mirror of
https://github.com/AuxXxilium/linux_dsm_epyc7002.git
synced 2024-11-25 03:20:53 +07:00
04ccc65cd1
Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Tejun Heo <tj@kernel.org> Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
87 lines
3.8 KiB
Plaintext
87 lines
3.8 KiB
Plaintext
|
|
Explicit volatile write back cache control
|
|
=====================================
|
|
|
|
Introduction
|
|
------------
|
|
|
|
Many storage devices, especially in the consumer market, come with volatile
|
|
write back caches. That means the devices signal I/O completion to the
|
|
operating system before data actually has hit the non-volatile storage. This
|
|
behavior obviously speeds up various workloads, but it means the operating
|
|
system needs to force data out to the non-volatile storage when it performs
|
|
a data integrity operation like fsync, sync or an unmount.
|
|
|
|
The Linux block layer provides two simple mechanisms that let filesystems
|
|
control the caching behavior of the storage device. These mechanisms are
|
|
a forced cache flush, and the Force Unit Access (FUA) flag for requests.
|
|
|
|
|
|
Explicit cache flushes
|
|
----------------------
|
|
|
|
The REQ_FLUSH flag can be OR ed into the r/w flags of a bio submitted from
|
|
the filesystem and will make sure the volatile cache of the storage device
|
|
has been flushed before the actual I/O operation is started. This explicitly
|
|
guarantees that previously completed write requests are on non-volatile
|
|
storage before the flagged bio starts. In addition the REQ_FLUSH flag can be
|
|
set on an otherwise empty bio structure, which causes only an explicit cache
|
|
flush without any dependent I/O. It is recommend to use
|
|
the blkdev_issue_flush() helper for a pure cache flush.
|
|
|
|
|
|
Forced Unit Access
|
|
-----------------
|
|
|
|
The REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the
|
|
filesystem and will make sure that I/O completion for this request is only
|
|
signaled after the data has been committed to non-volatile storage.
|
|
|
|
|
|
Implementation details for filesystems
|
|
--------------------------------------
|
|
|
|
Filesystems can simply set the REQ_FLUSH and REQ_FUA bits and do not have to
|
|
worry if the underlying devices need any explicit cache flushing and how
|
|
the Forced Unit Access is implemented. The REQ_FLUSH and REQ_FUA flags
|
|
may both be set on a single bio.
|
|
|
|
|
|
Implementation details for make_request_fn based block drivers
|
|
--------------------------------------------------------------
|
|
|
|
These drivers will always see the REQ_FLUSH and REQ_FUA bits as they sit
|
|
directly below the submit_bio interface. For remapping drivers the REQ_FUA
|
|
bits need to be propagated to underlying devices, and a global flush needs
|
|
to be implemented for bios with the REQ_FLUSH bit set. For real device
|
|
drivers that do not have a volatile cache the REQ_FLUSH and REQ_FUA bits
|
|
on non-empty bios can simply be ignored, and REQ_FLUSH requests without
|
|
data can be completed successfully without doing any work. Drivers for
|
|
devices with volatile caches need to implement the support for these
|
|
flags themselves without any help from the block layer.
|
|
|
|
|
|
Implementation details for request_fn based block drivers
|
|
--------------------------------------------------------------
|
|
|
|
For devices that do not support volatile write caches there is no driver
|
|
support required, the block layer completes empty REQ_FLUSH requests before
|
|
entering the driver and strips off the REQ_FLUSH and REQ_FUA bits from
|
|
requests that have a payload. For devices with volatile write caches the
|
|
driver needs to tell the block layer that it supports flushing caches by
|
|
doing:
|
|
|
|
blk_queue_flush(sdkp->disk->queue, REQ_FLUSH);
|
|
|
|
and handle empty REQ_FLUSH requests in its prep_fn/request_fn. Note that
|
|
REQ_FLUSH requests with a payload are automatically turned into a sequence
|
|
of an empty REQ_FLUSH request followed by the actual write by the block
|
|
layer. For devices that also support the FUA bit the block layer needs
|
|
to be told to pass through the REQ_FUA bit using:
|
|
|
|
blk_queue_flush(sdkp->disk->queue, REQ_FLUSH | REQ_FUA);
|
|
|
|
and the driver must handle write requests that have the REQ_FUA bit set
|
|
in prep_fn/request_fn. If the FUA bit is not natively supported the block
|
|
layer turns it into an empty REQ_FLUSH request after the actual write.
|