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

[Zdenek Kabelac <zkabelac@redhat.com>]

Dne 28.3.2012 20:26, Stuart D Gathman napsal(a):
> 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)?

You may check the man page for dmsetup suspend operation - options  nolockfs
and noflush.

For lvm creation of snapshot I guess everyone wants to get the filesystem in
'stable' condition - so all in-flight operation before creation of snapshot
happened should hit the disk - and if you run fsck you should get pretty
consistent results.

Or thin it would be preferable that flush & lockfs part should be skipped at
this moment and users would get their snapshot fs in quite broken state ?

zdenek