Re: [PATCH resend] btrfs: mount: call btrfs_check_rw_degradable only if there is a missing device

[David Sterba <dsterba@suse.cz>]

On Tue, Oct 19, 2021 at 06:43:38PM +0800, Anand Jain wrote:
> In open_ctree() in btrfs_check_rw_degradable() [1], we check each block
> group individually if at least the minimum number of devices is available
> for that profile. If all the devices are available, then we don't have to
> check degradable.
> 
> [1]
> open_ctree()
> ::
> 3559 if (!sb_rdonly(sb) && !btrfs_check_rw_degradable(fs_info, NULL)) {
> 
> Also before calling btrfs_check_rw_degradable() in open_ctee() at the
> line number shown below [2] we call btrfs_read_chunk_tree() and down to
> add_missing_dev() to record number of missing devices.
> 
> [2]
> open_ctree()
> ::
> 3454         ret = btrfs_read_chunk_tree(fs_info);
> 
> btrfs_read_chunk_tree()
>  read_one_chunk() / read_one_dev()
>   add_missing_dev()
> 
> So, check if there is any missing device before btrfs_check_rw_degradable()
> in open_ctree().
> 
> With this, in an example, the mount command could save ~16ms.[3] in the
> most common case, that is no device is missing.
> 
> [3]
>  1) * 16934.96 us | btrfs_check_rw_degradable [btrfs]();
> 
> Signed-off-by: Anand Jain <anand.jain@oracle.com>

Added to misc-next, thanks.