Fix synchronous behavior in __vdev_disk_physio() #3833
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Commit b39c22b set the READ_SYNC and WRITE_SYNC flags for a bio based on the ZIO_PRIORITY_* flag passed in. This had the unnoticed side-effect of making the vdev_disk_io_start() synchronous for certain I/Os. This in turn resulted in vdev_disk_io_start() being able to re-dispatch zio's which would result in a RCU stalls when a disk was removed from the system. Additionally, this could negatively impact performance and may explain the performance regressions reported in both openzfs#3829 and openzfs#3780. This patch resolves the issue by making the blocking behavior dependant on a 'wait' flag being passed rather than overloading the passed bio flags. Finally, the WRITE_SYNC and READ_SYNC behavior is restricted to non-rotational devices where there is no benefit to queuing to aggregate the I/O. Signed-off-by: Brian Behlendorf <[email protected]> Issue openzfs#3780 Issue openzfs#3829 Issue openzfs#3652
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@bprotopopov could you please review this change. Your earlier commit b39c22b resulted in a few unexpected side effects this patch is designed to address. |
This was referenced Sep 25, 2015
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Merged as: 5592404 Fix synchronous behavior in __vdev_disk_physio() |
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@behlendorf, the change looks good. Needless to say, I am a bit surprised with the outcome of the original commit. It seems that this behavior is kernel version dependent as on Centos 6.5/6.6, I did not see a slowdown when ZIL is not on SLOG. |
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Commit b39c22b set the READ_SYNC and WRITE_SYNC flags for a bio
based on the ZIO_PRIORITY_* flag passed in. This had the unnoticed
side-effect of making the vdev_disk_io_start() synchronous for
certain I/Os.
This in turn resulted in vdev_disk_io_start() being able to
re-dispatch zio's which would result in a RCU stalls when a disk
was removed from the system. Additionally, this could negatively
impact performance and may explain the performance regressions
reported in both #3829 and #3780.
This patch resolves the issue by making the blocking behavior
dependant on a 'wait' flag being passed rather than overloading
the passed bio flags.
Finally, the WRITE_SYNC and READ_SYNC behavior is restricted to
non-rotational devices where there is no benefit to queuing to
aggregate the I/O.
Signed-off-by: Brian Behlendorf [email protected]
Issue #3780
Issue #3829
Issue #3652