Stripping and mirroring

[Mariel Gonzales Faylona]

I would like to know what are the difference between stripping and mirroring a disk. What are their importance, advantages and significance. How do I apply stripping and mirroring on an HP-UX L-class server. Do you have any documentation on stripping and mirroring.

Regards,
Allan

[vinod_25 · Accepted solution]

hi allan

Benefits of striping:

1) Increase the performance of applications that read and write large sequentially accessed files. Striping with randomly accessed files yields inconsistent performance.

2) If each of the disk has their own controller instead of using a common
controller, each can process and transmit data simultaneously, resulting in further performance gains.

3) You can use familiar, standard comands to manage your striped disks, i.e. lvcreate, diskinfo, newfs, fsck, mount, all work with striped disk.

Drawbacks of Striping:

1) Since disk striping uses multiple disks, there is a greater likelihood of disk failure for any one of them, thus requiring restoring from backup (if not mirrored).

2) More complexity occurs when some of the disks have both striped and non-striped logical volumes. It is recommended, but not required that all the logical volumes within the same volume group be striped.

3) Since it is recommended that all the logical volumes within the same volume group be striped, if one of the disks goes bad, then recovery of multiple filesystems may be needed.

4) Performance could actually decrease if your disks are on the same controller and the I/O for one disk has to wait for completion of I/O on another disk.


Other considerations:
----------------------
1) Best performance occurs from a striped logical volume that spans similar disks. The more closely you match the disks in terms of speed, capacity, and interface type, the better the performance. For example, when striping across several disks of varying speeds, performance will be no faster than the slowest disk.

2) If you have more than one interface card or bus to which you can connect the disks, distribute the disks as evenly as possible among them to achieve best I/O performance.

Much of this
information came from Tom Madell's excellent book, "Disks and File Management Tasks on HP-UX".

hope this helps...

Regards

Vinod K

[vinod_25]

hi allan

on mirroring:...

Mirroring permits you to store identical copies of data on separate disks.

The advantages of mirroring are:

* Increased safety of data.

Data is not lost if a single disk fails, or because of media errors local to one part of a disk. Even when using logical volumes spanning multiple disks, you run the risk of losing all your data if a single disk fails; if your logical volume is mirrored, such risk of total failure is considerably reduced.
* Increased system availability.

The system can be kept running even if either the root or primary swap volumes fail when these logical volumes are mirrored.
Together, increased safety of data and increased system availability promote high availability in the event an I/O channel fails.
* Less administrative downtime.

Backups can be done on one copy of the data while the other copy is still in use.
* Improved performance.

Hardware can read data from the most convenient mirror copy. However, writes may take longer. Therefore, if you access data frequently while not performing many writes, your performance will improve.

When using lvextend(1m) to add a mirror to an existing volume, the entire contents of the volume will be copied. When first establishing a mirrored volume it is very important to decide on the allocation policies that you whish to use. Once the volume is mirrored it will not be possible to use lvchange(1m) to tighten up these policies.

Regards

Vinod K

[Indira Aramandla]

Hi Allan,

Mirrored disk operations require the installation of the optional HP MirrorDisk/UX software, which is not included in the standard HP-UX operating system.

The basic reason for striping is performance. Mirroring is for data redundancy/protection. The requirement of your environment / business and the types of disks you are using makes a large difference. Also remember that mirroring creates twice the number of writes.

You can mirror and strip with "extent-based mirrored strips". A good discussion can acutally be found in the man pages for 'lvcreate'. You can adjust the extent size on the disk during the 'pvcreate' if you wish. 'lvcreate's distributed allocation policy is incompatible with the striped scheduling policy ('-i stripes').

lvcreate cannot be performed if the volume group is activated in shared mode. Logical volumes that were created using the striped option are not supported in shared mode.

IA

[Joseph Loo]

hi,

refer to this doc, KBRC00012000:

http://www2.itrc.hp.com/service/cki/search.do?category=c0&mode=id&searchString=KBRC00012000&searchCrit=allwords&docType=Security&docType=Patch&docType=EngineerNotes&docType=BugReports&docType=Hardware&docType=ReferenceMaterials&docType=ThirdParty&search.x=33&search.y=6

through it reference to 11.0 but it is still in place for 11i.

regards.

[Mahesh Kumar Malik]

Hi Allan

Please view the following links to get more details on subject

Striping
http://docs.hp.com/en/B2355-90672/ch08s02.html

http://docs.hp.com/en/B2355-90672/ch08s05.html

Mirroring
http://docs.hp.com/en/B7961-90026/ch04s08.html

You may have combination of striping & mirroring

Regards
Mahesh

[Yogeeraj_1]

hi allan,

also bookmark this one:
http://www.raid5.com/04_00.html

regards
yogeeraj