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Re: [linux-lvm] How big are PEs by default?

On Tue, Mar 11, 2003 at 11:22:20AM +0000, Paul Furness wrote:
> Hi.
> I'm trying to get LVM 1.0.7 going on our new production server. I've
> never used LVM before, but based on reccommendations and looking at what
> it can do, it looks like exactly what I need.
> For the reccord, it's a SMP Xeon system, running RedHat 7.3 plus current
> RH patches.
> I am confused about the defualt setting for Physical Extents when you
> run vgcreate on a new PV. 
> Looking through the man page for vgcreate is says this:
>        -s, --physicalextentsize PhysicalExtentSize[kKmMgGtT]
>               Sets  the  physical extent size on physical volumes
>               of this volume group. ................. The default
>               of 32 MB allows LV sizes of ~2TB because as many as
>               ~64k  extents  are supported per LV.
> A bit later, under "Examples." it says this:
>        To create a volume group named test_vg using physical vol-
>        umes /dev/hdk1,  /dev/hdl1,  and  /dev/hdm1  with  default
>        physical extent size of 4MB:
>             vgcreate test_vg /dev/sd[k-m]1
> So what is the default? 4M or 32M? Or something else? Or should I simply
> forget about the default and pick a number?

The default is 32MB, the example has not been updated. It used to be 4MB but
limiting LVs to 256MB by default soon became a problem for people.

> Finally, is there any documentation that's more detailed than the HOWTO,
> and explains a bit more about the theory of LVM - what happens when you
> change the size of existing file systems, and so on? The HowTo is ok
> (although not all that great if you've never patched a kernel before),
> but necessarily brief. :)

Not that I know of. But if you want to know what a filesystem does when it's
extended then you need filesystem documentation, not LVM documentation.



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