Disk and Extent-Based Striping

[Emily S]

What is the different between disk striping and extent-based (distributed) striping?

In term of the number of disks to be striped are the same, eg four disks. Which method will give better performance?

[Rajeev Shukla]

extent-based striping could be done on a single disk by creating say 2 subdisks on a single disk.
And when it comes to disk striping you do the striping accross the disk which surely gives you performance if you have disks accross multiple controllers.

[RAC_1]

Disk stripping is done with -i and -I options to lvcreate. In this you create strip over num,ber of disks. For such lvol, if you want to add space, you will have to add same number od disks, with which it was creatd.

Extent based stripping is with use of /etc/lvmpvg. Here lvol is created in such way that an extend is taken from one disk, then second and so on.

About performance, I have really not checked it. But if disks are from SAN/NAS they may already be stripped based on what raid level it is. So you would not want to stripp that again. Also with disk based stippring, you can not do mirroring.

[A. Clay Stephenson · Accepted solution]

They, in essence, do the same thing although at a different level of granularity. Conventional LVM striping allows a number of blocks that make up a logical volume to be written to disk0, then the next chunk of blocks gets written to disk1, and so on in round-robin fashion. Mirroring is not allowed under this mode of striping. Extent-based striping does the same thing except that rather than a "number of blocks" the stripe size if changed to 1 physical extent (PE) and mirroring is allowed.

Now here's the rub since you asked a performance question. Ideally, the stripe size should be somewhere in the 64-128 (and maybe 256) KB range to efficiently spread the i/o. Conventional striping can use these stripe chunks but the smallest possible PE is 1MB. The smallest possible PE size is much too large to be a good stripe size and I have never seen a significant gain from extent-based striping but conventional striping using 64KB (or so) blocks does yield significant and measureable performance gains.

In general, you are going to find that striping across more than about 3-4 disks yields diminishing gains.

[Senthil Kumar .A_1]

Hi,

In LVM, there is nothing called "disk striping"...

There are 2 type of stiping in LVM...

1) Extent striping(distributed).

This is created using "-D" option to lvcreate command. The disadvantage is the stripe size is equal to the PE size / one extent. Hence this is not a optimized solution for disk striping. Because, the extent size cannot be reduced less than 1MB and hence cannot allign to the default block size of certain application. The only advantage of this striping is ,It helps us implement "RAID 1/0" in LVM, that is mirroring a stiped LV. In general this itself is not considered as optimized implementation of RAID 1/0 per say.

2) Block striping.

This is created using "-i -I" option to lvcreate. Here the block can be less than a single PE extent size. Hence you can optimize as per the application block size. A stipe size could be even 8kb for instance. The advantage is ,due to this factor it is called true striping because it acheives the optimization required by stripping. The disadvantage being you can't mirror these striped LV's. Because Mirror uses 256kb LTG for keeping tab of syncronization, since in block striping , the stripe size could be less than 256kb, by default mirroing is disable for stripes created using "block striping".

Regards,
Senthil Kumar .A

[Bharat Katkar]

In addition to this thought this PV_LV_VG admin doc would be helpful. :)

Regards,