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

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

[Austin Chang]

Inform dmsetup users to zero the first 4k of the metadata device at
cache creation in examples, just as mentioned in thin-provisioning
documentation. Instructions to use lvmcache for end users may be added
as well.

Link: https://www.redhat.com/archives/dm-devel/2013-April/msg00124.html
Signed-off-by: Austin Chang <austin880625@gmail.com>
---
 Documentation/admin-guide/device-mapper/cache.rst | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/admin-guide/device-mapper/cache.rst b/Documentation/admin-guide/device-mapper/cache.rst
index f15e5254d05b..ef9a82d50d19 100644
--- a/Documentation/admin-guide/device-mapper/cache.rst
+++ b/Documentation/admin-guide/device-mapper/cache.rst
@@ -330,6 +330,10 @@ https://github.com/jthornber/device-mapper-test-suite
 
 ::
 
+  # 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
   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

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

[Matthew Wilcox]

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
> 

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

[Austin Chang]

On Sun, Jul 12, 2020 at 08:59:32PM +0100, Matthew Wilcox wrote:
> 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?

Oops, I copied the command I just used since zeroing the whole metadata
device didn't harm. But 'count=1' should match what I've just written.
I will send a v2 patch later

[PATCH v2] dm-cache: document zeroing metadata device step

[Austin Chang]

Inform dmsetup users to zero the first 4k of the metadata device at
cache creation in examples, just as mentioned in thin-provisioning
documentation. Instructions to use lvmcache for end users may be added
as well.

Link: https://www.redhat.com/archives/dm-devel/2013-April/msg00124.html
Signed-off-by: Austin Chang <austin880625@gmail.com>
---
Changes in v2:
- Correct the dd arguments

 Documentation/admin-guide/device-mapper/cache.rst | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/admin-guide/device-mapper/cache.rst b/Documentation/admin-guide/device-mapper/cache.rst
index f15e5254d05b..16a1c96946a6 100644
--- a/Documentation/admin-guide/device-mapper/cache.rst
+++ b/Documentation/admin-guide/device-mapper/cache.rst
@@ -330,6 +330,10 @@ https://github.com/jthornber/device-mapper-test-suite
 
 ::
 
+  # 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 count=1
   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