writeback_cache_control.rst (3996B)
1========================================== 2Explicit volatile write back cache control 3========================================== 4 5Introduction 6------------ 7 8Many storage devices, especially in the consumer market, come with volatile 9write back caches. That means the devices signal I/O completion to the 10operating system before data actually has hit the non-volatile storage. This 11behavior obviously speeds up various workloads, but it means the operating 12system needs to force data out to the non-volatile storage when it performs 13a data integrity operation like fsync, sync or an unmount. 14 15The Linux block layer provides two simple mechanisms that let filesystems 16control the caching behavior of the storage device. These mechanisms are 17a forced cache flush, and the Force Unit Access (FUA) flag for requests. 18 19 20Explicit cache flushes 21---------------------- 22 23The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from 24the filesystem and will make sure the volatile cache of the storage device 25has been flushed before the actual I/O operation is started. This explicitly 26guarantees that previously completed write requests are on non-volatile 27storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can be 28set on an otherwise empty bio structure, which causes only an explicit cache 29flush without any dependent I/O. It is recommend to use 30the blkdev_issue_flush() helper for a pure cache flush. 31 32 33Forced Unit Access 34------------------ 35 36The REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the 37filesystem and will make sure that I/O completion for this request is only 38signaled after the data has been committed to non-volatile storage. 39 40 41Implementation details for filesystems 42-------------------------------------- 43 44Filesystems can simply set the REQ_PREFLUSH and REQ_FUA bits and do not have to 45worry if the underlying devices need any explicit cache flushing and how 46the Forced Unit Access is implemented. The REQ_PREFLUSH and REQ_FUA flags 47may both be set on a single bio. 48 49 50Implementation details for bio based block drivers 51-------------------------------------------------------------- 52 53These drivers will always see the REQ_PREFLUSH and REQ_FUA bits as they sit 54directly below the submit_bio interface. For remapping drivers the REQ_FUA 55bits need to be propagated to underlying devices, and a global flush needs 56to be implemented for bios with the REQ_PREFLUSH bit set. For real device 57drivers that do not have a volatile cache the REQ_PREFLUSH and REQ_FUA bits 58on non-empty bios can simply be ignored, and REQ_PREFLUSH requests without 59data can be completed successfully without doing any work. Drivers for 60devices with volatile caches need to implement the support for these 61flags themselves without any help from the block layer. 62 63 64Implementation details for request_fn based block drivers 65--------------------------------------------------------- 66 67For devices that do not support volatile write caches there is no driver 68support required, the block layer completes empty REQ_PREFLUSH requests before 69entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from 70requests that have a payload. For devices with volatile write caches the 71driver needs to tell the block layer that it supports flushing caches by 72doing:: 73 74 blk_queue_write_cache(sdkp->disk->queue, true, false); 75 76and handle empty REQ_OP_FLUSH requests in its prep_fn/request_fn. Note that 77REQ_PREFLUSH requests with a payload are automatically turned into a sequence 78of an empty REQ_OP_FLUSH request followed by the actual write by the block 79layer. For devices that also support the FUA bit the block layer needs 80to be told to pass through the REQ_FUA bit using:: 81 82 blk_queue_write_cache(sdkp->disk->queue, true, true); 83 84and the driver must handle write requests that have the REQ_FUA bit set 85in prep_fn/request_fn. If the FUA bit is not natively supported the block 86layer turns it into an empty REQ_OP_FLUSH request after the actual write.