On 3.04.19 г. 10:05 ч., Qu Wenruo wrote:
>
>
> 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.
Yeah, backup roots should really be perceived as a "best-effort" type of
mechanism rather than something guaranteeing you can go back 4
generations w.r.t fs state.
>
> 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
>>
>>
>>
>>
>
