On 2019/4/3 上午11:58, Chris Murphy wrote: > I'm sometimes seeing the same backup_tree_root used more than once. > Below you'll see backup 0 and backup 2 have the same address, > different generation. The concern is if this suggests backup 2 is > actually stale and not useful? Currently, backup root of btrfs only has a life time of one generation. That's to say, any backup older than current generation - 1 is already stale. Filipe had tried to make btrfs to pin down extents up to 4 generations. Thus btrfs could make all 4 backup roots valid, but that leaded to a lot of ENOSPC. So he gave up that idea. Thanks, Qu > > We've previously seen that Btrfs with discard mount option can very > quickly cause the locations pointed to by backup roots to return no > data (metadata deallocated and trimmed). I wonder if this is similar > where Btrfs has no concept of delaying metadata deallocation for a > while so that the backup trees remain valid for a while (few minutes?) > Anyway if these backup roots can become stale, if they're ever needed > they're not going to be useful. > > superblock: bytenr=65536, device=/dev/mapper/sdc > --------------------------------------------------------- > csum_type 0 (crc32c) > csum_size 4 > csum 0xe0c01b78 [match] > bytenr 65536 > flags 0x1 > ( WRITTEN ) > magic _BHRfS_M [match] > fsid ecd5e90e-6fb7-4b42-ab57-515a733c01f2 > metadata_uuid ecd5e90e-6fb7-4b42-ab57-515a733c01f2 > label third > generation 686 > root 3355115520 > sys_array_size 129 > chunk_root_generation 685 > root_level 1 > chunk_root 4325376000 > chunk_root_level 1 > log_root 0 > log_root_transid 0 > log_root_level 0 > total_bytes 1000207335424 > bytes_used 439356653568 > sectorsize 4096 > nodesize 32768 > leafsize (deprecated) 32768 > stripesize 4096 > root_dir 6 > num_devices 2 > compat_flags 0x0 > compat_ro_flags 0x3 > ( FREE_SPACE_TREE | > FREE_SPACE_TREE_VALID ) > incompat_flags 0x171 > ( MIXED_BACKREF | > COMPRESS_ZSTD | > BIG_METADATA | > EXTENDED_IREF | > SKINNY_METADATA ) > cache_generation 18446744073709551615 > uuid_tree_generation 686 > dev_item.uuid de7a23cd-ec25-47d4-bbdb-fcf9c4dc705a > dev_item.fsid ecd5e90e-6fb7-4b42-ab57-515a733c01f2 [match] > dev_item.type 0 > dev_item.total_bytes 500103667712 > dev_item.bytes_used 442415185920 > dev_item.io_align 4096 > dev_item.io_width 4096 > dev_item.sector_size 4096 > dev_item.devid 2 > dev_item.dev_group 0 > dev_item.seek_speed 0 > dev_item.bandwidth 0 > dev_item.generation 0 > sys_chunk_array[2048]: > item 0 key (FIRST_CHUNK_TREE CHUNK_ITEM 4325376000) > length 33554432 owner 2 stripe_len 65536 type SYSTEM|RAID1 > io_align 65536 io_width 65536 sector_size 4096 > num_stripes 2 sub_stripes 1 > stripe 0 devid 2 offset 1083179008 > dev_uuid de7a23cd-ec25-47d4-bbdb-fcf9c4dc705a > stripe 1 devid 1 offset 1104150528 > dev_uuid 15e28877-dbc7-446d-9991-618504a758ca > backup_roots[4]: > backup 0: > backup_tree_root: 3355115520 gen: 686 level: 1 > backup_chunk_root: 4325376000 gen: 685 level: 1 > backup_extent_root: 3358523392 gen: 686 level: 2 > backup_fs_root: 3550511104 gen: 497 level: 0 > backup_dev_root: 3358752768 gen: 685 level: 1 > backup_csum_root: 3259400192 gen: 500 level: 2 > backup_total_bytes: 1000207335424 > backup_bytes_used: 439356653568 > backup_num_devices: 2 > > backup 1: > backup_tree_root: 3358752768 gen: 683 level: 1 > backup_chunk_root: 4325441536 gen: 677 level: 1 > backup_extent_root: 3359506432 gen: 683 level: 2 > backup_fs_root: 3550511104 gen: 497 level: 0 > backup_dev_root: 3358982144 gen: 677 level: 1 > backup_csum_root: 3259400192 gen: 500 level: 2 > backup_total_bytes: 1000207335424 > backup_bytes_used: 439356653568 > backup_num_devices: 2 > > backup 2: > backup_tree_root: 3355115520 gen: 684 level: 1 > backup_chunk_root: 4325441536 gen: 677 level: 1 > backup_extent_root: 3358523392 gen: 684 level: 2 > backup_fs_root: 3550511104 gen: 497 level: 0 > backup_dev_root: 3358982144 gen: 677 level: 1 > backup_csum_root: 3259400192 gen: 500 level: 2 > backup_total_bytes: 1000207335424 > backup_bytes_used: 439356653568 > backup_num_devices: 2 > > backup 3: > backup_tree_root: 3359834112 gen: 685 level: 1 > backup_chunk_root: 4325376000 gen: 685 level: 1 > backup_extent_root: 3361898496 gen: 685 level: 2 > backup_fs_root: 3550511104 gen: 497 level: 0 > backup_dev_root: 3358752768 gen: 685 level: 1 > backup_csum_root: 3259400192 gen: 500 level: 2 > backup_total_bytes: 1000207335424 > backup_bytes_used: 439356653568 > backup_num_devices: 2 > > > >
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