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Improve caching for dbuf prefetches

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To avoid read errors with transaction open dmu_tx_check_ioerr()
is used to read everything required in advance.  But there seems
to be a chance for the buffer to evicted from dbuf cache in
between, which result in immediate eviction from ARC, which may
require additional disk read later in a place where error handling
is problematic.

To partially workaround this introduce a new flag DMU_IS_PREFETCH,
relayed to ARC as ARC_FLAG_PREFETCH | ARC_FLAG_PRESCIENT_PREFETCH,
making ARC delay eviction by at least several seconds, or till the
actual read inside the transaction, that will promote it to demand
access.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #18160
This commit is contained in:
Alexander Motin 2026-02-04 13:12:32 -05:00 committed by GitHub
parent 00d69b0f72
commit 21bbe7cb67
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GPG Key ID: B5690EEEBB952194
3 changed files with 8 additions and 4 deletions
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1
include/sys/dmu.h
View File

@ -573,6 +573,7 @@ typedef enum dmu_flags {
DMU_PARTIAL_FIRST = 1 << 7, /* First partial access. */
DMU_PARTIAL_MORE = 1 << 8, /* Following partial access. */
DMU_KEEP_CACHING = 1 << 9, /* Don't affect caching. */
DMU_IS_PREFETCH = 1 << 10, /* This read is a prefetch. */
} dmu_flags_t;
/*

4
module/zfs/dbuf.c
View File

@ -1637,6 +1637,8 @@ dbuf_read_impl(dmu_buf_impl_t *db, dnode_t *dn, zio_t *zio, dmu_flags_t flags,
aflags |= ARC_FLAG_UNCACHED;
else if (dbuf_is_l2cacheable(db, bp))
aflags |= ARC_FLAG_L2CACHE;
if (flags & DMU_IS_PREFETCH)
aflags |= ARC_FLAG_PREFETCH | ARC_FLAG_PRESCIENT_PREFETCH;
dbuf_add_ref(db, NULL);
@ -1769,7 +1771,7 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *pio, dmu_flags_t flags)
mutex_enter(&db->db_mtx);
if (!(flags & (DMU_UNCACHEDIO | DMU_KEEP_CACHING)))
db->db_pending_evict = B_FALSE;
if (flags & DMU_PARTIAL_FIRST)
if (flags & (DMU_PARTIAL_FIRST | DMU_IS_PREFETCH))
db->db_partial_read = B_TRUE;
else if (!(flags & (DMU_PARTIAL_MORE | DMU_KEEP_CACHING)))
db->db_partial_read = B_FALSE;

7
module/zfs/dmu_tx.c
View File

@ -220,11 +220,12 @@ dmu_tx_check_ioerr(zio_t *zio, dnode_t *dn, int level, uint64_t blkid)
if (err != 0)
return (err);
/*
* PARTIAL_FIRST allows caching for uncacheable blocks. It will
* be cleared after dmu_buf_will_dirty() call dbuf_read() again.
* DMU_IS_PREFETCH keeps the buffer temporarily in DBUF cache and ARC
* to avoid immediate eviction after the check. It will be promoted
* to demand access when dmu_buf_will_dirty() read it again.
*/
err = dbuf_read(db, zio, DB_RF_CANFAIL | DMU_READ_NO_PREFETCH |
(level == 0 ? (DMU_UNCACHEDIO | DMU_PARTIAL_FIRST) : 0));
(level == 0 ? (DMU_KEEP_CACHING | DMU_IS_PREFETCH) : 0));
dbuf_rele(db, FTAG);
return (err);
}