linux-kernel - Re: [RFC PATCH v2 3/3] ceph: update documentation regarding snapshot naming limitations

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Date:   Wed, 16 Mar 2022 08:48:30 +0800
From:   Xiubo Li <xiubli@...hat.com>
To:     Luís Henriques <lhenriques@...e.de>,
        Jeff Layton <jlayton@...nel.org>,
        Ilya Dryomov <idryomov@...il.com>
Cc:     ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 3/3] ceph: update documentation regarding snapshot
 naming limitations


On 3/16/22 12:19 AM, Luís Henriques wrote:
> Signed-off-by: Luís Henriques <lhenriques@...e.de>
> ---
>   Documentation/filesystems/ceph.rst | 10 ++++++++++
>   1 file changed, 10 insertions(+)
>
> diff --git a/Documentation/filesystems/ceph.rst b/Documentation/filesystems/ceph.rst
> index 4942e018db85..d487cabe792d 100644
> --- a/Documentation/filesystems/ceph.rst
> +++ b/Documentation/filesystems/ceph.rst
> @@ -57,6 +57,16 @@ a snapshot on any subdirectory (and its nested contents) in the
>   system.  Snapshot creation and deletion are as simple as 'mkdir
>   .snap/foo' and 'rmdir .snap/foo'.
>   
> +Snapshot names have two limitations:
> +
> +* They can not start with an underscore ('_'), as these names are reserved
> +  for internal usage by the MDS.
> +* They can not exceed 240 characters in size.  This is because the MDS makes
> +  use of long snapshot names internally, which follow the format:
> +  `_<SNAPSHOT-NAME>_<INODE-NUMBER>`.  Since filenames in general can't have
> +  more than 255 characters, and `<node-id>` takes 13 characters, the long
> +  snapshot names can take as much as 255 - 1 - 1 - 13 = 240.
> +
>   Ceph also provides some recursive accounting on directories for nested
>   files and bytes.  That is, a 'getfattr -d foo' on any directory in the
>   system will reveal the total number of nested regular files and
>
LGTM.

Reviewed-by: Xiubo Li <xiubli@...hat.com>