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

From: Damien Le Moal <Damien.LeMoal@wdc.com>
To: Graham Cobb <g.btrfs@cobb.uk.net>,
	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 11:03:35 +0000	[thread overview]
Message-ID: <CH2PR04MB6522151758FE8FD0ECBCB09EE7F40@CH2PR04MB6522.namprd04.prod.outlook.com> (raw)
In-Reply-To: 1dc43899-82de-564f-6e52-bd5b990f3887@cobb.uk.net

On 2020/12/01 19:45, Graham Cobb wrote:
> On 01/12/2020 02:29, Damien Le Moal wrote:
>> 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 there will be no testing against HA drives? And no btrfs developers
> will have one? And they have very different timing and possibly failure
> modes from "normal" disks when they do GC?
> 
> I think there is no option but to disallow them. If HA drives start to
> appear in significant numbers then that would be easy enough to change,
> after suitable testing.

Works for me. Even simpler :)

> 
>> 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.
> 
> That is a bit of a shame. With that unfortunate decision in the block
> layer, system managers need to realise that partitioning an HA disk
> means they may be entering territory untested by their filesystem.

Well, not really. HA drives, per specifications, are backward compatible. If
they are partitioned, the block layer will force a regular drive mode use,
hiding their zoned interface, which is completely optional to use in the first
place.

If by "untested territory" you mean the possibility of hitting drive FW bugs
coming from the added complexity of internal GC, then I would argue that this is
a common territory for any FS on any drive, especially SSDs: device FW bugs do
exist and show up from time to time, even on the simplest of drives.

-- 
Damien Le Moal
Western Digital Research