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dir/file metadata consumes enourmous amount of ARC #13925

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devZer0 opened this issue Sep 19, 2022 · 9 comments
Open

dir/file metadata consumes enourmous amount of ARC #13925

devZer0 opened this issue Sep 19, 2022 · 9 comments
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Type: Defect Incorrect behavior (e.g. crash, hang)

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@devZer0
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devZer0 commented Sep 19, 2022 •
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Distribution Name | debian
Distribution Version | 11.5
Kernel Version | 5.10.0-18-amd64
Architecture | x86_64
OpenZFS Version | zfs-2.1.5-1~bpo11+1

Describe the problem you're observing

is it normal/expected that 1 mio empty dirs/files , which consume 161/136 Mb on disk (uncompressed) inflate to 2,2Gb of ARC in memory when you do a "find /zfs/dirs|files" across those ?

looks weird to me, thats more then 2kb of ram for each empty dir/file, especially as ARC is compressed, too - so why does caching of dirs/files expand by a factor of nearly 14 when in ram ?

Describe how to reproduce the problem

  • create empty zfs dataset /zfspool/dataset,
  • create 1 mio empty dirs or files in /zfspool/dataset
  • drop caches
  • do "find /zfspool/dataset"
  • watch arc grow with arcstat
@devZer0 devZer0 added the Type: Defect Incorrect behavior (e.g. crash, hang) label Sep 19, 2022
@devZer0
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devZer0 commented Sep 19, 2022 •
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interesting - crawling through 1mio cached empty dirs from ARC is an order of magnitude (about 25 times) slower then crawling trough 1mio files (all dirs and files are contained in one dir)

root@debian:/dirpool# time find /filepool/files/| wc -l
1000001

real	0m0,639s
user	0m0,458s
sys	0m0,182s

root@debian:/dirpool# time find /dirpool/dirs  | wc -l
1000001

real	0m16,283s
user	0m2,524s
sys	0m13,649s

@amotin
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amotin commented Sep 20, 2022

Are you sure that the 2.2GB are in ARC, not dbuf cache? dbuf cache unlike ARC is not compressed and micro zap structures used for small directories use 128KB blocks. dbuf cache is limited to only 3% of ARC, but you haven't specified how much ARC do you have for the beginning, so 2.2GB may or may not be within the 3%, in which case it is a correct behavior.

@devZer0
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devZer0 commented Sep 20, 2022 •
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i didn't know that dbuf cache is being separate from arc and uncompressed.

for me, directory entries are metadata. so are filenames, file acl's, timestamps etc....

the system is freshly booted.

after a directory crawl with "find /dir..." (and nothing more) , arc_summary looks like this

# arc_summary

------------------------------------------------------------------------
ZFS Subsystem Report                            Tue Sep 20 19:55:47 2022
Linux 5.10.0-18-amd64                                    2.1.5-1~bpo11+1
Machine: debian (x86_64)                                 2.1.5-1~bpo11+1

ARC status:                                                      HEALTHY
        Memory throttle count:                                         0

ARC size (current):                                    11.5 %    2.2 GiB
        Target size (adaptive):                         9.5 %    1.8 GiB
        Min size (hard limit):                          4.4 %  856.8 MiB
        Max size (high water):                           22:1   19.1 GiB
        Most Frequently Used (MFU) cache size:         99.6 %  595.5 MiB
        Most Recently Used (MRU) cache size:            0.4 %    2.6 MiB
        Metadata cache size (hard limit):              99.0 %   18.9 GiB
        Metadata cache size (current):                 11.6 %    2.2 GiB
        Dnode cache size (hard limit):                 80.0 %   15.1 GiB
        Dnode cache size (current):                     6.1 %  946.3 MiB

ARC hash breakdown:
        Elements max:                                              39.7k
        Elements current:                             100.0 %      39.7k
        Collisions:                                                  189
        Chain max:                                                     2
        Chains:                                                      188

ARC misc:
        Deleted:                                                      25
        Mutex misses:                                                243
        Eviction skips:                                              131
        Eviction skips due to L2 writes:                               0
        L2 cached evictions:                                     0 Bytes
        L2 eligible evictions:                                  80.5 MiB
        L2 eligible MFU evictions:                     96.6 %   77.8 MiB
        L2 eligible MRU evictions:                      3.4 %    2.7 MiB
        L2 ineligible evictions:                               110.1 MiB

ARC total accesses (hits + misses):                                 7.1M
        Cache hit ratio:                               99.4 %       7.0M
        Cache miss ratio:                               0.6 %      45.9k
        Actual hit ratio (MFU + MRU hits):             99.4 %       7.0M
        Data demand efficiency:                           n/a          0
        Data prefetch efficiency:                         n/a          0

Cache hits by cache type:
        Most frequently used (MFU):                    95.6 %       6.7M
        Most recently used (MRU):                       4.4 %     309.7k
        Most frequently used (MFU) ghost:               0.0 %          0
        Most recently used (MRU) ghost:                 0.1 %       6.1k

Cache hits by data type:
        Demand data:                                    0.0 %          0
        Demand prefetch data:                           0.0 %          0
        Demand metadata:                              100.0 %       7.0M
        Demand prefetch metadata:                     < 0.1 %         50

Cache misses by data type:
        Demand data:                                    0.0 %          0
        Demand prefetch data:                           0.0 %          0
        Demand metadata:                               79.8 %      36.6k
        Demand prefetch metadata:                      20.2 %       9.3k

DMU prefetch efficiency:                                               0
        Hit ratio:                                        n/a          0
        Miss ratio:                                       n/a          0

L2ARC not detected, skipping section

Solaris Porting Layer (SPL):
        spl_hostid                                                     0
        spl_hostid_path                                      /etc/hostid
        spl_kmem_alloc_max                                       1048576
        spl_kmem_alloc_warn                                        65536
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        spl_panic_halt                                                 0
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        spl_taskq_thread_bind                                          0
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        spl_taskq_thread_sequential                                    4

Tunables:
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        dbuf_cache_lowater_pct                                        10
        dbuf_cache_max_bytes                        18446744073709551615
        dbuf_cache_shift                                               5
        dbuf_metadata_cache_max_bytes               18446744073709551615
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        dmu_object_alloc_chunk_shift                                   7
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        zfs_dbgmsg_enable                                              1
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        zfs_deadman_enabled                                            1
        zfs_deadman_failmode                                        wait
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        zfs_dirty_data_max_percent                                    10
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        zfs_livelist_max_entries                                  500000
        zfs_livelist_min_percent_shared                               75
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        zfs_lua_max_memlimit                                   104857600
        zfs_max_async_dedup_frees                                 100000
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        zfs_metaslab_fragmentation_threshold                          70
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        zfs_metaslab_mem_limit                                        25
        zfs_metaslab_segment_weight_enabled                            1
        zfs_metaslab_switch_threshold                                  2
        zfs_metaslab_try_hard_before_gang                              0
        zfs_mg_fragmentation_threshold                                95
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        zfs_multihost_history                                          0
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        zfs_no_scrub_io                                                0
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        zfs_object_mutex_size                                         64
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        zfs_override_estimate_recordsize                               0
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        zfs_per_txg_dirty_frees_percent                                5
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        zfs_read_history_hits                                          0
        zfs_rebuild_max_segment                                  1048576
        zfs_rebuild_scrub_enabled                                      1
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        zfs_reconstruct_indirect_combinations_max                   4096
        zfs_recover                                                    0
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        zfs_recv_write_batch_size                                1048576
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        zfs_remove_max_segment                                  16777216
        zfs_resilver_disable_defer                                     0
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        zfs_scan_suspend_progress                                      0
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        zfs_scrub_min_time_ms                                       1000
        zfs_send_corrupt_data                                          0
        zfs_send_no_prefetch_queue_ff                                 20
        zfs_send_no_prefetch_queue_length                        1048576
        zfs_send_queue_ff                                             20
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        zfs_send_unmodified_spill_blocks                               1
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        zfs_spa_discard_memory_limit                            16777216
        zfs_special_class_metadata_reserve_pct                        25
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        zfs_sync_pass_dont_compress                                    8
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        zfs_vdev_async_read_max_active                                 3
        zfs_vdev_async_read_min_active                                 1
        zfs_vdev_async_write_active_max_dirty_percent                 60
        zfs_vdev_async_write_active_min_dirty_percent                 30
        zfs_vdev_async_write_max_active                               10
        zfs_vdev_async_write_min_active                                2
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        zfs_vdev_initializing_min_active                               1
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        zfs_vdev_raidz_impl cycle [fastest] original scalar sse2 ssse3 avx2
        zfs_vdev_read_gap_limit                                    32768
        zfs_vdev_rebuild_max_active                                    3
        zfs_vdev_rebuild_min_active                                    1
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        zfs_vdev_removal_min_active                                    1
        zfs_vdev_scheduler                                        unused
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        zfs_vdev_sync_read_min_active                                 10
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        zfs_vdev_sync_write_min_active                                10
        zfs_vdev_trim_max_active                                       2
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        zfs_vdev_write_gap_limit                                    4096
        zfs_vnops_read_chunk_size                                1048576
        zfs_zevent_len_max                                           512
        zfs_zevent_retain_expire_secs                                900
        zfs_zevent_retain_max                                       2000
        zfs_zil_clean_taskq_maxalloc                             1048576
        zfs_zil_clean_taskq_minalloc                                1024
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        zil_slog_bulk                                             786432
        zio_deadman_log_all                                            0
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        zio_slow_io_ms                                             30000
        zio_taskq_batch_pct                                           80
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        zvol_inhibit_dev                                               0
        zvol_major                                                   230
        zvol_max_discard_blocks                                    16384
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        zvol_request_sync                                              0
        zvol_threads                                                  32
        zvol_volmode                                                   1

VDEV cache disabled, skipping section

ZIL committed transactions:                                            0
        Commit requests:                                               0
        Flushes to stable storage:                                     0
        Transactions to SLOG storage pool:            0 Bytes          0
        Transactions to non-SLOG storage pool:        0 Bytes          0

@devZer0
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devZer0 commented Sep 20, 2022

i increased zfs_arc_meta_limit_percent and zfs_arc_dnode_limit_percent just to make sure, there is enough ram available for directory information

@devZer0
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devZer0 commented Sep 20, 2022

with zfs_arc_dnode_limit_percent removed from the boot options, it looks weird. how can dnode current cache size surpass the hard limit ?

ARC size (current):                                    11.6 %    2.2 GiB
        Target size (adaptive):                        11.5 %    2.2 GiB
        Min size (hard limit):                          4.4 %  856.8 MiB
        Max size (high water):                           22:1   19.1 GiB
        Most Frequently Used (MFU) cache size:         99.5 %  613.0 MiB
        Most Recently Used (MRU) cache size:            0.5 %    2.9 MiB
        Metadata cache size (hard limit):              99.0 %   18.9 GiB
        Metadata cache size (current):                 11.7 %    2.2 GiB
        Dnode cache size (hard limit):                  1.0 %  193.5 MiB
        Dnode cache size (current):                   488.9 %  946.2 MiB

@devZer0
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devZer0 commented Sep 20, 2022

for my own interest, i set VMs ram down to 4gb, did warm up the arc again so that it contains 2,2gb of dir information , did a hibernate of the VM and tried to compress the resulting hibernation file.

as we can see, the ram contents are very well compressible.

ok, this type of compression test may not really be valid, as we need to take the data structures' blocksize into account, but we see there may perhaps be room for improvement.

-rw-r----- 1 root root  4194813952 Sep 20 20:10 vm-123-state-suspend-2022-09-20.raw
-rw-r--r-- 1 root root   813174635 Sep 20 20:11 vm-123-state-suspend-2022-09-20.raw.lz4
-rw-r--r-- 1 root root   383353154 Sep 20 20:13 vm-123-state-suspend-2022-09-20.raw.zstd

@amotin
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amotin commented Sep 20, 2022

As I see arc_dnode_size_limit limit is respected only during ARC data eviction phase. If your system has enough RAM, it does not apply.

@devZer0 devZer0 mentioned this issue Sep 20, 2022
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stale bot commented Oct 15, 2023

This issue has been automatically marked as "stale" because it has not had any activity for a while. It will be closed in 90 days if no further activity occurs. Thank you for your contributions.

@stale stale bot added the Status: Stale No recent activity for issue label Oct 15, 2023
@GregorKopka
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Bad bot.
@behlendorf

@stale stale bot removed the Status: Stale No recent activity for issue label Oct 16, 2023
@devZer0 devZer0 closed this as completed Oct 16, 2023
@devZer0 devZer0 reopened this Oct 16, 2023
@devZer0 devZer0 mentioned this issue Dec 7, 2023
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