Fix synchronicity for ZVOLs.

zvol_write() assumes that the write request must be written to stable storage
if rq_is_sync() is true. Unfortunately, this assumption is incorrect. Indeed,
"sync" does *not* mean what we think it means in the context of the Linux
block layer. This is well explained in linux/fs.h:

    WRITE:       A normal async write. Device will be plugged.
    WRITE_SYNC:  Synchronous write. Identical to WRITE, but passes down
                 the hint that someone will be waiting on this IO
                 shortly.
    WRITE_FLUSH: Like WRITE_SYNC but with preceding cache flush.
    WRITE_FUA:   Like WRITE_SYNC but data is guaranteed to be on
                 non-volatile media on completion.

In other words, SYNC does not *mean* that the write must be on stable storage
on completion. It just means that someone is waiting on us to complete the
write request. Thus triggering a ZIL commit for each SYNC write request on a
ZVOL is unnecessary and harmful for performance. To make matters worse, ZVOL
users have no way to express that they actually want data to be written to
stable storage, which means the ZIL is broken for ZVOLs.

The request for stable storage is expressed by the FUA flag, so we must
commit the ZIL after the write if the FUA flag is set. In addition, we must
commit the ZIL before the write if the FLUSH flag is set.

Also, we must inform the block layer that we actually support FLUSH and FUA.

Signed-off-by: Brian Behlendorf <[email protected]>

12 files changed, 12 insertions, 0 deletions

diff --git a/cmd/Makefile.in b/cmd/Makefile.in
index c57b7d4fe..ba5360941 100644
--- a/cmd/Makefile.in
+++ b/cmd/Makefile.in
@@ -49,6 +49,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/mount_zfs/Makefile.in b/cmd/mount_zfs/Makefile.in
index 1ed8e1be5..94e316004 100644
--- a/cmd/mount_zfs/Makefile.in
+++ b/cmd/mount_zfs/Makefile.in
@@ -52,6 +52,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/sas_switch_id/Makefile.in b/cmd/sas_switch_id/Makefile.in
index ebe9fb00a..09826f2c9 100644
--- a/cmd/sas_switch_id/Makefile.in
+++ b/cmd/sas_switch_id/Makefile.in
@@ -51,6 +51,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/zdb/Makefile.in b/cmd/zdb/Makefile.in
index eb3d70f48..7dd5dfc5f 100644
--- a/cmd/zdb/Makefile.in
+++ b/cmd/zdb/Makefile.in
@@ -52,6 +52,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/zfs/Makefile.in b/cmd/zfs/Makefile.in
index 3e8b26020..54f63c75f 100644
--- a/cmd/zfs/Makefile.in
+++ b/cmd/zfs/Makefile.in
@@ -52,6 +52,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/zinject/Makefile.in b/cmd/zinject/Makefile.in
index d8a8d6e43..59d67f096 100644
--- a/cmd/zinject/Makefile.in
+++ b/cmd/zinject/Makefile.in
@@ -52,6 +52,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/zpios/Makefile.in b/cmd/zpios/Makefile.in
index 7103c01a8..f357559f3 100644
--- a/cmd/zpios/Makefile.in
+++ b/cmd/zpios/Makefile.in
@@ -52,6 +52,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/zpool/Makefile.in b/cmd/zpool/Makefile.in
index 2d025b6c4..cb15fee8e 100644
--- a/cmd/zpool/Makefile.in
+++ b/cmd/zpool/Makefile.in
@@ -52,6 +52,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/zpool_id/Makefile.in b/cmd/zpool_id/Makefile.in
index 509d9177e..a6277ef81 100644
--- a/cmd/zpool_id/Makefile.in
+++ b/cmd/zpool_id/Makefile.in
@@ -51,6 +51,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/zpool_layout/Makefile.in b/cmd/zpool_layout/Makefile.in
index 9ff60f07f..12f21e51b 100644
--- a/cmd/zpool_layout/Makefile.in
+++ b/cmd/zpool_layout/Makefile.in
@@ -51,6 +51,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/ztest/Makefile.in b/cmd/ztest/Makefile.in
index fb8979172..85fdde032 100644
--- a/cmd/ztest/Makefile.in
+++ b/cmd/ztest/Makefile.in
@@ -52,6 +52,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \
diff --git a/cmd/zvol_id/Makefile.in b/cmd/zvol_id/Makefile.in
index 83fc24f71..2c22c80dc 100644
--- a/cmd/zvol_id/Makefile.in
+++ b/cmd/zvol_id/Makefile.in
@@ -52,6 +52,7 @@ am__aclocal_m4_deps =  \
 	$(top_srcdir)/config/kernel-bio-rw-syncio.m4 \
 	$(top_srcdir)/config/kernel-blk-end-request.m4 \
 	$(top_srcdir)/config/kernel-blk-fetch-request.m4 \
+	$(top_srcdir)/config/kernel-blk-queue-flush.m4 \
 	$(top_srcdir)/config/kernel-blk-requeue-request.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-bytes.m4 \
 	$(top_srcdir)/config/kernel-blk-rq-pos.m4 \