--- /dev/null
+----------------------------------------------------------------------
+1. INTRODUCTION
+
+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.
+
+----------------------------------------------------------------------
+2. THE DATA INTEGRITY EXTENSIONS
+
+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
+controllers.
+
+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
+similar.
+
+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.
+
+----------------------------------------------------------------------
+3. KERNEL CHANGES
+
+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. BLOCK LAYER IMPLEMENTATION DETAILS
+
+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.
+
+
+4.2 BLOCK DEVICE
+
+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.
+
+
+----------------------------------------------------------------------
+5.0 BLOCK LAYER INTEGRITY API
+
+5.1 NORMAL FILESYSTEM
+
+ 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.
+
+
+5.2 INTEGRITY-AWARE FILESYSTEM
+
+ 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().
+
+
+6.3 PASSING EXISTING INTEGRITY METADATA
+
+ 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.
+
+
+6.4 REGISTERING A BLOCK DEVICE AS CAPABLE OF EXCHANGING INTEGRITY
+ METADATA
+
+ 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 = {
+ .name = "STANDARDSBODY-TYPE-VARIANT-CSUM",
+ .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 <martin.petersen@oracle.com>
projects / linux-2.6 / commitdiff