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