the DBWR process
blocks (waits) until the write has been completed. If aio is used,
DBWR can continue almost straight away. aio is activated by the
relevant init.ora parameter, which will either be ASYNC_WRITE or
USE_ASYNC_IO, depending on the platform. If you use aio, there is
no need to have multiple DBWRs.
16. What are its advantages and disadvantages?
In the above DBWR example, the idle time is eliminated, resulting
in more efficient DBWR operation. However, aio availability and
configuration is very platform-dependent: whilst many UNIX versions
support it, some do not. Raw devices must be used to store the files,
so use of aio adds some complexity to the system administrator's job.
As well as taking steps to set it up, be must ensure that the key
applications on the system are able to take advantage of it.
17. What is List I/O?
List I/O is a feature found on many SVR4 UNIX versions. As the
name implies, it allows a number of I/O requests to be batched
into a "list", which is then read or written in a single
operation.
18. What are its advantages and disadvantages?
I/O should be much more efficient when done in this manner. You
also get the benefits of aio, so this is not needed if listio is
available. However, listio is only available on some UNIX systems,
and as in the case of aio, the system administrator needs to set
it up and make sure key applications are configured to use it.
19. How do Logical Volume Managers (LVMs) affect use of RAID-5?
Many UNIX vendors now include support for an LVM in their standard
product; under AIX, all filesystems must reside on logical volumes.
Performance of a UNIX system using logical volumes can be very good
compared with standard UNIX filesystems, particularly if the stripe
size (size of the chunks files are divided into) is small. Performance
will not be as good as RAID-5, given that the latter uses dedicated
hardware with fast interconnects. In practice, however, many small
and medium sized systems will find use of logical volumes (with a
suitable stripe size for the type of application) gives performance
just as good as RAID-5. This particularly applies to systems where
there is no I/O problem. Larger systems, though, are more likely to
need the extra performance benefits of RAID-5.
20. How can I tell if my strategy to improve I/O performance is working?
At the UNIX level, there are several commands which can tell you
if a disk device is contributing to I/O problems. On SVR4, use the
sar command with the appropriate flag, usually -d. On BSD, use the
iostat command. You are looking for disks whose request queue
average length is short, ideally zero. Disks with more than a few
entries in the queue may need attention. Also check the percent
busy value, as a disk might have a short average queue length yet
be very active.
It is essential to obtain baseline figures for normal system
operation, so you will know when a performance problem develops,
and when your corrective action has restored (or improved upon)
the performance normally expected.
References:
Installation and Configuration Guide for Oracle7 (platform-specific)
Oracle for UNIX Performance Tuning Tips, part number A22535-1