Re: [PATCH v10 11/41] btrfs: implement log-structured superblock for ZONED mode

[Hannes Reinecke <hare@suse.de>]

On 11/24/20 7:46 AM, Anand Jain wrote:
> On 10/11/20 7:26 pm, Naohiro Aota wrote:
>> Superblock (and its copies) is the only data structure in btrfs which 
>> has a
>> fixed location on a device. Since we cannot overwrite in a sequential 
>> write
>> required zone, we cannot place superblock in the zone. One easy 
>> solution is
>> limiting superblock and copies to be placed only in conventional zones.
>> However, this method has two downsides: one is reduced number of 
>> superblock
>> copies. The location of the second copy of superblock is 256GB, which 
>> is in
>> a sequential write required zone on typical devices in the market today.
>> So, the number of superblock and copies is limited to be two.  Second
>> downside is that we cannot support devices which have no conventional 
>> zones
>> at all.
>>
> 
> 
>> To solve these two problems, we employ superblock log writing. It uses 
>> two
>> zones as a circular buffer to write updated superblocks. Once the first
>> zone is filled up, start writing into the second buffer. Then, when the
>> both zones are filled up and before start writing to the first zone 
>> again,
>> it reset the first zone.
>>
>> We can determine the position of the latest superblock by reading write
>> pointer information from a device. One corner case is when the both zones
>> are full. For this situation, we read out the last superblock of each
>> zone, and compare them to determine which zone is older.
>>
>> The following zones are reserved as the circular buffer on ZONED btrfs.
>>
>> - The primary superblock: zones 0 and 1
>> - The first copy: zones 16 and 17
>> - The second copy: zones 1024 or zone at 256GB which is minimum, and next
>>    to it
> 
> Superblock log approach needs a non-deterministic and inconsistent
> number of blocks to be read to find copy #0. And, to use 4K bytes
> we are reserving a lot more space. But I don't know any better way.
> I am just checking with you...
> 
> At the time of mkfs, is it possible to format the block device to
> add conventional zones as needed to support our sb LBAs?

No. The number of conventional zones (if any) are a drive characteristic 
and one cannot assume that the number can be modified.

>   OR
> For superblock zones why not reset the write pointer before the
> transaction commit?
> 
A write pointer reset is equivalent to clearing the contents of the 
zone, so we would lose the previous information there.

HTH.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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