Documentation: Remove mentioning of block barriers
Remove mentioning of block barriers since they were removed. Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Leonid V. Fedorenchik <leonidsbox@gmail.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>wifi-calibration
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@ -48,8 +48,7 @@ Description of Contents:
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- Highmem I/O support
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- Highmem I/O support
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- I/O scheduler modularization
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- I/O scheduler modularization
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1.2 Tuning based on high level requirements/capabilities
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1.2 Tuning based on high level requirements/capabilities
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1.2.1 I/O Barriers
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1.2.1 Request Priority/Latency
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1.2.2 Request Priority/Latency
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1.3 Direct access/bypass to lower layers for diagnostics and special
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1.3 Direct access/bypass to lower layers for diagnostics and special
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device operations
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device operations
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1.3.1 Pre-built commands
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1.3.1 Pre-built commands
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@ -255,29 +254,12 @@ some control over i/o ordering.
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What kind of support exists at the generic block layer for this ?
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What kind of support exists at the generic block layer for this ?
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The flags and rw fields in the bio structure can be used for some tuning
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The flags and rw fields in the bio structure can be used for some tuning
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from above e.g indicating that an i/o is just a readahead request, or for
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from above e.g indicating that an i/o is just a readahead request, or priority
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marking barrier requests (discussed next), or priority settings (currently
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settings (currently unused). As far as user applications are concerned they
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unused). As far as user applications are concerned they would need an
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would need an additional mechanism either via open flags or ioctls, or some
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additional mechanism either via open flags or ioctls, or some other upper
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other upper level mechanism to communicate such settings to block.
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level mechanism to communicate such settings to block.
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1.2.1 I/O Barriers
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1.2.1 Request Priority/Latency
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There is a way to enforce strict ordering for i/os through barriers.
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All requests before a barrier point must be serviced before the barrier
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request and any other requests arriving after the barrier will not be
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serviced until after the barrier has completed. This is useful for higher
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level control on write ordering, e.g flushing a log of committed updates
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to disk before the corresponding updates themselves.
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A flag in the bio structure, BIO_BARRIER is used to identify a barrier i/o.
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The generic i/o scheduler would make sure that it places the barrier request and
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all other requests coming after it after all the previous requests in the
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queue. Barriers may be implemented in different ways depending on the
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driver. For more details regarding I/O barriers, please read barrier.txt
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in this directory.
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1.2.2 Request Priority/Latency
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Todo/Under discussion:
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Todo/Under discussion:
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Arjan's proposed request priority scheme allows higher levels some broad
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Arjan's proposed request priority scheme allows higher levels some broad
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@ -906,8 +888,8 @@ queue and specific I/O schedulers. Unless stated otherwise, elevator is used
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to refer to both parts and I/O scheduler to specific I/O schedulers.
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to refer to both parts and I/O scheduler to specific I/O schedulers.
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Block layer implements generic dispatch queue in block/*.c.
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Block layer implements generic dispatch queue in block/*.c.
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The generic dispatch queue is responsible for properly ordering barrier
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The generic dispatch queue is responsible for requeueing, handling non-fs
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requests, requeueing, handling non-fs requests and all other subtleties.
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requests and all other subtleties.
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Specific I/O schedulers are responsible for ordering normal filesystem
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Specific I/O schedulers are responsible for ordering normal filesystem
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requests. They can also choose to delay certain requests to improve
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requests. They can also choose to delay certain requests to improve
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