BAD BLOCK RELOCATION

[Nobody's Hero]

I was in a web training class last week. The instructor said that when creating logical volumes via lvm, to not use bad block relocation. He claims it was not needed and that there is a performance loss when used. Can anyone explain why this is not needed. I can't remember his reason on this.

[S.K. Chan · Accepted solution]

In theory here will be some performance losses, how much and how big the impact? .. I don't know. By default if you create a non-root logical volume bad block relocation is turned on automatically and I have been leaving them like that without seeing any significant impact, well it could be I do not have many bad blocks yet on the disk. Now on a root volume group (when you install the OS) the root, boot and swap/dump logical volume will have its bad block relocation turned off by default. This is because these logical volumes does not support bad block relocation because of their contigous nature. Turning it OFF will only cause the bad lock to be marked as bad but not relocated. Now to answer you question "why is it not needed", in my opinion I tend to leave bad block relocation ON ... UNLESS ...my logical volumes resides on a disk array (eg: EMC) that has a built-in bad block relocation mechanism already. In this case you want to turn the bad block relocation to NONE instead so that not only the bad block will not be relocated, it will not be recorded or mark as bad.

[Robert-Jan Goossens]

Hi,

Some more info BBR.

http://www5.itrc.hp.com/service/cki/docDisplay.do?docLocale=en_US&docId=200000063198322

And look at this discussion.

http://forums.itrc.hp.com/cm/QuestionAnswer/1,,0xa6d9a848deccd61190050090279cd0f9,00.html

Robert-Jan.

[Michael Tully]

The bad block relocation option will depend largely on the type of disks being used.
If your using an EMC Symmetrix for example, the you would have this setting as 'NONE'. There are quite a few that have this. My advice would be to set the BBR as stated by the disk vendor.

I have never seen where performance has been hindered, because of a different setting.

[harry d brown jr]

You were is a web training class and you were discussing lvm setup?

If you are using disks that are basically jbod, then you should use bad block reallocation.

live free or die
harry

[Jean-Louis Phelix]

Hi,

SK Chan was really clear. I could also add that systems lvols like /stand, / and swap should be available even if LVM is out (hpux -lm). In this case, a relocated block would be hard to access. So rules should be :

/, /stand, primary swap : OFF
Other except Arrays like EMC, XP, even if swap : ON
Arrays : NONE

Regards.

[Bill McNAMARA_1]

xp arrays do bbr at a hardware level, it is not a good idea to do sw bbr via lmv for lvols based on these arrays.

Later,
Bill

[Nobody's Hero]

Thanks, everyone.