Re: Volume Managers in Linux

Florian Lohoff (flo@quit.mediaways.net)
Wed, 4 Nov 1998 11:51:53 +0100


On Tue, Nov 03, 1998 at 03:01:11PM -0500, Theodore Y. Ts'o wrote:
> The reason why I think the LVM approach is more complex than it needs to
> be is its approach of taking a disk, and chopping it into thousands of
> 4MB pieces, as if LVM were a Ginsu knife, and then having to stich those
> pieces back together into LVM partitions.

But this isnt complex - It just costs performance in seeking time
on Harddrives as those stayed the same over the last years
unlike throughtput.

> Does the 4M ginsu-knife approach buy you something? Yes, it allows you
> to have infinitely configurable partitions, which can be scattered
> across the entire disk in a non-contiguous fashion. Whether or not this
> is a good thing or not can be debated. I will say that if the
> filesystem isn't involved, some of its optimizations to reduce seek
> times get thrown out the door since there are no guarantees whether
> adjacent 4M blocks are anywhere near each other or not. Then again,
> some people may prefer the ability to create partitions without needing
> any kind of advance planning as being more important than performance.
> (I don't, but clearly some people do.)

I dont see the problems with fragmentation in the LVM layer as

1. I dont think the fragmentation on end user machines with a simple
"one shot" installation gets that big that it will cost a lot performance.
2. Even when you get lvm-layer-fragmentation you may defrag on the fly
in io-idle times or offline.
3. lvm-layer-fragmentation if once defragmented will not come back while using
the fs unlike the fs-layer-fragmentation.

Flo

-- 
Florian.Lohoff@mediaWays.net			+49-5241-80-7085
Good, Fast, Cheap: Pick any two (you can't have all three). (RFC 1925)

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