Re: [PATCH v6 03/28] btrfs: Check and enable HMZONED mode

[Josef Bacik <josef@toxicpanda.com>]

On 12/12/19 11:08 PM, Naohiro Aota wrote:
> HMZONED mode cannot be used together with the RAID5/6 profile for now.
> Introduce the function btrfs_check_hmzoned_mode() to check this. This
> function will also check if HMZONED flag is enabled on the file system and
> if the file system consists of zoned devices with equal zone size.
> 
> Additionally, as updates to the space cache 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 completely disabling the space cache.  This does not
> introduce any problems in HMZONED mode: all the free space is located after
> the allocation pointer and no free space is located before the pointer.
> There is no need to have such cache.
> 
> 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, HMZONED will disable:
> 
> | Disabled features | Reason                                              |
> |-------------------+-----------------------------------------------------|
> | RAID5/6           | 1) Non-full stripe write cause overwriting of       |
> |                   | parity block                                        |
> |                   | 2) Rebuilding on high capacity volume (usually with |
> |                   | SMR) can lead to higher failure rate                |
> |-------------------+-----------------------------------------------------|
> | 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                                         |
> | async checksum    | Not to mix up bios by multiple workers              |
> 
> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>

I assume the progs will be updated to account for these limitations as well?

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

Thanks,

Josef