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docs/ZIO-Scheduler.rst

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+ZFS I/O (ZIO) Scheduler
+=======================
+
+ZFS issues I/O operations to leaf vdevs (usually devices) to satisfy and
+complete I/Os. The ZIO scheduler determines when and in what order those
+operations are issued. Operations into five I/O classes prioritized in
+the following order:
+
++----------+-------------+-------------------------------------------+
+| Priority | I/O Class   | Description                               |
++==========+=============+===========================================+
+| highest  | sync read   | most reads                                |
++----------+-------------+-------------------------------------------+
+|          | sync write  | as defined by application or via 'zfs'    |
+|          |             | 'sync' property                           |
++----------+-------------+-------------------------------------------+
+|          | async read  | prefetch reads                            |
++----------+-------------+-------------------------------------------+
+|          | async write | most writes                               |
++----------+-------------+-------------------------------------------+
+| lowest   | scrub read  | scan read: includes both scrub and        |
+|          |             | resilver                                  |
++----------+-------------+-------------------------------------------+
+
+Each queue defines the minimum and maximum number of concurrent
+operations issued to the device. In addition, the device has an
+aggregate maximum, zfs_vdev_max_active. Note that the sum of the
+per-queue minimums must not exceed the aggregate maximum. If the sum of
+the per-queue maximums exceeds the aggregate maximum, then the number of
+active I/Os may reach zfs_vdev_max_active, in which case no further I/Os
+are issued regardless of whether all per-queue minimums have been met.
+
++-------------+--------------------------+--------------------------+
+| I/O Class   | Min Active Parameter     | Max Active Parameter     |
++=============+==========================+==========================+
+| sync read   | zfs_v                    | zfs_v                    |
+|             | dev_sync_read_min_active | dev_sync_read_max_active |
++-------------+--------------------------+--------------------------+
+| sync write  | zfs_vd                   | zfs_vd                   |
+|             | ev_sync_write_min_active | ev_sync_write_max_active |
++-------------+--------------------------+--------------------------+
+| async read  | zfs_vd                   | zfs_vd                   |
+|             | ev_async_read_min_active | ev_async_read_max_active |
++-------------+--------------------------+--------------------------+
+| async write | zfs_vde                  | zfs_vde                  |
+|             | v_async_write_min_active | v_async_write_max_active |
++-------------+--------------------------+--------------------------+
+| scrub read  | z                        | z                        |
+|             | fs_vdev_scrub_min_active | fs_vdev_scrub_max_active |
++-------------+--------------------------+--------------------------+
+
+For many physical devices, throughput increases with the number of
+concurrent operations, but latency typically suffers. Further, physical
+devices typically have a limit at which more concurrent operations have
+no effect on throughput or can actually cause it to performance to
+decrease.
+
+The ZIO scheduler selects the next operation to issue by first looking
+for an I/O class whose minimum has not been satisfied. Once all are
+satisfied and the aggregate maximum has not been hit, the scheduler
+looks for classes whose maximum has not been satisfied. Iteration
+through the I/O classes is done in the order specified above. No further
+operations are issued if the aggregate maximum number of concurrent
+operations has been hit or if there are no operations queued for an I/O
+class that has not hit its maximum. Every time an I/O is queued or an
+operation completes, the I/O scheduler looks for new operations to
+issue.
+
+In general, smaller max_active's will lead to lower latency of
+synchronous operations. Larger max_active's may lead to higher overall
+throughput, depending on underlying storage and the I/O mix.
+
+The ratio of the queues' max_actives determines the balance of
+performance between reads, writes, and scrubs. For example, when there
+is contention, increasing zfs_vdev_scrub_max_active will cause the scrub
+or resilver to complete more quickly, but reads and writes to have
+higher latency and lower throughput.
+
+All I/O classes have a fixed maximum number of outstanding operations
+except for the async write class. Asynchronous writes represent the data
+that is committed to stable storage during the syncing stage for
+transaction groups (txgs). Transaction groups enter the syncing state
+periodically so the number of queued async writes quickly bursts up and
+then reduce down to zero. The zfs_txg_timeout tunable (default=5
+seconds) sets the target interval for txg sync. Thus a burst of async
+writes every 5 seconds is a normal ZFS I/O pattern.
+
+Rather than servicing I/Os as quickly as possible, the ZIO scheduler
+changes the maximum number of active async write I/Os according to the
+amount of dirty data in the pool. Since both throughput and latency
+typically increase as the number of concurrent operations issued to
+physical devices, reducing the burstiness in the number of concurrent
+operations also stabilizes the response time of operations from other
+queues. This is particular important for the sync read and write queues,
+where the periodic async write bursts of the txg sync can lead to
+device-level contention. In broad strokes, the ZIO scheduler issues more
+concurrent operations from the async write queue as there's more dirty
+data in the pool.