On Thu, Apr 08, 2021 at 05:25:28PM +0900, Naohiro Aota wrote:
> This commit moves the location of the superblock logging zones. The new
> locations of the logging zones are now determined based on fixed block
> addresses instead of on fixed zone numbers.
>
> The old placement method based on fixed zone numbers causes problems when
> one needs to inspect a file system image without access to the drive zone
> information. In such case, the super block locations cannot be reliably
> determined as the zone size is unknown. By locating the superblock logging
> zones using fixed addresses, we can scan a dumped file system image without
> the zone information since a super block copy will always be present at or
> after the fixed location.
>
> This commit introduces 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 a logging zone is outside of the disk capacity, we do not record the
> superblock copy.
>
> 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 sizes, up to 32GB. Such large zone size is
> unrealistic and very unlikely to ever be seen in real devices. Currently,
> SMR disks have a zone size of 256MB, and we are expecting ZNS drives to be
> in the 1-4GB range, so this 32GB limit gives us room to breathe. For now,
> we only allow zone sizes up to 8GB, below this hard limit of 32GB.
>
> The fixed location addresses are somewhat arbitrary, but with the intent of
> maintaining superblock reliability even for smaller devices. For this
> reason, the superblock fixed locations do not exceed 1TB.
Yeah, so how much should we adjust the offsets in favor of smaller
devices instead of larger ones? I think this is going the wrong
direction, the capacity will grow, the zones are supposed to be larger.
> The superblock logging zones are reserved for superblock logging and never
> used for data or metadata blocks. Note that we only reserve the two zones
> per primary/copy actually used for superblock logging. We do not reserve
> the ranges of zones possibly containing superblocks with the largest
> supported zone size (0-16GB, 64G-80GB, 256G-272GB).
>
> The zones containing the fixed location offsets used to store superblocks
> in a regular btrfs volume (no zoned case) are also reserved to avoid
> confusion.
>
> Signed-off-by: Naohiro Aota <naohiro.a...@wdc.com>
> ---
> fs/btrfs/zoned.c | 43 +++++++++++++++++++++++++++++++++++--------
> 1 file changed, 35 insertions(+), 8 deletions(-)
>
> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> index 1f972b75a9ab..a4b195fe08a0 100644
> --- a/fs/btrfs/zoned.c
> +++ b/fs/btrfs/zoned.c
> @@ -21,9 +21,28 @@
> /* 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
> + */
> +#define BTRFS_PRIMARY_SB_LOG_ZONE 0ULL
> +#define BTRFS_FIRST_SB_LOG_ZONE (64ULL * SZ_1G)
> +#define BTRFS_SECOND_SB_LOG_ZONE (256ULL * SZ_1G)
> +#define BTRFS_FIRST_SB_LOG_ZONE_SHIFT const_ilog2(BTRFS_FIRST_SB_LOG_ZONE)
> +#define BTRFS_SECOND_SB_LOG_ZONE_SHIFT const_ilog2(BTRFS_SECOND_SB_LOG_ZONE)
This should be named like BTRFS_SB_LOG_* ie. "namespaces" or common
prefixes for the same class of valuesd.
> +
> /* Number of superblock log zones */
> #define BTRFS_NR_SB_LOG_ZONES 2
>
> +/*
> + * Maximum size of zones. Currently, SMR disks have a zone size of 256MB,
> + * and we are expecting ZNS drives to be in the 1-4GB range. We do not
> + * expect the zone size to become larger than 8GB in the near future.
> + */
> +#define BTRFS_MAX_ZONE_SIZE SZ_8G
> +
> static int copy_zone_info_cb(struct blk_zone *zone, unsigned int idx, void
> *data)
> {
> struct blk_zone *zones = data;
> @@ -111,11 +130,8 @@ 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 +139,8 @@ 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 +316,21 @@ 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. */
> + if (zone_info->zone_size > BTRFS_MAX_ZONE_SIZE) {
> + btrfs_err_in_rcu(fs_info,
> + "zoned: %s: zone size %llu is too large",
> + rcu_str_deref(device->name),
> + zone_info->zone_size);
> + ret = -EINVAL;
> + goto out;
> + }
> +
> + nr_sectors = bdev_nr_sectors(bdev);
> zone_info->zone_size_shift = ilog2(zone_info->zone_size);
> zone_info->max_zone_append_size =
> (u64)queue_max_zone_append_sectors(queue) << SECTOR_SHIFT;
> --
> 2.31.1
