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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. Seconds between L2ARC writing.
. .
.It Sy l2arc_headroom Ns = Ns Sy 8 Pq u64 .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. expressed as a multiplier of the effective write size.
ARC persistence across reboots can be achieved with persistent L2ARC Setting to
by setting this parameter to .Sy 0
.Sy 0 , disables the per-cycle headroom limit.
allowing the full length of ARC lists to be searched for cacheable content. 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 .It Sy l2arc_headroom_boost Ns = Ns Sy 200 Ns % Pq u64
Scales 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. L2ARC operates in one of two modes depending on total cache capacity.
When total L2ARC capacity is less than twice When total L2ARC capacity is less than twice
.Sy arc_c_max , .Sy arc_c_max ,
L2ARC uses exclusive caching, L2ARC writes buffers to cache as they are evicted from ARC.
writing buffers to cache as they are evicted from ARC.
When total capacity is at least twice When total capacity is at least twice
.Sy arc_c_max , .Sy arc_c_max ,
L2ARC switches to inclusive caching with persistent markers L2ARC uses persistent markers that track scan positions across iterations,
that track scan positions, writing cacheable content as write throughput allows.
attempting to duplicate ARC contents as much as write throughput allows. A depth cap
Setting .Pq Sy l2arc_ext_headroom_pct
.Sy l2arc_headroom Ns = Ns Sy 0 limits how far markers advance from the tail, keeping them focused on
will result in scanning the full-length ARC lists for cacheable content to be buffers soon to be evicted where caching adds the most value.
written in L2ARC (persistent ARC).
In inclusive mode, markers progress toward the head across iterations,
naturally covering the full list.
.Pp .Pp
If a cache device is added with If a cache device is added with
.Nm zpool Cm add , .Nm zpool Cm add ,