What is the difference between Striping and PVG Distributed/Strict

[white221g]

Please explain the difference between the following terms:
"Filesystem is Striped across 4 disks"
&
"Filesystem is PVG Distributed/Strict across 4 disks"

Please explain from a storage perspective. How many luns are needed to build/extend:
1) a Filesystem with 4 way striping
2) a Filesystem which is PVG Distributed/Strict across 4 disks.

If there is a doc available that explains the basics between these two please provide link.

thanks

[Pete Randall]

With striping, your data is spread across the four disks sequentially (record 1 on disk 1, record 2 on disk 2 - though it's actually blocks of data, not individual records). This can speed access to the data.

With PVG, the data is written to disk 1 until it's space is full, then it goes to disk 2, then 3, etc.


Pete

[Torsten.]

In general, striping and distributed allocation policy does the same.

An "amount of data" is written to the first, the second, the third ... disk.

The difference is the "amount of data", 8k per default with striping, but 1PE (some megabyte!) with distribution.

Compare

http://docs.hp.com/en/B3921-60631/lvcreate.1M.html

[white221g]

Thanks for your replies. Here is the situation. The arrays have a minimum LUN size of 15gb. Please answer:
If i need to extend a 4 way striped Filesystem by 2gb, then the minimum number of luns i would need is 4, correct?

If i need to extend a Filesystem that is PVG Strict/Distributed by 2gb, then will i still need 4 luns, or just 1 lun will suffice?

[Torsten.]

You will need 4 in both cases.

What type of array is it?

Most arrays will do the striping transparently anyway by itself.

[Jozef_Novak]

Hello,

in general, for extension of a distributed logical volume, you need at least two disks (so striping can actually take place). If you already have 4-way distribution then extending across 4 disks is ideal for your case. PVG-strict-distributed allocation means that the logical volume is striped across all disks in a physical volume group (PVG) and moreover, the mirrors of the logical volume cannot share the same PVG.

Hope this helps to explain your question.
Jozef