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Date: Fri, 30 May 2014 15:26:02 +0100
From: "Richard W.M. Jones" <rjones@redhat.com>
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Subject: Re: [linux-lvm] Testing the new LVM cache feature
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To: Heinz Mauelshagen <heinzm@redhat.com>
Cc: Zdenek Kabelac <zkabelac@redhat.com>, thornber@redhat.com, Mike Snitzer <snitzer@redhat.com>, LVM general discussion and development <linux-lvm@redhat.com>

On Fri, May 30, 2014 at 03:54:49PM +0200, Heinz Mauelshagen wrote:
> On 05/30/2014 03:46 PM, Richard W.M. Jones wrote:
> >I have now set both read_promote_adjustment ==
> >write_promote_adjustment == 0 and used drop_caches between runs.
> 
> Did you adjust "sequential_threshold 0" as well?
> 
> dm-cache tries to avoid promoting large sequential files to the cache,
> because spindles have good bandwidth.
> 
> This is again because of the hot spot caching nature of dm-cache.

Setting this had no effect.

I starting to wonder if my settings are having any effect at all.

Here are the device-mapper tables:

$ sudo dmsetup table
vg_guests-lv_cache_cdata: 0 419430400 linear 8:33 2099200
vg_guests-lv_cache_cmeta: 0 2097152 linear 8:33 2048
vg_guests-home: 0 209715200 linear 9:127 2048
vg_guests-libvirt--images: 0 1677721600 cache 253:1 253:0 253:2 128 0 default 0
vg_guests-libvirt--images_corig: 0 1677721600 linear 9:127 2055211008

And here is the command I used to set sequential_threshold to 0
(there was no error and no other output):

$ sudo dmsetup message vg_guests-libvirt--images 0 sequential_threshold 0

Is there a way to print the current settings?

Could writethrough be enabled?  (I'm supposed to be using writeback).
How do I find out?

Rich.

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