Explicit volatile write back cache control


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_PREFLUSH 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_PREFLUSH 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_PREFLUSH 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_PREFLUSH and REQ_FUA flags may both be set on a single bio.

Feature settings for block drivers

For devices that do not support volatile write caches there is no driver support required, the block layer completes empty REQ_PREFLUSH requests before entering the driver and strips off the REQ_PREFLUSH 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 setting the


flag in the queue_limits feature field. For devices that also support the FUA bit the block layer needs to be told to pass on the REQ_FUA bit by also setting the


flag in the features field of the queue_limits structure.

Implementation details for bio based block drivers

For bio based drivers the REQ_PREFLUSH and REQ_FUA bit are simply passed on to the driver if the driver sets the BLK_FEAT_WRITE_CACHE flag and the driver needs to handle them.

NOTE: The REQ_FUA bit also gets passed on when the BLK_FEAT_FUA flags is _not_ set. Any bio based driver that sets BLK_FEAT_WRITE_CACHE also needs to handle REQ_FUA.

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_PREFLUSH bit set.

Implementation details for blk-mq drivers

When the BLK_FEAT_WRITE_CACHE flag is set, REQ_OP_WRITE | REQ_PREFLUSH requests with a payload are automatically turned into a sequence of a REQ_OP_FLUSH request followed by the actual write by the block layer.

When the BLK_FEAT_FUA flags is set, the REQ_FUA bit is simply passed on for the REQ_OP_WRITE request, else a REQ_OP_FLUSH request is sent by the block layer after the completion of the write request for bio submissions with the REQ_FUA bit set.