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authorRichard Yao <ryao@gentoo.org>2014-07-04 18:43:47 -0400
committerRichard Yao <ryao@gentoo.org>2015-09-04 15:30:24 -0400
commit37f9dac592bf5889c3efb305c48ac39b4c7dd140 (patch)
tree367b1a78b28df3c585f5c0489517686c28783935 /config/kernel.m4
parent782b2c326ea445c5cab0c1b0373d64d5e83cc5d4 (diff)
zvol processing should use struct bio
Internally, zvols are files exposed through the block device API. This is intended to reduce overhead when things require block devices. However, the ZoL zvol code emulates a traditional block device in that it has a top half and a bottom half. This is an unnecessary source of overhead that does not exist on any other OpenZFS platform does this. This patch removes it. Early users of this patch reported double digit performance gains in IOPS on zvols in the range of 50% to 80%. Comments in the code suggest that the current implementation was done to obtain IO merging from Linux's IO elevator. However, the DMU already does write merging while arc_read() should implicitly merge read IOs because only 1 thread is permitted to fetch the buffer into ARC. In addition, commercial ZFSOnLinux distributions report that regular files are more performant than zvols under the current implementation, and the main consumers of zvols are VMs and iSCSI targets, which have their own elevators to merge IOs. Some minor refactoring allows us to register zfs_request() as our ->make_request() handler in place of the generic_make_request() function. This eliminates the layer of code that broke IO requests on zvols into a top half and a bottom half. This has several benefits: 1. No per zvol spinlocks. 2. No redundant IO elevator processing. 3. Interrupts are disabled only when actually necessary. 4. No redispatching of IOs when all taskq threads are busy. 5. Linux's page out routines will properly block. 6. Many autotools checks become obsolete. An unfortunate consequence of eliminating the layer that generic_make_request() is that we no longer calls the instrumentation hooks for block IO accounting. Those hooks are GPL-exported, so we cannot call them ourselves and consequently, we lose the ability to do IO monitoring via iostat. Since zvols are internally files mapped as block devices, this should be okay. Anyone who is willing to accept the performance penalty for the block IO layer's accounting could use the loop device in between the zvol and its consumer. Alternatively, perf and ftrace likely could be used. Also, tools like latencytop will still work. Tools such as latencytop sometimes provide a better view of performance bottlenecks than the traditional block IO accounting tools do. Lastly, if direct reclaim occurs during spacemap loading and swap is on a zvol, this code will deadlock. That deadlock could already occur with sync=always on zvols. Given that swap on zvols is not yet production ready, this is not a blocker. Signed-off-by: Richard Yao <ryao@gentoo.org>
Diffstat (limited to 'config/kernel.m4')
-rw-r--r--config/kernel.m44
1 files changed, 4 insertions, 0 deletions
diff --git a/config/kernel.m4 b/config/kernel.m4
index 09d8003f1..b3dd7232c 100644
--- a/config/kernel.m4
+++ b/config/kernel.m4
@@ -7,6 +7,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
ZFS_AC_TEST_MODULE
ZFS_AC_KERNEL_CONFIG
ZFS_AC_KERNEL_DECLARE_EVENT_CLASS
+ ZFS_AC_KERNEL_CURRENT_BIO_TAIL
ZFS_AC_KERNEL_BDEV_BLOCK_DEVICE_OPERATIONS
ZFS_AC_KERNEL_BLOCK_DEVICE_OPERATIONS_RELEASE_VOID
ZFS_AC_KERNEL_TYPE_FMODE_T
@@ -22,6 +23,8 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
ZFS_AC_KERNEL_BIO_FAILFAST_DTD
ZFS_AC_KERNEL_REQ_FAILFAST_MASK
ZFS_AC_KERNEL_BIO_END_IO_T_ARGS
+ ZFS_AC_KERNEL_BIO_RW_BARRIER
+ ZFS_AC_KERNEL_BIO_RW_DISCARD
ZFS_AC_KERNEL_BIO_RW_SYNC
ZFS_AC_KERNEL_BIO_RW_SYNCIO
ZFS_AC_KERNEL_REQ_SYNC
@@ -100,6 +103,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
ZFS_AC_KERNEL_VFS_RW_ITERATE
ZFS_AC_KERNEL_KMAP_ATOMIC_ARGS
ZFS_AC_KERNEL_FOLLOW_DOWN_ONE
+ ZFS_AC_KERNEL_MAKE_REQUEST_FN
AS_IF([test "$LINUX_OBJ" != "$LINUX"], [
KERNELMAKE_PARAMS="$KERNELMAKE_PARAMS O=$LINUX_OBJ"