Problem: RAID and Oracle - 20 Common Questions and
Answers
RAID and Oracle - 20 Common Questions and
Answers
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1.
What is RAID?
RAID is an acronym for Redundant Array of Independent Disks. A
RAID
system consists of an enclosure containing a number of disk
volumes,
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 several disks, and RAID-1 is a mirrored set of two or more
disks.
The only other widely-used level is RAID-5, which is the subject
of
this article. Other RAID levels exist, but tend to be
vendor-specific,
and there is no generally accepted standard for features
included.
2.
What platforms is RAID available for?
Third-party vendors supply RAID systems for most of the popular
UNIX
(including Linux) platforms, and for Windows. Hardware vendors
often
provide their own RAID options.
3.
What does RAID do?
The main feature of RAID-5 is prevention of data loss. 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, 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?
Depending on the application, performance may be better or
worse.
The basic principle of RAID-5 is that files are not stored on
a
single disk, but are divided into sections, 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.
Only hardware-based striping should be used on Windows.
Software
striping, from Disk Administrator, gives very poor
performance.
5.
How does RAID-5 differ from RAID-1?
RAID-1 (mirroring) is a strategy that aims to prevent downtime
due
to loss of a disk, 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
the required 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 practical if disk space is not plentiful. On a
system
where uptime must be maximised, Oracle recommends mirroring
at
least the control files, and preferably 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 that 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. Some vendors, for example EMC, allow
striping
in their RAID systems, but this generally has to be set up by
the
vendor. There is an additional consideration for Oracle, in that
if
a database file needs recovery several physical disks may 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
- The algorithms used to specify how reads and writes are
done.
12. What types of files 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
benefits 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.
Another reason redo logs should not be placed on RAID-5 devices
is
related to the type of caching (if any) being done by the
RAID
system. Given the critical nature of the contents of the redo
logs,
catastrophic loss of data could ensue if the contents of the
cache
were not written to disk, e.g. because of a power failure,
when
Oracle was notified they had been written. This is
particularly
true of write-back caching, where the write is regarded as
having
been written to disk when it has only been written to the
cache.
Write-through caching, where the write is only regarded as
having
completed when it has reached the disk, is much safer, but
still
not recommended for redo logs for the reason mentioned earlier.
13. What about using multiple Database
Writers as an alternative to RAID-5?
Using at least as many DBWR processes as you have database disks
will
maximise synchronous write
capability, by avoiding one disk having to
wait for a DBWR process that 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?
Three 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).
These are briefly described in the following points.
In addition, recent Oracle Database releases (10g and 11g) offer
a
number of powerful and sophisticated features for managing
storage.
For more information on these, see the books listed in
the
References section.
15. What is Asynchronous
I/O?
Asynchronous I/O (aio) 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 aio ia used, there
is
no need to have multiple DBWRs.
Asynchronous I/O is optional on many UNIX platforms. It is used
by
default on Windows.
16. What are the advantages and
disadvantages of Asynchronous I/O?
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; while many UNIX
versions
support it, some do not. Raw devices must be used to store the
files
so the use of aio adds some complexity to the system
administrator's
job. Also, the applications must be able to utilise aio.
17. What is List I/O?
List I/O is a feature found on many SVR4 UNIX variants. 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. It does not exist on Windows.
18. What are the advantages and
disadvantages of List I/O?
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 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, many small and
medium-sized systems will find that the use of logical volumes (with
a
suitable stripe size for the type of application)
performs
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?
On UNIX, there are several commands that can help you
determine
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.
On Windows, the Performance Monitor allows I/O statistics to be
monitored easily and in a graphical manner.
On any platform, 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 expected performance.
References:
RAID and Oracle - 20 Common Questions and Answers
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