Sau*_*nda 3 zfs zfs-l2arc zfsonlinux
我正在尝试针对我的工作负载（Postgres 和同一物理机器上的文件服务器 [1]）在 Linux 上调整 ZFS，并想了解我是否真的需要 L2ARC。
如果https://www.zfsbuild.com/2010/04/15/explanation-of-arc-and-l2arc/ （写于 2010 年，当时我猜 SSD 很贵）给出的信息是正确的，应该我不是在禁用 L2ARC 吗？如果 ARC 上存在缓存未命中，则从 L2ARC 和主数据集读取将花费相同的时间（两者都是 SSD）。我的理解正确吗？
一个相关的问题——如何查看 L2ARC 的摘要？我认为没有arc_summary提供有关 L2ARC 的任何信息，对吗？
L2ARC 是二级自适应替换缓存。L2ARC 在 ZFS 系统中通常称为“缓存驱动器”。
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这些缓存驱动器是物理上的 MLC 式 SSD 驱动器。这些 SSD 驱动器比系统内存慢，但仍比硬盘驱动器快得多。更重要的是，SSD 驱动器比系统内存便宜得多。
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当 ZFS 池中存在缓存驱动器时，缓存驱动器将缓存不适合 ARC 的经常访问的数据。当读取请求进入系统时，ZFS 将尝试为来自 ARC 的这些请求提供服务。如果数据不在 ARC 中，ZFS 将尝试为来自 L2ARC 的请求提供服务。仅当 ARC 或 L2ARC 中不存在数据时才访问硬盘驱动器。
[1] 硬件配置：https : //www.hetzner.com/dedicated-rootserver/px61-nvme
zpool status pool: firstzfs
state: ONLINE
scan: none requested
config:
NAME STATE READ WRITE CKSUM
firstzfs ONLINE 0 0 0
nvme0n1p3 ONLINE 0 0 0
errors: No known data errors
arc_summaryZFS Subsystem Report Wed Jan 30 09:26:07 2019
ARC Summary: (HEALTHY)
Memory Throttle Count: 0
ARC Misc:
Deleted: 43.56k
Mutex Misses: 0
Evict Skips: 0
ARC Size: 65.51% 20.54 GiB
Target Size: (Adaptive) 100.00% 31.35 GiB
Min Size (Hard Limit): 6.25% 1.96 GiB
Max Size (High Water): 16:1 31.35 GiB
ARC Size Breakdown:
Recently Used Cache Size: 86.54% 16.66 GiB
Frequently Used Cache Size: 13.46% 2.59 GiB
ARC Hash Breakdown:
Elements Max: 4.64m
Elements Current: 89.55% 4.16m
Collisions: 83.96m
Chain Max: 8
Chains: 721.73k
ARC Total accesses: 985.94m
Cache Hit Ratio: 95.94% 945.94m
Cache Miss Ratio: 4.06% 40.00m
Actual Hit Ratio: 93.33% 920.18m
Data Demand Efficiency: 87.42% 313.82m
Data Prefetch Efficiency: 100.00% 25.94m
CACHE HITS BY CACHE LIST:
Anonymously Used: 2.72% 25.76m
Most Recently Used: 27.97% 264.53m
Most Frequently Used: 69.31% 655.65m
Most Recently Used Ghost: 0.00% 0
Most Frequently Used Ghost: 0.00% 0
CACHE HITS BY DATA TYPE:
Demand Data: 29.00% 274.35m
Prefetch Data: 2.74% 25.94m
Demand Metadata: 68.21% 645.27m
Prefetch Metadata: 0.04% 379.71k
CACHE MISSES BY DATA TYPE:
Demand Data: 98.68% 39.47m
Prefetch Data: 0.00% 0
Demand Metadata: 1.32% 527.28k
Prefetch Metadata: 0.00% 0
DMU Prefetch Efficiency: 865.60m
Hit Ratio: 9.64% 83.45m
Miss Ratio: 90.36% 782.14m
ZFS Tunable:
dbuf_cache_hiwater_pct 10
dbuf_cache_lowater_pct 10
dbuf_cache_max_bytes 104857600
dbuf_cache_max_shift 5
dmu_object_alloc_chunk_shift 7
ignore_hole_birth 1
l2arc_feed_again 1
l2arc_feed_min_ms 200
l2arc_feed_secs 1
l2arc_headroom 2
l2arc_headroom_boost 200
l2arc_noprefetch 1
l2arc_norw 0
l2arc_write_boost 8388608
l2arc_write_max 8388608
metaslab_aliquot 524288
metaslab_bias_enabled 1
metaslab_debug_load 0
metaslab_debug_unload 0
metaslab_fragmentation_factor_enabled 1
metaslab_lba_weighting_enabled 1
metaslab_preload_enabled 1
metaslabs_per_vdev 200
send_holes_without_birth_time 1
spa_asize_inflation 24
spa_config_path /etc/zfs/zpool.cache
spa_load_verify_data 1
spa_load_verify_maxinflight 10000
spa_load_verify_metadata 1
spa_slop_shift 5
zfetch_array_rd_sz 1048576
zfetch_max_distance 8388608
zfetch_max_streams 8
zfetch_min_sec_reap 2
zfs_abd_scatter_enabled 1
zfs_abd_scatter_max_order 10
zfs_admin_snapshot 1
zfs_arc_average_blocksize 8192
zfs_arc_dnode_limit 0
zfs_arc_dnode_limit_percent 10
zfs_arc_dnode_reduce_percent 10
zfs_arc_grow_retry 0
zfs_arc_lotsfree_percent 10
zfs_arc_max 0
zfs_arc_meta_adjust_restarts 4096
zfs_arc_meta_limit 0
zfs_arc_meta_limit_percent 75
zfs_arc_meta_min 0
zfs_arc_meta_prune 10000
zfs_arc_meta_strategy 1
zfs_arc_min 0
zfs_arc_min_prefetch_lifespan 0
zfs_arc_p_aggressive_disable 1
zfs_arc_p_dampener_disable 1
zfs_arc_p_min_shift 0
zfs_arc_pc_percent 0
zfs_arc_shrink_shift 0
zfs_arc_sys_free 0
zfs_autoimport_disable 1
zfs_compressed_arc_enabled 1
zfs_dbgmsg_enable 0
zfs_dbgmsg_maxsize 4194304
zfs_dbuf_state_index 0
zfs_deadman_checktime_ms 5000
zfs_deadman_enabled 1
zfs_deadman_synctime_ms 1000000
zfs_dedup_prefetch 0
zfs_delay_min_dirty_percent 60
zfs_delay_scale 500000
zfs_delete_blocks 20480
zfs_dirty_data_max 4294967296
zfs_dirty_data_max_max 4294967296
zfs_dirty_data_max_max_percent 25
zfs_dirty_data_max_percent 10
zfs_dirty_data_sync 67108864
zfs_dmu_offset_next_sync 0
zfs_expire_snapshot 300
zfs_flags 0
zfs_free_bpobj_enabled 1
zfs_free_leak_on_eio 0
zfs_free_max_blocks 100000
zfs_free_min_time_ms 1000
zfs_immediate_write_sz 32768
zfs_max_recordsize 1048576
zfs_mdcomp_disable 0
zfs_metaslab_fragmentation_threshold 70
zfs_metaslab_segment_weight_enabled 1
zfs_metaslab_switch_threshold 2
zfs_mg_fragmentation_threshold 85
zfs_mg_noalloc_threshold 0
zfs_multihost_fail_intervals 5
zfs_multihost_history 0
zfs_multihost_import_intervals 10
zfs_multihost_interval 1000
zfs_multilist_num_sublists 0
zfs_no_scrub_io 0
zfs_no_scrub_prefetch 0
zfs_nocacheflush 0
zfs_nopwrite_enabled 1
zfs_object_mutex_size 64
zfs_pd_bytes_max 52428800
zfs_per_txg_dirty_frees_percent 30
zfs_prefetch_disable 0
zfs_read_chunk_size 1048576
zfs_read_history 0
zfs_read_history_hits 0
zfs_recover 0
zfs_resilver_delay 2
zfs_resilver_min_time_ms 3000
zfs_scan_idle 50
zfs_scan_min_time_ms 1000
zfs_scrub_delay 4
zfs_send_corrupt_data 0
zfs_sync_pass_deferred_free 2
zfs_sync_pass_dont_compress 5
zfs_sync_pass_rewrite 2
zfs_sync_taskq_batch_pct 75
zfs_top_maxinflight 32
zfs_txg_history 0
zfs_txg_timeout 5
zfs_vdev_aggregation_limit 131072
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
zfs_vdev_cache_bshift 16
zfs_vdev_cache_max 16384
zfs_vdev_cache_size 0
zfs_vdev_max_active 1000
zfs_vdev_mirror_non_rotating_inc 0
zfs_vdev_mirror_non_rotating_seek_inc 1
zfs_vdev_mirror_rotating_inc 0
zfs_vdev_mirror_rotating_seek_inc 5
zfs_vdev_mirror_rotating_seek_offset 1048576
zfs_vdev_queue_depth_pct 1000
zfs_vdev_raidz_impl [fastest] original scalar sse2 ssse3 avx2
zfs_vdev_read_gap_limit 32768
zfs_vdev_scheduler noop
zfs_vdev_scrub_max_active 2
zfs_vdev_scrub_min_active 1
zfs_vdev_sync_read_max_active 10
zfs_vdev_sync_read_min_active 10
zfs_vdev_sync_write_max_active 10
zfs_vdev_sync_write_min_active 10
zfs_vdev_write_gap_limit 4096
zfs_zevent_cols 80
zfs_zevent_console 0
zfs_zevent_len_max 128
zfs_zil_clean_taskq_maxalloc 1048576
zfs_zil_clean_taskq_minalloc 1024
zfs_zil_clean_taskq_nthr_pct 100
zil_replay_disable 0
zil_slog_bulk 786432
zio_delay_max 30000
zio_dva_throttle_enabled 1
zio_requeue_io_start_cut_in_line 1
zio_taskq_batch_pct 75
zvol_inhibit_dev 0
zvol_major 230
zvol_max_discard_blocks 16384
zvol_prefetch_bytes 131072
zvol_request_sync 0
zvol_threads 32
zvol_volmode 1
L2ARC 仅在使用比主池设备更快的设备时才有用，并且仅在您将缓存设备显式附加到池时才有效。
arc_summary 清楚地报告你的 L2ARC 统计数据，但显然只有当你将它附加到主池时。
如果您没有看到 L2ARC 统计信息，则意味着您现在没有L2 缓存。可以肯定的是，请发布输出zpool status
编辑：zpool status确认您没有L2ARC。的输出也arcstat没有显示 L2ARC 的迹象；唯一的参考是关于可调参数，在这种情况下，它没有任何影响。
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