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man: Update L2ARC documentation for depth cap and write budget fairness

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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
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Ameer Hamza 2026-03-05 20:26:12 +05:00 committed by Brian Behlendorf
parent b27a87f399
commit be5d36919a
2 changed files with 14 additions and 16 deletions
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12
man/man4/zfs.4
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@ -108,12 +108,14 @@ and only applicable in related situations.
Seconds between L2ARC writing.
.
.It Sy l2arc_headroom Ns = Ns Sy 8 Pq u64
How far through the ARC lists to search for L2ARC cacheable content,
How far through the ARC lists to search for L2ARC cacheable content per cycle,
expressed as a multiplier of the effective write size.
ARC persistence across reboots can be achieved with persistent L2ARC
by setting this parameter to
.Sy 0 ,
allowing the full length of ARC lists to be searched for cacheable content.
Setting to
.Sy 0
disables the per-cycle headroom limit.
Scan depth is also bounded by
.Sy l2arc_ext_headroom_pct
when persistent markers are active.
.
.It Sy l2arc_headroom_boost Ns = Ns Sy 200 Ns % Pq u64
Scales

18
man/man7/zpoolconcepts.7
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@ -430,19 +430,15 @@ is updated even if no metadata structures are written.
L2ARC operates in one of two modes depending on total cache capacity.
When total L2ARC capacity is less than twice
.Sy arc_c_max ,
L2ARC uses exclusive caching,
writing buffers to cache as they are evicted from ARC.
L2ARC writes buffers to cache as they are evicted from ARC.
When total capacity is at least twice
.Sy arc_c_max ,
L2ARC switches to inclusive caching with persistent markers
that track scan positions,
attempting to duplicate ARC contents as much as write throughput allows.
Setting
.Sy l2arc_headroom Ns = Ns Sy 0
will result in scanning the full-length ARC lists for cacheable content to be
written in L2ARC (persistent ARC).
In inclusive mode, markers progress toward the head across iterations,
naturally covering the full list.
L2ARC uses persistent markers that track scan positions across iterations,
writing cacheable content as write throughput allows.
A depth cap
.Pq Sy l2arc_ext_headroom_pct
limits how far markers advance from the tail, keeping them focused on
buffers soon to be evicted where caching adds the most value.
.Pp
If a cache device is added with
.Nm zpool Cm add ,