Cheers.

This post is more targeted towards end-users, small companies, schools, small labs, where requirements are often not written down and where the filesystem layout is often not taken into consideration.
About the LVM overhead, do you have any reference(s)? I am pretty much interested in this area.

Thanks for your valuable input.

“this becomes more difficult to predict when you have different raid volumes over different spindles”

should read:

“this becomes more difficult to predict when you have different raid volumes over the same spindles”

There are some enterprise storage systems that do this in order to make use of the performance characteristics of hard disk drives, but you require pretty intelligent controllers and they typically spread these out over a large number of spindles in order to minimize contention. HP’s EVA/HSV series does volume virtualization over large pools of disks and their performance best practice guide is to have as many disks in the same disk group as possible. This buys you more spindles and less chance of contention.

Linux (IIRC) and HD command queues will themselves attempt to reorder actual requests to the drive in order to minimize seeks and rotational latency, but it can only do so much.

Additionally, it should be mentioned that LVM technology adds flexibility, but at the cost of performance. e.g. LVM on Linux can come with a cost around 30%.

Following that idea of performance at all cost, up to the extreme — and stupid! — case, having one file/directory per hard disks would also be the best from a strict performance point of view. Obviously, this is not a reasonable option. Therefore, you are always facing a deal between price versus performance.

By the way, RAID has been specifically set up to address this compromise: RAID historically meant Redundant Array of *Inexpensive* Disks.

Finally, I would be confident in the linux kernel to deal with the spindle contention problem. I mean, in the face of contention, linux may order requests to hard drives in a “smart” way, reducing contention to a minimum (well I hope it can do that actually, I do not have any evidence that it actually does it! ).