[linux-lvm] LVM commands extremely slow during raid check/resync

[Ray Morris]

> A question (or minor nit): how could lvm possibly require a fs sync
> to create a snapshot?  I could see this for Xen, where guest OS has
> to support a com channel to host.  But for full virtualization, LVM
> doesn't know in general what OS is running,

LVM doesn't have anything whatever to do with virtualization. There are
some cool ways to use them together, but actually most systems using
LVM aren't using virtualization. Sure, you could put a disk image on 
a filesystem which is on a logical volume, but the snapshot and the
sync are in the OS where the logical volume is. Whether files in the FS
on the LV are used by Firefox or by qemu doesn't matter.
-- 
Ray Morris
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On Wed, 28 Mar 2012 14:26:11 -0400
Stuart D Gathman <stuart at bmsi.com> wrote:

> Long ago, Nostradamus foresaw that on 03/28/2012 03:53 AM, Zdenek 
> Kabelac would write:
> > It seems that your cfq scheduler should be tuned better for raid 
> > arrays - I assume you allow the system to create very large queues
> > of buffers and your mdraid isn't fast enough to store dirty pages
> > on disk 
> > - I'd probably suggest to significantly lower the maximum amount of 
> > dirty pages - as creation of snapshot requires fs sync operation it 
> > will need to wait till all buffers before the operation are in
> > place.
> A question (or minor nit): how could lvm possibly require a fs sync
> to create a snapshot?  I could see this for Xen, where guest OS has
> to support a com channel to host.  But for full virtualization, LVM
> doesn't know in general what OS is running, or how to suggest an FS
> sync.  Or is this something an admin does, run a script that tells
> guest to sync before creating shapshot through lvm (to maximize the
> amount of useful data in the snapshot)?
> 
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