Re: [PATCH v10 05/41] btrfs: check and enable ZONED mode

From: Damien Le Moal <Damien.LeMoal@wdc.com>
To: Anand Jain <anand.jain@oracle.com>,
	"dsterba@suse.cz" <dsterba@suse.cz>,
	Naohiro Aota <Naohiro.Aota@wdc.com>,
	"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
	"dsterba@suse.com" <dsterba@suse.com>,
	"hare@suse.com" <hare@suse.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	Jens Axboe <axboe@kernel.dk>,
	"hch@infradead.org" <hch@infradead.org>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
	Josef Bacik <josef@toxicpanda.com>
Subject: Re: [PATCH v10 05/41] btrfs: check and enable ZONED mode
Date: Tue, 1 Dec 2020 06:09:27 +0000	[thread overview]
Message-ID: <CH2PR04MB652224DE2A682FADE28F443DE7F40@CH2PR04MB6522.namprd04.prod.outlook.com> (raw)
In-Reply-To: 47fffa8d-a495-5588-f970-1ab04ece19b6@oracle.com

On 2020/12/01 14:54, Anand Jain wrote:
> On 1/12/20 10:29 am, Damien Le Moal wrote:
>> On 2020/12/01 11:20, Anand Jain wrote:
>>> On 30/11/20 9:15 pm, Damien Le Moal wrote:
>>>> On 2020/11/30 21:13, Anand Jain wrote:
>>>>> On 28/11/20 2:44 am, David Sterba wrote:
>>>>>> On Wed, Nov 18, 2020 at 07:29:20PM +0800, Anand Jain wrote:
>>>>>>> On 10/11/20 7:26 pm, Naohiro Aota wrote:
>>>>>>>> This commit introduces the function btrfs_check_zoned_mode() to check if
>>>>>>>> ZONED flag is enabled on the file system and if the file system consists of
>>>>>>>> zoned devices with equal zone size.
>>>>>>>>
>>>>>>>> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
>>>>>>>> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
>>>>>>>> Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
>>>>>>>> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
>>>>>>>> ---
>>>>>>>>      fs/btrfs/ctree.h       | 11 ++++++
>>>>>>>>      fs/btrfs/dev-replace.c |  7 ++++
>>>>>>>>      fs/btrfs/disk-io.c     | 11 ++++++
>>>>>>>>      fs/btrfs/super.c       |  1 +
>>>>>>>>      fs/btrfs/volumes.c     |  5 +++
>>>>>>>>      fs/btrfs/zoned.c       | 81 ++++++++++++++++++++++++++++++++++++++++++
>>>>>>>>      fs/btrfs/zoned.h       | 26 ++++++++++++++
>>>>>>>>      7 files changed, 142 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
>>>>>>>> index aac3d6f4e35b..453f41ca024e 100644
>>>>>>>> --- a/fs/btrfs/ctree.h
>>>>>>>> +++ b/fs/btrfs/ctree.h
>>>>>>>> @@ -948,6 +948,12 @@ struct btrfs_fs_info {
>>>>>>>>      	/* Type of exclusive operation running */
>>>>>>>>      	unsigned long exclusive_operation;
>>>>>>>>      
>>>>>>>> +	/* Zone size when in ZONED mode */
>>>>>>>> +	union {
>>>>>>>> +		u64 zone_size;
>>>>>>>> +		u64 zoned;
>>>>>>>> +	};
>>>>>>>> +
>>>>>>>>      #ifdef CONFIG_BTRFS_FS_REF_VERIFY
>>>>>>>>      	spinlock_t ref_verify_lock;
>>>>>>>>      	struct rb_root block_tree;
>>>>>>>> @@ -3595,4 +3601,9 @@ static inline int btrfs_is_testing(struct btrfs_fs_info *fs_info)
>>>>>>>>      }
>>>>>>>>      #endif
>>>>>>>>      
>>>>>>>> +static inline bool btrfs_is_zoned(struct btrfs_fs_info *fs_info)
>>>>>>>> +{
>>>>>>>> +	return fs_info->zoned != 0;
>>>>>>>> +}
>>>>>>>> +
>>>>>>>>      #endif
>>>>>>>> diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
>>>>>>>> index 6f6d77224c2b..db87f1aa604b 100644
>>>>>>>> --- a/fs/btrfs/dev-replace.c
>>>>>>>> +++ b/fs/btrfs/dev-replace.c
>>>>>>>> @@ -238,6 +238,13 @@ static int btrfs_init_dev_replace_tgtdev(struct btrfs_fs_info *fs_info,
>>>>>>>>      		return PTR_ERR(bdev);
>>>>>>>>      	}
>>>>>>>>      
>>>>>>>> +	if (!btrfs_check_device_zone_type(fs_info, bdev)) {
>>>>>>>> +		btrfs_err(fs_info,
>>>>>>>> +			  "dev-replace: zoned type of target device mismatch with filesystem");
>>>>>>>> +		ret = -EINVAL;
>>>>>>>> +		goto error;
>>>>>>>> +	}
>>>>>>>> +
>>>>>>>>      	sync_blockdev(bdev);
>>>>>>>>      
>>>>>>>>      	list_for_each_entry(device, &fs_info->fs_devices->devices, dev_list) {
>>>>>>>
>>>>>>>      I am not sure if it is done in some other patch. But we still have to
>>>>>>>      check for
>>>>>>>
>>>>>>>      (model == BLK_ZONED_HA && incompat_zoned))
>>>>>>
>>>>>> Do you really mean BLK_ZONED_HA, ie. host-aware (HA)?
>>>>>> btrfs_check_device_zone_type checks for _HM.
>>>>>
>>>>>
>>>>> Still confusing to me. The below function, which is part of this
>>>>> patch, says we don't support BLK_ZONED_HM. So does it mean we
>>>>> allow BLK_ZONED_HA only?
>>>>>
>>>>> +static inline bool btrfs_check_device_zone_type(struct btrfs_fs_info
>>>>> *fs_info,
>>>>> +						struct block_device *bdev)
>>>>> +{
>>>>> +	u64 zone_size;
>>>>> +
>>>>> +	if (btrfs_is_zoned(fs_info)) {
>>>>> +		zone_size = (u64)bdev_zone_sectors(bdev) << SECTOR_SHIFT;
>>>>> +		/* Do not allow non-zoned device */
>>>>
>>>> This comment does not make sense. It should be:
>>>>
>>>> 		/* Only allow zoned devices with the same zone size */
>>>>
>>>>> +		return bdev_is_zoned(bdev) && fs_info->zone_size == zone_size;
>>>>> +	}
>>>>> +
>>>>> +	/* Do not allow Host Manged zoned device */
>>>>> +	return bdev_zoned_model(bdev) != BLK_ZONED_HM;
>>>>
>>>> The comment is also wrong. It should read:
>>>>
>>>> 	/* Allow only host managed zoned devices */
>>>>
>>>> This is because we decided to treat host aware devices in the same way as
>>>> regular block devices, since HA drives are backward compatible with regular
>>>> block devices.
>>>
>>>
>>> Yeah, I read about them, but I have questions like do an FS work on top
>>> of a BLK_ZONED_HA without modification?
>>
>> Yes. These drives are fully backward compatible and accept random writes
>> anywhere. Performance however is potentially a different story as the drive will
>> eventually need to do internal garbage collection of some sort, exactly like an
>> SSD, but definitely not at SSD speeds :)
>>
>>>    Are we ok to replace an HM device with a HA device? Or add a HA device
>>> to a btrfs on an HM device.
>>
>> We have a choice here: we can treat HA drives as regular devices or treat them
>> as HM devices. Anything in between does not make sense. I am fine either way,
>> the main reason being that there are no HA drive on the market today that I know
>> of (this model did not have a lot of success due to the potentially very
>> unpredictable performance depending on the use case).
>>
>> So the simplest thing to do is, in my opinion, to ignore their "zone"
>> characteristics and treat them as regular disks. But treating them as HM drives
>> is a simple to do too.
>>> Of note is that a host-aware drive will be reported by the block layer as
>> BLK_ZONED_HA only as long as the drive does not have any partition. If it does,
>> then the block layer will treat the drive as a regular disk.
> 
> IMO. For now, it is better to check for the BLK_ZONED_HA explicitly in a 
> non-zoned-btrfs. And check for BLK_ZONED_HM explicitly in a zoned-btrfs. 

Sure, we can. But since HA drives are backward compatible, not sure the HA check
for non-zoned make sense. As long as the zoned flag is not set, the drive can be
used like a regular disk. If the user really want to use it as a zoned drive,
then it can format with force selecting the zoned flag in btrfs super. Then the
HA drive will be used as a zoned disk, exactly like HM disks.

> This way, if there is another type of BLK_ZONED_xx in the future, we 
> have the opportunity to review to support it. As below [1]...

It is very unlikely that we will see any other zone model. ZNS adopted the HM
model in purpose, to avoid multiplying the possible models, making the ecosystem
effort a nightmare.

> 
> [1]
> bool btrfs_check_device_type()
> {
> 	if (bdev_is_zoned()) {
> 		if (btrfs_is_zoned())
> 			if (bdev_zoned_model == BLK_ZONED_HM)
> 			/* also check the zone_size. */
> 				return true;
> 		else
> 			if (bdev_zoned_model == BLK_ZONED_HA)
> 			/* a regular device and FS, no zone_size to check I think? */
> 				return true;
> 	} else {
> 		if (!btrfs_is_zoned())
> 			return true
> 	}
> 
> 	return false;
> }
> 
> Thanks.

Works for me. May be reverse the conditions to make things easier to read and
understand:

bool btrfs_check_device_type()
{
	if (btrfs_is_zoned()) {
		if (bdev_is_zoned()) {
			/* also check the zone_size. */
			return true;
		}

		/*
		 * Regular device: emulate zones with zone size equal
		 * to device extent size.
		 */
		return true;
	}

	if (bdev_zoned_model == BLK_ZONED_HM) {
		/* Zoned HM device require zoned btrfs */
		return false;
	}

	/* Regular device or zoned HA device used as a regular device */
	return true;
}

-- 
Damien Le Moal
Western Digital Research