Signed-off-by: Eric Wheeler <bcache@xxxxxxxxxxxxxxxxxx>
---
Documentation/bcache.txt | 25 +++++++++++++++++++++++++
1 file changed, 25 insertions(+)
diff --git a/Documentation/bcache.txt b/Documentation/bcache.txt
index a9259b5..c87d73f 100644
--- a/Documentation/bcache.txt
+++ b/Documentation/bcache.txt
@@ -133,6 +133,31 @@ the backing devices to passthrough mode.
writeback mode). It currently doesn't do anything intelligent if it fails to
read some of the dirty data, though.
+PER-PROCESS CACHE HINTING BY PRIORITY
+-------------------------------------
+
+Processes can be assigned an IO priority using `ionice` and bcache will
+either try to writeback or bypass the cache based on the IO priority
+level assigned to the process.
+
+Cache hinting is configurable by writing 'class,level' pairs to sysfs.
+These are the defaults:
+ echo 2,7 > /sys/block/bcache0/bcache/ioprio_bypass
+ echo 2,0 > /sys/block/bcache0/bcache/ioprio_writeback
+
+By default, Linux assigns processes the best-effort class with a class
+level of 4. Thus, if you do nothing, your default processes will follow
+the original bcache should_writeback/should_bypass logic.
+
+ (-p) IO Class (-n) Class level Action
+ -----------------------------------------------------
+ (1) Realtime 0-7 Writeback
+ (2) Best-effort 0 Writeback
+ (2) Best-effort 1-6 Original bcache logic
+ (2) Best-effort 7 Bypass cache
+ (3) Idle n/a Bypass cache
+
+See `man ionice` for more detail about process IO priority in Linux.
HOWTO/COOKBOOK
--------------
--
1.8.3.1
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