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Allow and prefer special vdevs as ZIL

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Before this change ZIL blocks were allocated only from normal or
SLOG vdevs.  In typical situation when special vdevs are SSDs and
normal are HDDs it could cause weird inversions when data blocks
are written to SSDs, but ZIL referencing them to HDDs.

This change assumes that special vdevs typically have much better
(or at least not worse) latency than normal, and so in absence of
SLOGs should store ZIL blocks.  It means similar to normal vdevs
introduction of special embedded log allocation class and updating
the allocation fallback order to: SLOG -> special embedded log ->
special -> normal embedded log -> normal.

The code tries to guess whether data block is going to be written
to normal or special vdev (it can not be done precisely before
compression) and prefer indirect writes for blocks written to a
special vdev to avoid double-write.  For blocks that are going to
be written to normal vdev, special vdev by default plays as SLOG,
reducing write latency by the cost of higher special vdev wear,
but it is tunable via module parameter.

This should allow HDD pools with decent SSD as special vdev to
work under synchronous workloads without requiring additional
SLOG SSD, impractical in many scenarios.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Rob Norris <rob.norris@klarasystems.com>
Reviewed-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #17505
This commit is contained in:
Alexander Motin
2025-07-18 21:44:14 -04:00
committed by GitHub
parent 2669b00f13
commit be1e991a1a
13 changed files with 164 additions and 44 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/sys/spa.h
+1
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@@ -1065,6 +1065,7 @@ extern metaslab_class_t *spa_normal_class(spa_t *spa);
extern metaslab_class_t *spa_log_class(spa_t *spa);
extern metaslab_class_t *spa_embedded_log_class(spa_t *spa);
extern metaslab_class_t *spa_special_class(spa_t *spa);
extern metaslab_class_t *spa_special_embedded_log_class(spa_t *spa);
extern metaslab_class_t *spa_dedup_class(spa_t *spa);
extern metaslab_class_t *spa_preferred_class(spa_t *spa, const zio_t *zio);
extern boolean_t spa_special_has_ddt(spa_t *spa);
include/sys/spa_impl.h
+1
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@@ -246,6 +246,7 @@ struct spa {
metaslab_class_t *spa_log_class; /* intent log data class */
metaslab_class_t *spa_embedded_log_class; /* log on normal vdevs */
metaslab_class_t *spa_special_class; /* special allocation class */
metaslab_class_t *spa_special_embedded_log_class; /* log on special */
metaslab_class_t *spa_dedup_class; /* dedup allocation class */
uint64_t spa_first_txg; /* first txg after spa_open() */
uint64_t spa_final_txg; /* txg of export/destroy */
include/sys/zil.h
+4
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@@ -635,6 +635,8 @@ extern void zil_set_logbias(zilog_t *zilog, uint64_t slogval);
extern uint64_t zil_max_copied_data(zilog_t *zilog);
extern uint64_t zil_max_log_data(zilog_t *zilog, size_t hdrsize);
extern itx_wr_state_t zil_write_state(zilog_t *zilog, uint64_t size,
uint32_t blocksize, boolean_t o_direct, boolean_t commit);
extern void zil_sums_init(zil_sums_t *zs);
extern void zil_sums_fini(zil_sums_t *zs);
@@ -642,6 +644,8 @@ extern void zil_kstat_values_update(zil_kstat_values_t *zs,
zil_sums_t *zil_sums);
extern int zil_replay_disable;
extern uint_t zfs_immediate_write_sz;
extern int zil_special_is_slog;
#ifdef __cplusplus
}