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SubjectRe: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush
On 08/20/2010 09:22 AM, Christoph Hellwig wrote:
> FYI: here's a little writeup to document the new cache flushing scheme,
> intended to replace Documentation/block/barriers.txt. Any good
> suggestion for a filename in the kernel tree?
>
> ---

I was thinking that we might be better off using the "durable writes" term more
since it is well documented (at least in the database world, where it is the "D"
in ACID properties). Maybe "durable_writes_support.txt" ?

>
> Explicit volatile write 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 physical medium. This
> behavior obviously speeds up various workloads, but it means the operating
> system needs to force data out to the physical medium when it performs
> a data integrity operation like fsync, sync or an unmount.
>
> The Linux block layer provides a two simple mechanism that lets 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.
>

Should we mention that users can also disable the write cache on the target device?

It might also be worth mentioning that storage needs to be properly configured -
i.e., an internal hardware RAID card with battery backing needs can expose
itself as a writethrough cache *only if* it actually has control over all of the
backend disks and can flush/disable their write caches.

Maybe that is too much detail, but I know that people have lost data with some
of these setups.

The rest of the write up below sounds good, thanks for pulling this together!

Ric


>
> 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. The explicit
> guarantees write requests that have completed before the bio was submitted
> actually are on the physical medium before this request has started.
> 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 requests is not
> signaled before the data has made it to non-volatile storage on the
> physical medium.
>
>
> Implementation details for filesystems
> --------------------------------------
>
> Filesystem 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 needs to be propagate 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 device 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 it's prep_fn/request_fn. Note that
> REQ_FLUSH requests with a payload are automatically turned into a sequence
> of empty REQ_FLUSH and 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 that bit using:
>
> blk_queue_flush(sdkp->disk->queue, REQ_FLUSH | REQ_FUA);
>
> and handle write requests that have the REQ_FUA bit set properly in it's
> prep_fn/request_fn. If the FUA bit is not natively supported the block
> layer turns it into an empty REQ_FLUSH requests after the actual write.



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