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L2ARC: Restrict write size to 1/4 of the device #15519

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Nov 14, 2023
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30 changes: 4 additions & 26 deletions module/zfs/arc.c
Original file line number Diff line number Diff line change
Expand Up @@ -8035,9 +8035,8 @@ l2arc_write_size(l2arc_dev_t *dev)
*/
size = l2arc_write_max;
if (size == 0) {
cmn_err(CE_NOTE, "Bad value for l2arc_write_max, value must "
"be greater than zero, resetting it to the default (%d)",
L2ARC_WRITE_SIZE);
cmn_err(CE_NOTE, "l2arc_write_max must be greater than zero, "
"resetting it to the default (%d)", L2ARC_WRITE_SIZE);
size = l2arc_write_max = L2ARC_WRITE_SIZE;
}

Expand All @@ -8060,30 +8059,9 @@ l2arc_write_size(l2arc_dev_t *dev)
* device. This is important in l2arc_evict(), otherwise infinite
* iteration can occur.
*/
if (size > dev->l2ad_end - dev->l2ad_start) {
cmn_err(CE_NOTE, "l2arc_write_max or l2arc_write_boost "
"plus the overhead of log blocks (persistent L2ARC, "
"%llu bytes) exceeds the size of the cache device "
"(guid %llu), resetting them to the default (%d)",
(u_longlong_t)l2arc_log_blk_overhead(size, dev),
(u_longlong_t)dev->l2ad_vdev->vdev_guid, L2ARC_WRITE_SIZE);
size = MIN(size, (dev->l2ad_end - dev->l2ad_start) / 4);

size = l2arc_write_max = l2arc_write_boost = L2ARC_WRITE_SIZE;

if (l2arc_trim_ahead > 1) {
cmn_err(CE_NOTE, "l2arc_trim_ahead set to 1");
l2arc_trim_ahead = 1;
}

if (arc_warm == B_FALSE)
size += l2arc_write_boost;

size += l2arc_log_blk_overhead(size, dev);
if (dev->l2ad_vdev->vdev_has_trim && l2arc_trim_ahead > 0) {
size += MAX(64 * 1024 * 1024,
(size * l2arc_trim_ahead) / 100);
}
}
size = P2ROUNDUP(size, 1ULL << dev->l2ad_vdev->vdev_ashift);

return (size);

Expand Down
20 changes: 6 additions & 14 deletions tests/zfs-tests/tests/functional/cache/cache_012_pos.ksh
Original file line number Diff line number Diff line change
Expand Up @@ -31,15 +31,13 @@
# 2. Set l2arc_write_max to a value larger than the cache device.
# 3. Create a file larger than the cache device and random read
# for 10 sec.
# 4. Verify that l2arc_write_max is set back to the default.
# 5. Set l2arc_write_max to a value less than the cache device size but
# 4. Set l2arc_write_max to a value less than the cache device size but
# larger than the default (256MB).
# 6. Record the l2_size.
# 7. Random read for 1 sec.
# 8. Record the l2_size again.
# 9. If (6) <= (8) then we have not looped around yet.
# 10. If (6) > (8) then we looped around. Break out of the loop and test.
# 11. Destroy pool.
# 5. Record the l2_size.
# 6. Random read for 1 sec.
# 7. Record the l2_size again.
# 8. If (5) <= (7) then we have not looped around yet.
# 9. Destroy pool.
#

verify_runnable "global"
Expand Down Expand Up @@ -93,10 +91,6 @@ log_must zfs set relatime=off $TESTPOOL
log_must fio $FIO_SCRIPTS/mkfiles.fio
log_must fio $FIO_SCRIPTS/random_reads.fio

typeset write_max2=$(get_tunable L2ARC_WRITE_MAX)

log_must test $write_max2 -eq $write_max

log_must set_tunable32 L2ARC_WRITE_MAX $(( 256 * 1024 * 1024 ))
export RUNTIME=1

Expand All @@ -108,8 +102,6 @@ while $do_once || [[ $l2_size1 -le $l2_size2 ]]; do
do_once=false
done

log_must test $l2_size1 -gt $l2_size2

log_must zpool destroy $TESTPOOL

log_pass "Looping around a cache device succeeds."