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-rw-r--r--Documentation/device-mapper/cache-policies.txt16
1 files changed, 11 insertions, 5 deletions
diff --git a/Documentation/device-mapper/cache-policies.txt b/Documentation/device-mapper/cache-policies.txt
index 7746e5dbfd40..0d124a971801 100644
--- a/Documentation/device-mapper/cache-policies.txt
+++ b/Documentation/device-mapper/cache-policies.txt
@@ -47,16 +47,22 @@ Message and constructor argument pairs are:
'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
+required before a stream is treated as sequential. Once a stream is
+considered sequential it will bypass the cache. The random threshold
is the number of intervening non-contiguous I/Os that must be seen
before the stream is treated as random again.
The sequential and random thresholds default to 512 and 4 respectively.
-Large, sequential ios are probably better left on the origin device
-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.
+Large, sequential I/Os are probably better left on the origin device
+since spindles tend to have good sequential I/O bandwidth. The
+io_tracker counts contiguous I/Os to try to spot when the I/O is in one
+of these sequential modes. But there are use-cases for wanting to
+promote sequential blocks to the cache (e.g. fast application startup).
+If sequential threshold is set to 0 the sequential I/O detection is
+disabled and sequential I/O will no longer implicitly bypass the cache.
+Setting the random threshold to 0 does _not_ disable the random I/O
+stream detection.
Internally the mq policy determines a promotion threshold. If the hit
count of a block not in the cache goes above this threshold it gets