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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