freebsd-nq/config/kernel-blk-queue-discard.m4
Etienne Dechamps 30930fba21 Add support for DISCARD to ZVOLs.
DISCARD (REQ_DISCARD, BLKDISCARD) is useful for thin provisioning.
It allows ZVOL clients to discard (unmap, trim) block ranges from
a ZVOL, thus optimizing disk space usage by allowing a ZVOL to
shrink instead of just grow.

We can't use zfs_space() or zfs_freesp() here, since these functions
only work on regular files, not volumes. Fortunately we can use the
low-level function dmu_free_long_range() which does exactly what we
want.

Currently the discard operation is not added to the log. That's not
a big deal since losing discard requests cannot result in data
corruption. It would however result in disk space usage higher than
it should be. Thus adding log support to zvol_discard() is probably
a good idea for a future improvement.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
2012-02-09 16:19:38 -08:00

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dnl #
dnl # 2.6.32 API change
dnl # Discard requests were moved to the normal I/O path.
dnl #
AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE_DISCARD], [
AC_MSG_CHECKING([whether blk_queue_discard() is available])
tmp_flags="$EXTRA_KCFLAGS"
EXTRA_KCFLAGS="-Wno-unused-but-set-variable"
ZFS_LINUX_TRY_COMPILE([
#include <linux/blkdev.h>
],[
struct request_queue *q = NULL;
(void) blk_queue_discard(q);
],[
AC_MSG_RESULT(yes)
AC_DEFINE(HAVE_BLK_QUEUE_DISCARD, 1,
[blk_queue_discard() is available])
],[
AC_MSG_RESULT(no)
])
EXTRA_KCFLAGS="$tmp_flags"
])