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zfs/config/kernel-blk-queue-nonrot.m4

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Improve ZVOL queue behavior. The Linux block device queue subsystem exposes a number of configurable settings described in Linux block/blk-settings.c. The defaults for these settings are tuned for hard drives, and are not optimized for ZVOLs. Proper configuration of these options would allow upper layers (I/O scheduler) to take better decisions about write merging and ordering. Detailed rationale: - max_hw_sectors is set to unlimited (UINT_MAX). zvol_write() is able to handle writes of any size, so there's no reason to impose a limit. Let the upper layer decide. - max_segments and max_segment_size are set to unlimited. zvol_write() will copy the requests' contents into a dbuf anyway, so the number and size of the segments are irrelevant. Let the upper layer decide. - physical_block_size and io_opt are set to the ZVOL's block size. This has the potential to somewhat alleviate issue #361 for ZVOLs, by warning the upper layers that writes smaller than the volume's block size will be slow. - The NONROT flag is set to indicate this isn't a rotational device. Although the backing zpool might be composed of rotational devices, the resulting ZVOL often doesn't exhibit the same behavior due to the COW mechanisms used by ZFS. Setting this flag will prevent upper layers from making useless decisions (such as reordering writes) based on incorrect assumptions about the behavior of the ZVOL. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
2011-09-05 13:15:45 +00:00
dnl #
dnl # 2.6.27 API change
dnl # The blk_queue_nonrot() function and QUEUE_FLAG_NONROT flag were
dnl # added so non-rotational devices could be identified. These devices
dnl # have no seek time which the higher level elevator uses to optimize
dnl # how the I/O issued to the device.
dnl #
AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE_NONROT], [
AC_MSG_CHECKING([whether blk_queue_nonrot() is available])
tmp_flags="$EXTRA_KCFLAGS"
Only use gcc -Wunused-but-set-variable when available Certain versions of gcc generate an 'unrecognized command line option' error message when -Wunused-but-set-variable is used unconditionally. This in turn can cause several of the autoconf tests to misdetect an interface. Now, the use of -Wunused-but-set-variable in the autoconf tests was introduced by commit b9c59ec8 to address a gcc 4.6 compatibility problem. So we really only need to pass this option for version of gcc which are known to support it. Therefore, the tests have been updated to use the result of the existing ZFS_AC_CONFIG_ALWAYS_NO_UNUSED_BUT_SET_VARIABLE which determines if gcc supports this option. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #1004
2013-01-11 00:09:31 +00:00
EXTRA_KCFLAGS="${NO_UNUSED_BUT_SET_VARIABLE}"
Improve ZVOL queue behavior. The Linux block device queue subsystem exposes a number of configurable settings described in Linux block/blk-settings.c. The defaults for these settings are tuned for hard drives, and are not optimized for ZVOLs. Proper configuration of these options would allow upper layers (I/O scheduler) to take better decisions about write merging and ordering. Detailed rationale: - max_hw_sectors is set to unlimited (UINT_MAX). zvol_write() is able to handle writes of any size, so there's no reason to impose a limit. Let the upper layer decide. - max_segments and max_segment_size are set to unlimited. zvol_write() will copy the requests' contents into a dbuf anyway, so the number and size of the segments are irrelevant. Let the upper layer decide. - physical_block_size and io_opt are set to the ZVOL's block size. This has the potential to somewhat alleviate issue #361 for ZVOLs, by warning the upper layers that writes smaller than the volume's block size will be slow. - The NONROT flag is set to indicate this isn't a rotational device. Although the backing zpool might be composed of rotational devices, the resulting ZVOL often doesn't exhibit the same behavior due to the COW mechanisms used by ZFS. Setting this flag will prevent upper layers from making useless decisions (such as reordering writes) based on incorrect assumptions about the behavior of the ZVOL. Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
2011-09-05 13:15:45 +00:00
ZFS_LINUX_TRY_COMPILE([
#include <linux/blkdev.h>
],[
struct request_queue *q = NULL;
(void) blk_queue_nonrot(q);
],[
AC_MSG_RESULT(yes)
AC_DEFINE(HAVE_BLK_QUEUE_NONROT, 1,
[blk_queue_nonrot() is available])
],[
AC_MSG_RESULT(no)
])
EXTRA_KCFLAGS="$tmp_flags"
])