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Prior tob39c22b, which was first generally available in the 0.6.5 release asb39c22b, ZoL never actually submitted synchronous read or write requests to the Linux block layer. This means the vdev_disk_dio_is_sync() function had always returned false and, therefore, the completion in dio_request_t.dr_comp was never actually used. Inb39c22b, synchronous ZIO operations were translated to synchronous BIO requests in vdev_disk_io_start(). The follow-on commits5592404andaa159affixed several problems introduced byb39c22b. In particular,5592404introduced the new flag parameter "wait" to __vdev_disk_physio() but under ZoL, since vdev_disk_physio() is never actually used, the wait flag was always zero so the new code had no effect other than to cause a bug in the use of the dio_request_t.dr_comp which was fixed byaa159af. The original rationale for introducing synchronous operations inb39c22bwas to hurry certains requests through the BIO layer which would have otherwise been subject to its unplug timer which would increase the latency. This behavior of the unplug timer, however, went away during the transition of the plug/unplug system between kernels 2.6.32 and 2.6.39. To handle the unplug timer behavior on 2.6.32-2.6.35 kernels the BIO_RW_UNPLUG flag is used as a hint to suppress the plugging behavior. For kernels 2.6.36-2.6.38, the REQ_UNPLUG macro will be available and ise used for the same purpose. Signed-off-by: Tim Chase <tim@chase2k.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #4858
24 lines
596 B
Plaintext
24 lines
596 B
Plaintext
dnl #
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dnl # 2.6.32-2.6.35 API - The BIO_RW_UNPLUG enum can be used as a hint
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dnl # to unplug the queue.
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dnl #
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AC_DEFUN([ZFS_AC_KERNEL_BLK_QUEUE_HAVE_BIO_RW_UNPLUG], [
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AC_MSG_CHECKING([whether the BIO_RW_UNPLUG enum is available])
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tmp_flags="$EXTRA_KCFLAGS"
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EXTRA_KCFLAGS="${NO_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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extern enum bio_rw_flags rw;
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rw = BIO_RW_UNPLUG;
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],[
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AC_MSG_RESULT(yes)
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AC_DEFINE(HAVE_BLK_QUEUE_HAVE_BIO_RW_UNPLUG, 1,
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[BIO_RW_UNPLUG 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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