I'm trying to tune ZFS on Linux for my workload (Postgres and a fileserver on the same physical machine [1]), and wanted to understand if I really need L2ARC, or not.
If the information given at https://www.zfsbuild.com/2010/04/15/explanation-of-arc-and-l2arc/ (written in 2010, when, I guess, SSDs were expensive), is correct, shouldn't I be disabling L2ARC? If there is a cache-miss on the ARC, reading from L2ARC and the main data-set will take the same time (both will be SSD). Is my understanding correct?
A related question -- how do I check the summary of L2ARC? I don't think arc_summary gives any information about L2ARC, right?
The L2ARC is the second level adaptive replacement cache. The L2ARC is often called “cache drives” in the ZFS systems.
[..]
These cache drives are physically MLC style SSD drives. These SSD drives are slower than system memory, but still much faster than hard drives. More importantly, the SSD drives are much cheaper than system memory.
[..]
When cache drives are present in the ZFS pool, the cache drives will cache frequently accessed data that did not fit in ARC. When read requests come into the system, ZFS will attempt to serve those requests from the ARC. If the data is not in the ARC, ZFS will attempt to serve the requests from the L2ARC. Hard drives are only accessed when data does not exist in either the ARC or L2ARC.
[1] Hardware config: https://www.hetzner.com/dedicated-rootserver/px61-nvme
- Two 512 GB NVMe Gen3 x4 SSDs
- 64 GB DDR4 ECC RAM
- Intel® Xeon® E3-1275 v5 Quad-Core Skylake processor (4-core / 8 thread)
Output of 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
Output of arc_summary
ZFS 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
Answer
L2ARC is only useful when using a faster than main pool device, and it is only active when you explicitly attach a cache device to the pool.
arc_summary clearly reports your L2ARC stats, but obviously only if you attached it to the main pool.
If you see no L2ARC stats, it means you have no L2 cache right now. To be sure, please post the output of zpool status
EDIT: zpool status confirms you have no L2ARC. The output of arcstat shows no signs of L2ARC either; the only references are about tunables which, in this case, have no effects.
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