On 3/15/21 1:53 AM, Naohiro Aota wrote:
This commit moves the location of superblock logging zones. The location of
the logging zones are determined based on fixed block addresses instead of
on fixed zone numbers.

By locating the superblock zones using fixed addresses, we can scan a
dumped file system image without the zone information. And, no drawbacks

We use the following three pairs of zones containing fixed offset
locations, regardless of the device zone size.

   - Primary superblock: zone starting at offset 0 and the following zone
   - First copy: zone containing offset 64GB and the following zone
   - Second copy: zone containing offset 256GB and the following zone

If the location of the zones are outside of disk, we don't record the
superblock copy.

These addresses are arbitrary, but using addresses that are too large
reduces superblock reliability for smaller devices, so we do not want to
exceed 1T to cover all case nicely.

Also, LBAs are generally distributed initially across one head (platter
side) up to one or more zones, then go on the next head backward (the other
side of the same platter), and on to the following head/platter. Thus using
non sequential fixed addresses for superblock logging, such as 0/64G/256G,
likely result in each superblock copy being on a different head/platter
which improves chances of recovery in case of superblock read error.

These zones are reserved for superblock logging and never used for data or
metadata blocks. Zones containing the offsets used to store superblocks in
a regular btrfs volume (no zoned case) are also reserved to avoid

Note that we only reserve the 2 zones per primary/copy actually used for
superblock logging. We don't reserve the ranges possibly containing
superblock with the largest supported zone size (0-16GB, 64G-80GB,

The first copy position is much larger than for a regular btrfs volume
(64M).  This increase is to avoid overlapping with the log zones for the
primary superblock. This higher location is arbitrary but allows supporting
devices with very large zone size, up to 32GB. But we only allow zone sizes
up to 8GB for now.

Ok it took me a few reads to figure out what's going on.

The problem is that with large zone sizes, our current choices put the back up super blocks waaaayyyyyy out on the disk, correct? So instead you've picked arbitrary byte offsets, hoping that they'll be closer to the front of the disk and thus actually be useful.

And then you've introduced the 8gib zone size as a way to avoid problems where we get the same zone for the backup supers.

Are these statements correct? If so the changelog should be updated to make this clear up front, because it took me a while to work that out.

Something at the beginning like the following

"With larger zone sizes, for example 8gib, the 3rd backup super would be located 8tib into the device. However not all zoned block devices are this large. In order to fix this limitation set the zones to a static byte offset, and calculate the zone number from there based on the devices zone size."

So that it's clear from the outset why we're making this change.

And this brings up another problem, in that what happens when we _do_ run into block devices that have huge zones, like 64gib zones? We have to change the disk format to support these devices. I'm not against that per-se, but it seems like a limitation, even if it's unlikely to ever happen. With the locations we currently have, any arbitrary zone size is going to work in the future, and the only drawback is you need a device of a certain size to take advantage of the back up super blocks. I would hope that we don't have 64gib zone size block devices that are only 128gib in size in the future.

Signed-off-by: Naohiro Aota <naohiro.a...@wdc.com>
  fs/btrfs/zoned.c | 39 +++++++++++++++++++++++++++++++--------
  1 file changed, 31 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
index 43948bd40e02..6a72ca1f7988 100644
--- a/fs/btrfs/zoned.c
+++ b/fs/btrfs/zoned.c
@@ -21,9 +21,24 @@
  /* Pseudo write pointer value for conventional zone */
  #define WP_CONVENTIONAL ((u64)-2)
+ * Location of the first zone of superblock logging zone pairs.
+ * - Primary superblock: the zone containing offset 0 (zone 0)
+ * - First superblock copy: the zone containing offset 64G
+ * - Second superblock copy: the zone containing offset 256G
+ */
  /* Number of superblock log zones */
+/* Max size of supported zone size */
  static int copy_zone_info_cb(struct blk_zone *zone, unsigned int idx, void 
        struct blk_zone *zones = data;
@@ -111,11 +126,8 @@ static int sb_write_pointer(struct block_device *bdev, 
struct blk_zone *zones,
- * 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
- *                     the following one
+ * Get the zone number of the first zone of a pair of contiguous zones used
+ * for superblock logging.
  static inline u32 sb_zone_number(int shift, int mirror)
@@ -123,8 +135,8 @@ static inline u32 sb_zone_number(int shift, int mirror)
switch (mirror) {
        case 0: return 0;
-       case 1: return 16;
-       case 2: return min_t(u64, btrfs_sb_offset(mirror) >> shift, 1024);
+       case 1: return 1 << (BTRFS_FIRST_SB_LOG_ZONE_SHIFT - shift);
+       case 2: return 1 << (BTRFS_SECOND_SB_LOG_ZONE_SHIFT - shift);
return 0;
@@ -300,10 +312,21 @@ int btrfs_get_dev_zone_info(struct btrfs_device *device)
                zone_sectors = bdev_zone_sectors(bdev);
- nr_sectors = bdev_nr_sectors(bdev);
        /* Check if it's power of 2 (see is_power_of_2) */
        ASSERT(zone_sectors != 0 && (zone_sectors & (zone_sectors - 1)) == 0);
        zone_info->zone_size = zone_sectors << SECTOR_SHIFT;
+       /* We reject devices with a zone size larger than 8GB. */

A longer explanation here of why it's important that we limit it to our MAX_ZONE_SIZE, and use MAX_ZONE_SIZE instead of 8gib, in case we increase the limit in the future.

For example

We reject devices with a zone size larger than MAX_ZONE_SIZE because we do not want the backup super block zone to overlap with the primary super block zone.

Or something along these lines. Again, I was confused why this was in the patch until I spent a lot more time thinking about it. Thanks,


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