Re: [PATCH v9 07/41] btrfs: disallow space_cache in ZONED mode

From: Josef Bacik <josef@toxicpanda.com>
To: Naohiro Aota <naohiro.aota@wdc.com>,
	linux-btrfs@vger.kernel.org, dsterba@suse.com
Cc: hare@suse.com, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v9 07/41] btrfs: disallow space_cache in ZONED mode
Date: Mon, 2 Nov 2020 12:02:02 -0500	[thread overview]
Message-ID: <4a2f90c0-e595-1a0f-5373-80517f9b9843@toxicpanda.com> (raw)
In-Reply-To: <f0a4ae9168940bf1756f89a140cabedb8972e0d1.1604065695.git.naohiro.aota@wdc.com>

On 10/30/20 9:51 AM, Naohiro Aota wrote:
> As updates to the space cache v1 are in-place, the space cache cannot be
> located over sequential zones and there is no guarantees that the device
> will have enough conventional zones to store this cache. Resolve this
> problem by disabling completely the space cache v1.  This does not
> introduces any problems with sequential block groups: all the free space is
> located after the allocation pointer and no free space before the pointer.
> There is no need to have such cache.
> 
> Note: we can technically use free-space-tree (space cache v2) on ZONED
> mode. But, since ZONED mode now always allocate extents in a block group
> sequentially regardless of underlying device zone type, it's no use to
> enable and maintain the tree.
> 
> For the same reason, NODATACOW is also disabled.
> 
> Also INODE_MAP_CACHE is also disabled to avoid preallocation in the
> INODE_MAP_CACHE inode.
> 
> In summary, ZONED will disable:
> 
> | Disabled features | Reason                                              |
> |-------------------+-----------------------------------------------------|
> | RAID/Dup          | Cannot handle two zone append writes to different   |
> |                   | zones                                               |
> |-------------------+-----------------------------------------------------|
> | space_cache (v1)  | In-place updating                                   |
> | NODATACOW         | In-place updating                                   |
> |-------------------+-----------------------------------------------------|
> | fallocate         | Reserved extent will be a write hole                |
> | INODE_MAP_CACHE   | Need pre-allocation. (and will be deprecated?)      |
> |-------------------+-----------------------------------------------------|
> | MIXED_BG          | Allocated metadata region will be write holes for   |
> |                   | data writes                                         |
> 
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
> ---
>   fs/btrfs/super.c | 12 ++++++++++--
>   fs/btrfs/zoned.c | 18 ++++++++++++++++++
>   fs/btrfs/zoned.h |  5 +++++
>   3 files changed, 33 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 3312fe08168f..9064ca62b0a0 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -525,8 +525,14 @@ int btrfs_parse_options(struct btrfs_fs_info *info, char *options,
>   	cache_gen = btrfs_super_cache_generation(info->super_copy);
>   	if (btrfs_fs_compat_ro(info, FREE_SPACE_TREE))
>   		btrfs_set_opt(info->mount_opt, FREE_SPACE_TREE);
> -	else if (cache_gen)
> -		btrfs_set_opt(info->mount_opt, SPACE_CACHE);
> +	else if (cache_gen) {
> +		if (btrfs_is_zoned(info)) {
> +			btrfs_info(info,
> +			"clearring existing space cache in ZONED mode");

'clearing', and then you can add

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef