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