RAID and Oracle - 20 Common Questions and Answers

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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 [ID 38281.1]

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