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-rw-r--r--Documentation/device-mapper/cache-policies.txt16
1 files changed, 14 insertions, 2 deletions
diff --git a/Documentation/device-mapper/cache-policies.txt b/Documentation/device-mapper/cache-policies.txt
index df52a849957f..66c2774c0c64 100644
--- a/Documentation/device-mapper/cache-policies.txt
+++ b/Documentation/device-mapper/cache-policies.txt
@@ -40,8 +40,11 @@ on hit count on entry. The policy aims to take different cache miss
costs into account and to adjust to varying load patterns automatically.
Message and constructor argument pairs are:
- 'sequential_threshold <#nr_sequential_ios>' and
- 'random_threshold <#nr_random_ios>'.
+ 'sequential_threshold <#nr_sequential_ios>'
+ 'random_threshold <#nr_random_ios>'
+ 'read_promote_adjustment <value>'
+ 'write_promote_adjustment <value>'
+ 'discard_promote_adjustment <value>'
The sequential threshold indicates the number of contiguous I/Os
required before a stream is treated as sequential. The random threshold
@@ -55,6 +58,15 @@ since spindles tend to have good bandwidth. The io_tracker counts
contiguous I/Os to try to spot when the io is in one of these sequential
modes.
+Internally the mq policy maintains a promotion threshold variable. If
+the hit count of a block not in the cache goes above this threshold it
+gets promoted to the cache. The read, write and discard promote adjustment
+tunables allow you to tweak the promotion threshold by adding a small
+value based on the io type. They default to 4, 8 and 1 respectively.
+If you're trying to quickly warm a new cache device you may wish to
+reduce these to encourage promotion. Remember to switch them back to
+their defaults after the cache fills though.
+
cleaner
-------