[linux-lvm] Snapshots
Mike Snitzer
snitzer at redhat.com
Fri Feb 17 13:56:24 UTC 2012
On Thu, Feb 16 2012 at 11:46pm -0500,
Stuart D. Gathman <stuart at bmsi.com> wrote:
> Long ago, Nostradamus foresaw that on Feb 16, Mike Snitzer would write:
>
> >>An elegant part of the LVM system is that the device mapper kernel support is
> >>very general, and new data structures can be experimented with entirely in user
> >>code - with a script language even. Metadata for experimental structures does
> >>not have to stored with the main metadata.
> >
> >Please note that the dm-thinp code has metadata in the kernel (on-disk
> >format for btrees, etc) much like a filesystem would have. So there is
> >both kernel and userspace (lvm2) metadata for dm-thinp.
>
> Yes, but isn't this loaded into the kernel via userland tools like
> device-mapper?
The 'thin-pool' and 'thin' DM device tables are loaded from userspace
via DM interfaces.
The kernel metadata isn't loaded from userspace. It is created and/or
changed by certain actions taken from userspace (via DM messages).
> So while a kernel feature would be required for a new
> type of kernel metadata, experimental uses of existing formats can
> be done in userland.
The kernel manages the kernel's metadata. But the LVM metadata that
userspace uses to coordinate and manage the thin devices can be
changed independently.
The thin-provisioning-tools know the kernel's metadata format and can
check it (and in the future repair it).
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