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    Subject[PATCH 3 of 3] block: Data integrity infrastructure documentation
    Signed-off-by: Martin K. Petersen <>

    2 files changed, 361 insertions(+)
    Documentation/ABI/testing/sysfs-block | 34 +++
    Documentation/block/data-integrity.txt | 327 ++++++++++++++++++++++++++++++++

    diff -r 91ceabd365c3 -r 936c875b13ee Documentation/ABI/testing/sysfs-block
    --- a/Documentation/ABI/testing/sysfs-block Fri Jun 13 19:24:49 2008 -0400
    +++ b/Documentation/ABI/testing/sysfs-block Fri Jun 13 19:24:50 2008 -0400
    @@ -26,3 +26,37 @@
    I/O statistics of partition <part>. The format is the
    same as the above-written /sys/block/<disk>/stat
    +What: /sys/block/<disk>/integrity/format
    +Date: June 2008
    +Contact: Martin K. Petersen <>
    + Metadata format for integrity capable block device.
    + E.g. T10-DIF-TYPE1-CRC.
    +What: /sys/block/<disk>/integrity/read_verify
    +Date: June 2008
    +Contact: Martin K. Petersen <>
    + Indicates whether the block layer should verify the
    + integrity of read requests serviced by devices that
    + support sending integrity metadata.
    +What: /sys/block/<disk>/integrity/tag_size
    +Date: June 2008
    +Contact: Martin K. Petersen <>
    + Number of bytes of integrity tag space available per
    + 512 bytes of data.
    +What: /sys/block/<disk>/integrity/write_generate
    +Date: June 2008
    +Contact: Martin K. Petersen <>
    + Indicates whether the block layer should automatically
    + generate checksums for write requests bound for
    + devices that support receiving integrity metadata.
    diff -r 91ceabd365c3 -r 936c875b13ee Documentation/block/data-integrity.txt
    --- /dev/null Thu Jan 01 00:00:00 1970 +0000
    +++ b/Documentation/block/data-integrity.txt Fri Jun 13 19:24:50 2008 -0400
    @@ -0,0 +1,327 @@
    +Modern filesystems feature checksumming of data and metadata to
    +protect against data corruption. However, the detection of the
    +corruption is done at read time which could potentially be months
    +after the data was written. At that point the original data that the
    +application tried to write is most likely lost.
    +The solution is to ensure that the disk is actually storing what the
    +application meant it to. Recent additions to both the SCSI family
    +protocols (SBC Data Integrity Field, SCC protection proposal) as well
    +as SATA/T13 (External Path Protection) try to remedy this by adding
    +support for appending integrity metadata to an I/O. The integrity
    +metadata (or protection information in SCSI terminology) includes a
    +checksum for each sector as well as an incrementing counter that
    +ensures the individual sectors are written in the right order. And
    +for some protection schemes also that the I/O is written to the right
    +place on disk.
    +Current storage controllers and devices implement various protective
    +measures, for instance checksumming and scrubbing. But these
    +technologies are working in their own isolated domains or at best
    +between adjacent nodes in the I/O path. The interesting thing about
    +DIF and the other integrity extensions is that the protection format
    +is well defined and every node in the I/O path can verify the
    +integrity of the I/O and reject it if corruption is detected. This
    +allows not only corruption prevention but also isolation of the point
    +of failure.
    +As written, the protocol extensions only protect the path between
    +controller and storage device. However, many controllers actually
    +allow the operating system to interact with the integrity metadata
    +(IMD). We have been working with several FC/SAS HBA vendors to enable
    +the protection information to be transferred to and from their
    +The SCSI Data Integrity Field works by appending 8 bytes of protection
    +information to each sector. The data + integrity metadata is stored
    +in 520 byte sectors on disk. Data + IMD are interleaved when
    +transferred between the controller and target. The T13 proposal is
    +Because it is highly inconvenient for operating systems to deal with
    +520 (and 4104) byte sectors, we approached several HBA vendors and
    +encouraged them to allow separation of the data and integrity metadata
    +scatter-gather lists.
    +The controller will interleave the buffers on write and split them on
    +read. This means that the Linux can DMA the data buffers to and from
    +host memory without changes to the page cache.
    +Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs
    +is somewhat heavy to compute in software. Benchmarks found that
    +calculating this checksum had a significant impact on system
    +performance for a number of workloads. Some controllers allow a
    +lighter-weight checksum to be used when interfacing with the operating
    +system. Emulex, for instance, supports the TCP/IP checksum instead.
    +The IP checksum received from the OS is converted to the 16-bit CRC
    +when writing and vice versa. This allows the integrity metadata to be
    +generated by Linux or the application at very low cost (comparable to
    +software RAID5).
    +The IP checksum is weaker than the CRC in terms of detecting bit
    +errors. However, the strength is really in the separation of the data
    +buffers and the integrity metadata. These two distinct buffers much
    +match up for an I/O to complete.
    +The separation of the data and integrity metadata buffers as well as
    +the choice in checksums is referred to as the Data Integrity
    +Extensions. As these extensions are outside the scope of the protocol
    +bodies (T10, T13), Oracle and its partners are trying to standardize
    +them within the Storage Networking Industry Association.
    +The data integrity framework in Linux enables protection information
    +to be pinned to I/Os and sent to/received from controllers that
    +support it.
    +The advantage to the integrity extensions in SCSI and SATA is that
    +they enable us to protect the entire path from application to storage
    +device. However, at the same time this is also the biggest
    +disadvantage. It means that the protection information must be in a
    +format that can be understood by the disk.
    +Generally Linux/POSIX applications are agnostic to the intricacies of
    +the storage devices they are accessing. The virtual filesystem switch
    +and the block layer make things like hardware sector size and
    +transport protocols completely transparent to the application.
    +However, this level of detail is required when preparing the
    +protection information to send to a disk. Consequently, the very
    +concept of an end-to-end protection scheme is a layering violation.
    +It is completely unreasonable for an application to be aware whether
    +it is accessing a SCSI or SATA disk.
    +The data integrity support implemented in Linux attempts to hide this
    +from the application. As far as the application (and to some extent
    +the kernel) is concerned, the integrity metadata is opaque information
    +that's attached to the I/O.
    +The current implementation allows the block layer to automatically
    +generate the protection information for any I/O. Eventually the
    +intent is to move the integrity metadata calculation to userspace for
    +user data. Metadata and other I/O that originates within the kernel
    +will still use the automatic generation interface.
    +Some storage devices allow each hardware sector to be tagged with a
    +16-bit value. The owner of this tag space is the owner of the block
    +device. I.e. the filesystem in most cases. The filesystem can use
    +this extra space to tag sectors as they see fit. Because the tag
    +space is limited, the block interface allows tagging bigger chunks by
    +way of interleaving. This way, 8*16 bits of information can be
    +attached to a typical 4KB filesystem block.
    +This also means that applications such as fsck and mkfs will need
    +access to manipulate the tags from user space. A passthrough
    +interface for this is being worked on.
    +4.1 BIO
    +The data integrity patches add a new field to struct bio when
    +CONFIG_BLK_DEV_INTEGRITY is enabled. bio->bi_integrity is a pointer
    +to a struct bip which contains the bio integrity payload. Essentially
    +a bip is a trimmed down struct bio which holds a bio_vec containing
    +the integrity metadata and the required housekeeping information (bvec
    +pool, vector count, etc.)
    +A kernel subsystem can enable data integrity protection on a bio by
    +calling bio_integrity_alloc(bio). This will allocate and attach the
    +bip to the bio.
    +Individual pages containing integrity metadata can subsequently be
    +attached using bio_integrity_add_page().
    +bio_free() will automatically free the bip.
    +Because the format of the protection data is tied to the physical
    +disk, each block device has been extended with a block integrity
    +profile (struct blk_integrity). This optional profile is registered
    +with the block layer using blk_integrity_register().
    +The profile contains callback functions for generating and verifying
    +the protection data, as well as getting and setting application tags.
    +The profile also contains a few constants to aid in completing,
    +merging and splitting the integrity metadata.
    +Layered block devices will need to pick a profile that's appropriate
    +for all subdevices. blk_integrity_compare() can help with that. DM
    +and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6
    +will require extra work due to the application tag.
    + The normal filesystem is unaware that the underlying block device
    + is capable of sending/receiving integrity metadata. The IMD will
    + be automatically generated by the block layer at submit_bio() time
    + in case of a WRITE. A READ request will cause the I/O integrity
    + to be verified upon completion.
    + IMD generation and verification can be toggled using the
    + /sys/block/<bdev>/integrity/write_generate
    + and
    + /sys/block/<bdev>/integrity/read_verify
    + flags.
    + A filesystem that is integrity-aware can prepare I/Os with IMD
    + attached. It can also use the application tag space if this is
    + supported by the block device.
    + int bdev_integrity_enabled(block_device, int rw);
    + bdev_integrity_enabled() will return 1 if the block device
    + supports integrity metadata transfer for the data direction
    + specified in 'rw'.
    + bdev_integrity_enabled() honors the write_generate and
    + read_verify flags in sysfs and will respond accordingly.
    + int bio_integrity_prep(bio);
    + To generate IMD for WRITE and to set up buffers for READ, the
    + filesystem must call bio_integrity_prep(bio).
    + Prior to calling this function, the bio data direction and start
    + sector must be set, and the bio should have all data pages
    + added. It is up to the caller to ensure that the bio does not
    + change while I/O is in progress.
    + bio_integrity_prep() should only be called if
    + bio_integrity_enabled() returned 1.
    + int bio_integrity_tag_size(bio);
    + If the filesystem wants to use the application tag space it will
    + first have to find out how much storage space is available.
    + Because tag space is generally limited (usually 2 bytes per
    + sector regardless of sector size), the integrity framework
    + supports interleaving the information between the sectors in an
    + I/O.
    + Filesystems can call bio_integrity_tag_size(bio) to find out how
    + many bytes of storage are available for that particular bio.
    + Another option is bdev_get_tag_size(block_device) which will
    + return the number of available bytes per hardware sector.
    + int bio_integrity_set_tag(bio, void *tag_buf, len);
    + After a successful return from bio_integrity_prep(),
    + bio_integrity_set_tag() can be used to attach an opaque tag
    + buffer to a bio. Obviously this only makes sense if the I/O is
    + a WRITE.
    + int bio_integrity_get_tag(bio, void *tag_buf, len);
    + Similarly, at READ I/O completion time the filesystem can
    + retrieve the tag buffer using bio_integrity_get_tag().
    + Filesystems that either generate their own integrity metadata or
    + are capable of transferring IMD from user space can use the
    + following calls:
    + struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);
    + Allocates the bio integrity payload and hangs it off of the bio.
    + nr_pages indicate how many pages of protection data need to be
    + stored in the integrity bio_vec list (similar to bio_alloc()).
    + The integrity payload will be freed at bio_free() time.
    + int bio_integrity_add_page(bio, page, len, offset);
    + Attaches a page containing integrity metadata to an existing
    + bio. The bio must have an existing bip,
    + i.e. bio_integrity_alloc() must have been called. For a WRITE,
    + the integrity metadata in the pages must be in a format
    + understood by the target device with the notable exception that
    + the sector numbers will be remapped as the request traverses the
    + I/O stack. This implies that the pages added using this call
    + will be modified during I/O! The first reference tag in the
    + integrity metadata must have a value of bip->bip_sector.
    + Pages can be added using bio_integrity_add_page() as long as
    + there is room in the bip bio_vec array (nr_pages).
    + Upon completion of a READ operation, the attached pages will
    + contain the integrity metadata received from the storage device.
    + It is up to the receiver to process them and verify data
    + integrity upon completion.
    + To enable integrity exchange on a block device the gendisk must be
    + registered as capable:
    + int blk_integrity_register(gendisk, blk_integrity);
    + The blk_integrity struct is a template and should contain the
    + following:
    + static struct blk_integrity my_profile = {
    + .generate_fn = my_generate_fn,
    + .verify_fn = my_verify_fn,
    + .get_tag_fn = my_get_tag_fn,
    + .set_tag_fn = my_set_tag_fn,
    + .tuple_size = sizeof(struct my_tuple_size),
    + .tag_size = <tag bytes per hw sector>,
    + };
    + 'name' is a text string which will be visible in sysfs. This is
    + part of the userland API so chose it carefully and never change
    + it. The format is standards body-type-variant.
    + E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC.
    + 'generate_fn' generates appropriate integrity metadata (for WRITE).
    + 'verify_fn' verifies that the data buffer matches the integrity
    + metadata.
    + 'tuple_size' must be set to match the size of the integrity
    + metadata per sector. I.e. 8 for DIF and EPP.
    + 'tag_size' must be set to identify how many bytes of tag space
    + are available per hardware sector. For DIF this is either 2 or
    + 0 depending on the value of the Control Mode Page ATO bit.
    + See 6.2 for a description of get_tag_fn and set_tag_fn.
    +2007-12-24 Martin K. Petersen <>

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