[linux-lvm] starting LVs on RAID stripe boundaries

[unlisted]

that sounds right... as long as your lvm PVs are entire disks and not inside
of another partition.  I spend months trying to understand all this.

Also, isn't your LVM metadata 192k ?

Sam


On Tue, Jan 20, 2009 at 9:11 AM, F-D. Cami <fcami at winsoft.fr> wrote:

>
> Hi Linux-LVM,
>
> I'm searching for some definitive information about how to make sure that
> logical
> volumes start exactly at the beginning of a RAID stripe.
>
> My hardware is an EMC Clariion CX3-40F array with a RHEL5 host.
> The default stripe element size is 64 KB (128 blocks) on the CX3.
>
> Both EMC and Red Hat recommend using fdisk or parted  to align the first
> partition
> to the start of a stripe boundary (128th block, 128x512=64KB) :
> Red Hat : http://kbase.redhat.com/faq/docs/DOC-2893
> EMC : "EMC CLARiiON Best Practices", PDF available on emc.com
>
> After doing that. the first partition is now aligned, next I need to make
> sure my logical
> volumes are similarly aligned. However, after reading the LVM Howto on
> tldp.org,
> the LVM wiki at redhat.com, and Alasdair Kergon's LinuxTag presentation at
> :
> http://people.redhat.com/agk/talks/LVM2-LinuxTag2006/
> I found no information on how to do that.
>
> I'll now use some reasonable (?) assumptions :
> * the physical extent size is a multiple of 64K (vgcreate's default is 4MB)
> ;
> * if physical extents are aligned, logical volumes are themselves aligned
>  (lvcreate -l, --extents LogicalExtentsNumber) ;
> * since PV, VG and LV infos are stored in PV metadata, the only possible PE
>   misalignment culprit is PV metadata ;
> * PV metadata size is a multiple of 64KB
>  (
> http://orezpraw.com/orezpraw.php/blogosphere/your-filesystem-starts-where)
>
> Since 64KB is my stripe size, and metadata size is a multiple of 64KB,
> pvcreate should do the RightThing(tm) with default options, i.e. start the
> first PE at a multiple of the stripe size.
>
> In other words, aligning the partition the PV uses to the stripe boundary
> and
> using a PE size multiple of the stripe size insure that LVs get themselves
> created
> on stripe boundaries.
>
> Comments, please ?
>
> Thanks,
>
> François Cami
>
> _______________________________________________
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> linux-lvm at redhat.com
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
>
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