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Enable Linux read-ahead for a single page on ZVOLs 

Linux has read-ahead logic designed to accelerate sequential workloads.
ZFS has its own read-ahead logic called zprefetch that operates on both
ZVOLs and datasets. Having two prefetchers active at the same time can
cause overprefetching, which unnecessarily reduces IOPS performance on
CoW filesystems like ZFS.

Testing shows that entirely disabling the Linux prefetch results in
a significant performance penalty for reads while commensurate benefits
are seen in random writes. It appears that read-ahead benefits are
inversely proportional to random write benefits, and so a single page
of Linux-layer read-ahead appears to offer the middle ground for both
workloads.

Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Issue #5902
This commit is contained in:
Richard Yao 2014-07-11 14:35:58 -04:00 committed by Brian Behlendorf
parent 5731140eaf
commit bc17f1047a
4 changed files with 35 additions and 0 deletions
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20
config/kernel-blk-queue-bdi.m4 Normal file
View File

@ -0,0 +1,20 @@
dnl #
dnl # 2.6.32 - 4.11, statically allocated bdi in request_queue
dnl # 4.12 - x.y, dynamically allocated bdi in request_queue
dnl #
AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE_BDI], [
AC_MSG_CHECKING([whether blk_queue bdi is dynamic])
ZFS_LINUX_TRY_COMPILE([
#include <linux/blkdev.h>
],[
struct request_queue q;
struct backing_dev_info bdi;
q.backing_dev_info = &bdi;
],[
AC_MSG_RESULT(yes)
AC_DEFINE(HAVE_BLK_QUEUE_BDI_DYNAMIC, 1,
[blk queue backing_dev_info is dynamic])
],[
AC_MSG_RESULT(no)
])
])

1
config/kernel.m4
View File

@ -32,6 +32,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
ZFS_AC_KERNEL_BIO_END_IO_T_ARGS
ZFS_AC_KERNEL_BIO_RW_BARRIER
ZFS_AC_KERNEL_BIO_RW_DISCARD
ZFS_AC_KERNEL_BLK_QUEUE_BDI
ZFS_AC_KERNEL_BLK_QUEUE_FLUSH
ZFS_AC_KERNEL_BLK_QUEUE_MAX_HW_SECTORS
ZFS_AC_KERNEL_BLK_QUEUE_MAX_SEGMENTS

11
include/linux/blkdev_compat.h
View File

@ -31,6 +31,7 @@
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/backing-dev.h>
#ifndef HAVE_FMODE_T
typedef unsigned __bitwise__ fmode_t;
@ -128,6 +129,16 @@ __blk_queue_max_segments(struct request_queue *q, unsigned short max_segments)
}
#endif
static inline void
blk_queue_set_read_ahead(struct request_queue *q, unsigned long ra_pages)
{
#ifdef HAVE_BLK_QUEUE_BDI_DYNAMIC
q->backing_dev_info->ra_pages = ra_pages;
#else
q->backing_dev_info.ra_pages = ra_pages;
#endif
}
#ifndef HAVE_GET_DISK_RO
static inline int
get_disk_ro(struct gendisk *disk)

3
module/zfs/zvol.c
View File

@ -1468,6 +1468,9 @@ zvol_alloc(dev_t dev, const char *name)
blk_queue_make_request(zv->zv_queue, zvol_request);
blk_queue_set_write_cache(zv->zv_queue, B_TRUE, B_TRUE);
/* Limit read-ahead to a single page to prevent over-prefetching. */
blk_queue_set_read_ahead(zv->zv_queue, 1);
/* Disable write merging in favor of the ZIO pipeline. */
queue_flag_set(QUEUE_FLAG_NOMERGES, zv->zv_queue);