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Robert Baldyga
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doc: Add requirements for sequential cutoff
Signed-off-by: Robert Baldyga <[email protected]>
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‎doc/requirements/sequential_cutoff‎

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---
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group: Sequential cutoff
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---
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Sequential cutoff feature allows to detect sequential data streams
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and redirect them to backend volume (bypassing the cache).
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title: Setting sequential cutoff policy for core
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id: policy_core
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---
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The management tool shall allow to specify sequential cutoff policy
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for given core.
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title: Setting sequential cutoff policy for all cores in given cache instance
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id: policy_all_cores
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---
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The management tool shall allow to specify sequential cutoff policy
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for all cores associated with given cache instance at once.
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title: Allowed sequential cutoff policies
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id: allowed_policies
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---
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There shall be three sequential cutoff policies:
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1. always
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2. full
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3. never
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title: Default sequential cutoff policy
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id: policy_default
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---
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The default sequential cutoff policy shall be **full**.
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title: Setting sequential cutoff threshold for core
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id: threshold_core
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---
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The management tool shall allow to specify sequential cutoff threshold
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for given core.
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title: Setting sequential cutoff threshold for all cores in given cache instance
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id: threshold_all_cores
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---
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The management tool shall allow to specify sequential cutoff threshold
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for all cores associated with given cache instance at once.
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title: Allowed sequential cutoff theshold values
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id: threshold_values
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---
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It shall be possible to specify sequential cutoff threshold value in range
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from 1 to 4194181 KiB inclusive.
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title: Default sequential cutoff threshold value
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id: threshold_default
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---
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The default sequential cutoff threshold shall be 1024 KiB.
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title: Setting sequential cutoff promotion count for core
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id: promotion_count_core
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---
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The management tool shall allow to specify sequential cutoff promotion count
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for given core.
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title: >-
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Setting sequential cutoff promotion count for all cores in given
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cache instance
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id: promotion_count_all_cores
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---
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The management tool shall allow to specify sequential cutoff promotion count
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for all cores associated with given cache instance at once.
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title: Allowed sequential cutoff promotion count values
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id: promotion_count_values
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---
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It shall be possible to specify sequential cutoff promotion count value
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in range from 0 to 65535 inclusive.
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title: Default sequential cutoff promotion count value
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id: promotion_count_default
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---
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The default sequential cutoff promotion count shall be 8.
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title: Loading sequential cutoff configuration on cache load
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id: loading_configuration
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---
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All sequential cutoff parameters shall be loaded from cache metadata for each
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core on cache load.
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title: Sequential cutoff stream detection
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id: operation
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---
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The sequential cutoff shall detect streams of sequential IO requests.
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title: Sequential cutoff operation
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id: operation
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---
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When lenght of the stream reaches the cutoff threshold, each next IO request
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which continues the stream and is not full dirty hit shall be handled
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in Pass-Through mode.
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title: Sequential cutoff for reads and writes
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id: reads_and_writes
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---
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The sequential cutoff shall track streams of read and write IO requests
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separately.
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title: Multiple per-CPU sequential cutoff streams
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id: per_cpu_multiple_streams
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---
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The sequential cutoff shall be able to detect up to 128 per-CPU streams.
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title: Multiple shared sequential cutoff streams
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id: shared_multiple_streams
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---
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The sequential cutoff shall be able to detect up to 64 shared streams.
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title: Sequential cutoff per-CPU stream continuation
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id: per_cpu_stream_continuation
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---
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When new IO request is being handled, the sequential cutoff shall first try to
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account it to one of per-CPU streams local to the CPU on which the IO request
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is being handled.
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title: Sequential cutoff shared stream continuation
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id: shared_stream_continuation
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---
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When new IO request is being handled and there is no per-CPU stream which can
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be continued by this IO request, the sequential cutoff shall try to account it
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to one of shared streams.
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title: Sequential cutoff stream creation
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id: stream_creation
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---
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When new IO request is being handled and there is neither per-CPU nor shared
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stream which can be countinued by this IO request, the sequential cutoff shall
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create new per-CPU stream starting with this IO request.
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title: Sequential cutoff stream promotion
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id: stream_promotion
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---
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When number of IO requests accounted to given per-CPU stream reaches
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the promotion count or stream length reaches the threshold, the stream
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shall be promoted to shared stream pool.
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title: Sequential cutoff policy **always**
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id: policy_always
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---
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When sequential cutoff policy is set to **always**, the sequential cutoff shall
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be enabled regardless to cache occupancy.
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title: Sequential cutoff policy **full**
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id: policy_full
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---
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When sequential cutoff policy is set to **full**, the sequential cutoff shall
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be enabled only when cache occupancy reaches 100%.
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title: Sequential cutoff policy **never**
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id: policy_never
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---
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When sequential cutoff policy is set to **never**, the sequential cutoff shall
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be disabled.

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