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block: add ability to flag write back caching on a device

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Add an internal helper and flag for setting whether a queue has
write back caching, or write through (or none). Add a sysfs file
to show this as well, and make it changeable from user space.

This will replace the (awkward) blk_queue_flush() interface that
drivers currently use to inform the block layer of write cache state
and capabilities.

Signed-off-by: Jens Axboe <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
This commit is contained in:
Jens Axboe 2016-04-12 12:32:46 -06:00
commit 93e9d8e836
4 changed files with 77 additions and 0 deletions
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9
Documentation/block/queue-sysfs.txt
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@ -141,6 +141,15 @@ control of this block device to that new IO scheduler. Note that writing
an IO scheduler name to this file will attempt to load that IO scheduler
module, if it isn't already present in the system.
write_cache (RW)
----------------
When read, this file will display whether the device has write back
caching enabled or not. It will return "write back" for the former
case, and "write through" for the latter. Writing to this file can
change the kernels view of the device, but it doesn't alter the
device state. This means that it might not be safe to toggle the
setting from "write back" to "write through", since that will also
eliminate cache flushes issued by the kernel.
Jens Axboe <[email protected]>, February 2009