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'generate_fn' generates appropriate integrity metadata (for WRITE).----------------------------------------------------------------------1. INTRODUCTIONModern filesystems feature checksumming of data and metadata toprotect against data corruption. However, the detection of thecorruption is done at read time which could potentially be monthsafter the data was written. At that point the original data that theapplication tried to write is most likely lost.The solution is to ensure that the disk is actually storing what theapplication meant it to. Recent additions to both the SCSI familyprotocols (SBC Data Integrity Field, SCC protection proposal) as wellas SATA/T13 (External Path Protection) try to remedy this by addingsupport for appending integrity metadata to an I/O. The integritymetadata (or protection information in SCSI terminology) includes achecksum for each sector as well as an incrementing counter thatensures the individual sectors are written in the right order. Andfor some protection schemes also that the I/O is written to the rightplace on disk.Current storage controllers and devices implement various protectivemeasures, for instance checksumming and scrubbing. But thesetechnologies are working in their own isolated domains or at bestbetween adjacent nodes in the I/O path. The interesting thing aboutDIF and the other integrity extensions is that the protection formatis well defined and every node in the I/O path can verify theintegrity of the I/O and reject it if corruption is detected. Thisallows not only corruption prevention but also isolation of the pointof failure.----------------------------------------------------------------------2. THE DATA INTEGRITY EXTENSIONSAs written, the protocol extensions only protect the path betweencontroller and storage device. However, many controllers actuallyallow the operating system to interact with the integrity metadata(IMD). We have been working with several FC/SAS HBA vendors to enablethe protection information to be transferred to and from theircontrollers.The SCSI Data Integrity Field works by appending 8 bytes of protectioninformation to each sector. The data + integrity metadata is storedin 520 byte sectors on disk. Data + IMD are interleaved whentransferred between the controller and target. The T13 proposal issimilar.Because it is highly inconvenient for operating systems to deal with520 (and 4104) byte sectors, we approached several HBA vendors andencouraged them to allow separation of the data and integrity metadatascatter-gather lists.The controller will interleave the buffers on write and split them onread. This means that Linux can DMA the data buffers to and fromhost memory without changes to the page cache.Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specsis somewhat heavy to compute in software. Benchmarks found thatcalculating this checksum had a significant impact on systemperformance for a number of workloads. Some controllers allow alighter-weight checksum to be used when interfacing with the operatingsystem. Emulex, for instance, supports the TCP/IP checksum instead.The IP checksum received from the OS is converted to the 16-bit CRCwhen writing and vice versa. This allows the integrity metadata to begenerated by Linux or the application at very low cost (comparable tosoftware RAID5).The IP checksum is weaker than the CRC in terms of detecting biterrors. However, the strength is really in the separation of the databuffers and the integrity metadata. These two distinct buffers mustmatch up for an I/O to complete.The separation of the data and integrity metadata buffers as well asthe choice in checksums is referred to as the Data IntegrityExtensions. As these extensions are outside the scope of the protocolbodies (T10, T13), Oracle and its partners are trying to standardizethem within the Storage Networking Industry Association.----------------------------------------------------------------------3. KERNEL CHANGESThe data integrity framework in Linux enables protection informationto be pinned to I/Os and sent to/received from controllers thatsupport it.The advantage to the integrity extensions in SCSI and SATA is thatthey enable us to protect the entire path from application to storagedevice. However, at the same time this is also the biggestdisadvantage. It means that the protection information must be in aformat that can be understood by the disk.Generally Linux/POSIX applications are agnostic to the intricacies ofthe storage devices they are accessing. The virtual filesystem switch