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L2ARC: Scan-based depth cap for persistent markers

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With persistent markers and inclusive scanning, the marker traverses the
entire ARC state across many feed cycles, writing buffers far from the
tail that may no longer be relevant.

Track cumulative bytes scanned per pass in l2arc_ext_scanned. When scans
reach l2arc_ext_headroom_pct (default 25%) of the ARC state size, reset
the pass markers to the tail via lazy reset flags. This keeps markers
focused on the tail zone where buffers soon to be evicted have the most
value for L2ARC.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Ameer Hamza <ahamza@ixsystems.com>
Closes #18289
This commit is contained in:
Ameer Hamza
2026-03-05 20:13:26 +05:00
committed by Brian Behlendorf
parent 15fc3d64c8
commit 62ca8f721b
4 changed files with 81 additions and 0 deletions
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module/zfs/arc.c
+64
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@@ -956,6 +956,13 @@ int l2arc_exclude_special = 0;
*/
static int l2arc_mfuonly = 0;
/*
* Depth cap as percentage of state size. Each pass resets its markers
* to tail after scanning this fraction of the state. Keeps markers
* focused on the tail zone where L2ARC adds the most value.
*/
static uint64_t l2arc_ext_headroom_pct = 25;
/*
* L2ARC TRIM
* l2arc_trim_ahead : A ZFS module parameter that controls how much ahead of
@@ -9083,6 +9090,8 @@ l2arc_pool_markers_init(spa_t *spa)
spa->spa_l2arc_info.l2arc_markers[pass][i]);
multilist_sublist_unlock(mls);
}
spa->spa_l2arc_info.l2arc_ext_scanned[pass] = 0;
}
}
@@ -9875,6 +9884,31 @@ l2arc_blk_fetch_done(zio_t *zio)
kmem_free(cb, sizeof (l2arc_read_callback_t));
}
/*
* Return the total size of the ARC state corresponding to the given
* L2ARC pass number (0..3).
*/
static uint64_t
l2arc_get_state_size(int pass)
{
switch (pass) {
case L2ARC_MFU_META:
return (zfs_refcount_count(
&arc_mfu->arcs_size[ARC_BUFC_METADATA]));
case L2ARC_MRU_META:
return (zfs_refcount_count(
&arc_mru->arcs_size[ARC_BUFC_METADATA]));
case L2ARC_MFU_DATA:
return (zfs_refcount_count(
&arc_mfu->arcs_size[ARC_BUFC_DATA]));
case L2ARC_MRU_DATA:
return (zfs_refcount_count(
&arc_mru->arcs_size[ARC_BUFC_DATA]));
default:
return (0);
}
}
/*
* Flag all sublists for a single pass for lazy marker reset to tail.
* Each sublist's marker will be reset when next visited by a feed thread.
@@ -9892,6 +9926,8 @@ l2arc_flag_pass_reset(spa_t *spa, int pass)
spa->spa_l2arc_info.l2arc_sublist_reset[pass][i] = B_TRUE;
multilist_sublist_unlock(mls);
}
spa->spa_l2arc_info.l2arc_ext_scanned[pass] = 0;
}
/*
@@ -10044,6 +10080,31 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
(spa->spa_l2arc_info.l2arc_next_sublist[pass] + 1) %
num_sublists;
/*
* Depth cap: track cumulative bytes scanned per pass
* and reset markers when the scan cap is reached.
* Keeps the marker near the tail where L2ARC adds
* the most value.
*/
if (save_position) {
mutex_enter(&spa->spa_l2arc_info.l2arc_sublist_lock);
spa->spa_l2arc_info.l2arc_ext_scanned[pass] +=
consumed_headroom;
uint64_t state_sz = l2arc_get_state_size(pass);
uint64_t scan_cap =
state_sz * l2arc_ext_headroom_pct / 100;
if (scan_cap > 0 &&
spa->spa_l2arc_info.l2arc_ext_scanned[pass] >=
scan_cap) {
l2arc_flag_pass_reset(spa, pass);
}
mutex_exit(&spa->spa_l2arc_info.l2arc_sublist_lock);
}
if (full == B_TRUE)
break;
}
@@ -11691,6 +11752,9 @@ ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, mfuonly, INT, ZMOD_RW,
ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, exclude_special, INT, ZMOD_RW,
"Exclude dbufs on special vdevs from being cached to L2ARC if set.");
ZFS_MODULE_PARAM(zfs_l2arc, l2arc_, ext_headroom_pct, U64, ZMOD_RW,
"Depth cap as percentage of state size for marker reset");
ZFS_MODULE_PARAM_CALL(zfs_arc, zfs_arc_, lotsfree_percent, param_set_arc_int,
param_get_uint, ZMOD_RW, "System free memory I/O throttle in bytes");