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From: Zdenek Kabelac <zdenek.kabelac@gmail.com>
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Date: Thu, 19 Nov 2015 09:56:41 +0100
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Subject: Re: [linux-lvm] Cache block size warning
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To: LVM general discussion and development <linux-lvm@redhat.com>

Dne 19.11.2015 v 08:27 Ciprian Hacman napsal(a):
> Hi,
>
> We are configuring lvm with SSD caching on some of our boxes in AWS. When the
> cache is initialised, we see this in logs:
>
>     device-mapper*: cache: You have created a cache device with a lot of
>     individual cache blocks (4915200)*
>
>     *               All these mappings can consume a lot of kernel memory, and
>     take some time to read/write.*
>
>     *               Please consider increasing the cache block size to reduce
>     the overall cache block count.*
>
>
> Is this something to worry? If so, how can we improve it?
>

If you have lots of kernel memory - it's probably ok.
Otherwise you may try to use bigger chunk size.

e.g.  --chunksize 512K   or even more:

lvcreate --type cache-pool -L300G vg/cpool

lvcreate -H --chunksize 512K -L1.5T   --name data  vg/cpool

It's possible future version of lvm2 will try to create this by default.

Zdenek