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

From: Naohiro Aota <Naohiro.Aota@wdc.com>
To: Anand Jain <anand.jain@oracle.com>
Cc: Damien Le Moal <Damien.LeMoal@wdc.com>,
	"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
	David Sterba <dsterba@suse.com>, Chris Mason <clm@fb.com>,
	Josef Bacik <josef@toxicpanda.com>,
	Nikolay Borisov <nborisov@suse.com>,
	Matias Bjorling <Matias.Bjorling@wdc.com>,
	Johannes Thumshirn <jthumshirn@suse.de>,
	Hannes Reinecke <hare@suse.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH v3 03/27] btrfs: Check and enable HMZONED mode
Date: Tue, 20 Aug 2019 14:07:37 +0900	[thread overview]
Message-ID: <20190820050737.ngyaamjkdmzvhlqj@naota.dhcp.fujisawa.hgst.com> (raw)
In-Reply-To: <86ef7944-0029-3d61-0ae3-874015726751@oracle.com>

On Sat, Aug 17, 2019 at 07:56:50AM +0800, 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.
>
>more below..

Sure. I will add a table in the next version.

btrfs.txt seems not to have much there. I'm considering to write a new
page in the wiki like:

https://btrfs.wiki.kernel.org/index.php/Feature:Skinny_Metadata

>>>>+	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?
>
>Thanks, Anand

I just post the updated version of userland side series:
https://lore.kernel.org/linux-btrfs/20190820045258.1571640-1-naohiro.aota@wdc.com/T/

Thanks,
Naohiro