Re: [PATCH v3 03/27] btrfs: Check and enable HMZONED mode

[Damien Le Moal <Damien.LeMoal@wdc.com>]

On 2019/08/16 16:57, Anand Jain wrote:
> 
> 
> On 8/16/19 10:23 PM, Damien Le Moal wrote:
>> On 2019/08/15 22:46, Anand Jain wrote:
>>> On 8/8/19 5:30 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 disabling completely the space cache.  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.
>>>>
>>>> For the same reason, NODATACOW is also disabled.
>>>>
>>>> Also INODE_MAP_CACHE is also disabled to avoid preallocation in the
>>>> INODE_MAP_CACHE inode.
>>>
>>>    A list of incompatibility features with zoned devices. This need better
>>>    documentation, may be a table and its reason is better.
>>
>> Are you referring to the format of the commit message itself ? Or would you like
>> to see a documentation added to Documentation/filesystems/btrfs.txt ?
> 
>   Documenting in the commit change log is fine. But it can be better
>   documented in a listed format as it looks like we have a list of
>   features which will be incompatible with zoned devices.

OK. We can update btrfs.txt doc file.

> 
> more below..
[...]>>>> +	if (!hmzoned_devices && incompat_hmzoned) {
>>>> +		/* No zoned block device found on HMZONED FS */
>>>> +		btrfs_err(fs_info, "HMZONED enabled file system should have zoned devices");
>>>> +		ret = -EINVAL;
>>>> +		goto out;
>>>
>>>
>>>    When does the HMZONED gets enabled? I presume during mkfs. Where are
>>>    the related btrfs-progs patches? Searching for the related btrfs-progs
>>>    patches doesn't show up anything in the ML. Looks like I am missing
>>>    something, nor the cover letter said anything about the progs part.
> 
> 
>   Any idea about this comment above?

Yep, the feature is set at format time if some of the devices in the volume are
zoned. The btrfs-progs changes to handle that are ready too.

Naohiro, please re-post btrfs-progs too !

> 
> Thanks, Anand

-- 
Damien Le Moal
Western Digital Research