A new iteration of this patchset, previously known as write streams. As before, this patchset aims at enabling applications split up writes into separate streams, based on the perceived life time of the data written. This is useful for a variety of reasons: - For NVMe, this feature is ratified and released with the NVMe 1.3 spec. Devices implementing Directives can expose multiple streams. Separating data written into streams based on life time can drastically reduce the write amplification. This helps device endurance, and increases performance. Testing just performed internally at Facebook with these patches showed up to a 25% reduction in NAND writes in a RocksDB setup. - Software caching solutions can make more intelligent decisions on how and where to place data. Contrary to previous patches, we're not exposing numeric stream values anymore. I've previously advocated for just doing a set of hints that makes sense instead. See the coverage from the LSFMM summit this year: https://lwn.net/Articles/717755/ This patchset attempts to do that. We define 4 flags for the pwritev2 system call: RWF_WRITE_LIFE_SHORT Data written with this flag is expected to have a high overwrite rate, or life time. RWF_WRITE_LIFE_MEDIUM Longer life time than SHORT RWF_WRITE_LIFE_LONG Longer life time than MEDIUM RWF_WRITE_LIFE_EXTREME Longer life time than LONG The idea is that these are relative values, so an application can use them as they see fit. The underlying device can then place data appropriately, or be free to ignore the hint. It's just a hint. Similarly, to query and set these values on the side, there's now an fcntl based interface. This exposes the WRITE_HINT_* values to userspace, and defines F_{GET,SET}_WRITE_LIFE commands to get and set them as well. A branch based on current master can be pulled from here: git://git.kernel.dk/linux-block write-stream.5 Changes since v4: - Add enum write_hint and the WRITE_HINT_* values. This is what we use internally (until transformed to req/bio flags), and what is exposed to user space with the fcntl() interface. Maps directly to the RWF_WRITE_LIFE_* values. - Add fcntl() interface for getting/setting hint values. - Get rid of inode ->i_write_hint, encode the 3 bits of hint info in the inode flags intead. - Allow a write with no hint to clear the old hint. Previously we only changed the hint if a new valid hint was given, not if no hint was passed in. - Shrink flag space grabbed from 4 to 3 bits for RWF_* and the inode flags. Changes since v3: - Change any naming of stream ID to write hint. - Various little API changes, suggested by Christoph - Cleanup the NVMe bits, dump the debug info. - Change NVMe to lazily allocate the streams. - Various NVMe error handling improvements and command checking. Changes since v2: - Get rid of bio->bi_stream and replace with four request/bio flags. These map directly to the RWF_WRITE_* flags that the user passes in. - Cleanup the NVMe stream setting. - Drivers now responsible for updating the queue stream write counter, as they determine what stream to map a given flag to. Changes since v1: - Guard queue stream stats to ensure we don't mess up memory, if bio_stream() ever were to return a larger value than we support. - NVMe: ensure we set the stream modulo the name space defined count. - Cleanup the RWF_ and IOCB_ flags. Set aside 4 bits, and just store the stream value in there. This makes the passing of stream ID from RWF_ space to IOCB_ (and IOCB_ to bio) more efficient, and cleans it up in general. - Kill the block internal definitions of the stream type, we don't need them anymore. See above. -- Jens Axboe
