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? OR For superblock zones why not reset the write pointer before the transaction commit? Thanks.
If these reserved zones are conventional, superblock is written fixed at the start of the zone without logging.
Signed-off-by: Naohiro Aota <naohiro.aota@xxxxxxx>
