topic Re: Linux fdisk & LVM in Operating System - Linux

Linux fdisk & LVM

SAM_24 — Sun, 20 Feb 2011 19:27:41 GMT

Hi,

If am going to you san disks to put under lvm, do I still need to create fdisk with option 8e?

Can you please shed light on this? Some people say it is required & some say not.

Thanks.

Re: Linux fdisk & LVM

Randy Jones_3 — Mon, 21 Feb 2011 00:46:55 GMT

fdisk is not required. You can do a pvcreate first and specify the path to your SAN allocation or the multipath name.

Some people use fdisk first to try to enhance performance by track-aligning the starting block of the empty space. The SANs I have used have been quite good at hiding the information on physical placement that is necessary for such alignment. The two times I went to the extra work of locating the space and using fdisk to adjust the starting block I found that the buffering and I/O reordering that the SAN controller was doing made the performance increase negligible.

Re: Linux fdisk & LVM

SAM_24 — Mon, 21 Feb 2011 11:34:07 GMT

Randy, Thanks.

Re: Linux fdisk & LVM

Don Mallory — Mon, 21 Feb 2011 18:26:21 GMT

If you are using an HP EVA or EMC CX array, if you do not align the partition, you can see up to a 50% loss of performance, or overhead on the array.

This is easily verifiable on the EMC arrays using the NaviAnalyser tools. As a result, I always align partitions using diskpart (Windows) or fdisk (Linux).

Re: Linux fdisk & LVM

Matti_Kurkela — Tue, 22 Feb 2011 09:45:31 GMT

pvcreate can align the data too.

With older Linux distributions using the 2.6 series kernel (and thus LVM2), you can use the "--metadatasize 250k" to achieve a 256k alignment for your data. That is not very intuitive, so newer versions of LVM utilities include two options for the purpose of explicitly specifying the data alignment: "--dataalignment" and "--dataalignmentoffset".

You can verify the alignment of each existing LVM PV with "pvs -o +pe_start".

If you use partitions to align your data, fdisk will require you to calculate the proper alignment yourself. On the other hand, if you use parted to create the partitions, it includes at least some support for automatic alignment (disclaimers: may depend on your distribution version, I haven't tested this yet).

MK