linux-kernel - Re: [PATCH] dm-cache: document zeroing metadata device step

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Message-ID: <20200712195932.GV12769@casper.infradead.org>
Date:   Sun, 12 Jul 2020 20:59:32 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Austin Chang <austin880625@...il.com>
Cc:     dm-devel@...hat.com, Alasdair Kergon <agk@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dm-cache: document zeroing metadata device step

On Mon, Jul 13, 2020 at 03:40:39AM +0800, Austin Chang wrote:
> +  # When using dmsetup directly instead of volume manager like lvm2,
> +  # the first 4k of the metadata device should be zeroed to indicate
> +  # empty metadata.
> +  dd if=/dev/zero of=/dev/mapper/metadata bs=4k conv=notrunc

... don't you need 'count=1' or you'll zero the entire device?

>    dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
>  	  /dev/mapper/ssd /dev/mapper/origin 512 1 writeback default 0'
>    dmsetup create my_cache --table '0 41943040 cache /dev/mapper/metadata \
> -- 
> 2.26.2
>