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

[Stuart D Gathman]

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)?