Documentation/block/data-integrity.txt

   1 ----------------------------------------------------------------------
   2 1. INTRODUCTION
   3
   4 Modern filesystems feature checksumming of data and metadata to
   5 protect against data corruption.  However, the detection of the
   6 corruption is done at read time which could potentially be months
   7 after the data was written.  At that point the original data that the
   8 application tried to write is most likely lost.
   9
  10 The solution is to ensure that the disk is actually storing what the
  11 application meant it to.  Recent additions to both the SCSI family
  12 protocols (SBC Data Integrity Field, SCC protection proposal) as well
  13 as SATA/T13 (External Path Protection) try to remedy this by adding
  14 support for appending integrity metadata to an I/O.  The integrity
  15 metadata (or protection information in SCSI terminology) includes a
  16 checksum for each sector as well as an incrementing counter that
  17 ensures the individual sectors are written in the right order.  And
  18 for some protection schemes also that the I/O is written to the right
  19 place on disk.
  20
  21 Current storage controllers and devices implement various protective
  22 measures, for instance checksumming and scrubbing.  But these
  23 technologies are working in their own isolated domains or at best
  24 between adjacent nodes in the I/O path.  The interesting thing about
  25 DIF and the other integrity extensions is that the protection format
  26 is well defined and every node in the I/O path can verify the
  27 integrity of the I/O and reject it if corruption is detected.  This
  28 allows not only corruption prevention but also isolation of the point
  29 of failure.
  30
  31 ----------------------------------------------------------------------
  32 2. THE DATA INTEGRITY EXTENSIONS
  33
  34 As written, the protocol extensions only protect the path between
  35 controller and storage device.  However, many controllers actually
  36 allow the operating system to interact with the integrity metadata
  37 (IMD).  We have been working with several FC/SAS HBA vendors to enable
  38 the protection information to be transferred to and from their
  39 controllers.
  40
  41 The SCSI Data Integrity Field works by appending 8 bytes of protection
  42 information to each sector.  The data + integrity metadata is stored
  43 in 520 byte sectors on disk.  Data + IMD are interleaved when
  44 transferred between the controller and target.  The T13 proposal is
  45 similar.
  46
  47 Because it is highly inconvenient for operating systems to deal with
  48 520 (and 4104) byte sectors, we approached several HBA vendors and
  49 encouraged them to allow separation of the data and integrity metadata
  50 scatter-gather lists.
  51
  52 The controller will interleave the buffers on write and split them on
  53 read.  This means that Linux can DMA the data buffers to and from
  54 host memory without changes to the page cache.
  55
  56 Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs
  57 is somewhat heavy to compute in software.  Benchmarks found that
  58 calculating this checksum had a significant impact on system
  59 performance for a number of workloads.  Some controllers allow a
  60 lighter-weight checksum to be used when interfacing with the operating
  61 system.  Emulex, for instance, supports the TCP/IP checksum instead.
  62 The IP checksum received from the OS is converted to the 16-bit CRC
  63 when writing and vice versa.  This allows the integrity metadata to be
  64 generated by Linux or the application at very low cost (comparable to
  65 software RAID5).
  66
  67 The IP checksum is weaker than the CRC in terms of detecting bit
  68 errors.  However, the strength is really in the separation of the data
  69 buffers and the integrity metadata.  These two distinct buffers must
  70 match up for an I/O to complete.
  71
  72 The separation of the data and integrity metadata buffers as well as
  73 the choice in checksums is referred to as the Data Integrity
  74 Extensions.  As these extensions are outside the scope of the protocol
  75 bodies (T10, T13), Oracle and its partners are trying to standardize
  76 them within the Storage Networking Industry Association.
  77
  78 ----------------------------------------------------------------------
  79 3. KERNEL CHANGES
  80
  81 The data integrity framework in Linux enables protection information
  82 to be pinned to I/Os and sent to/received from controllers that
  83 support it.
  84
  85 The advantage to the integrity extensions in SCSI and SATA is that
  86 they enable us to protect the entire path from application to storage
  87 device.  However, at the same time this is also the biggest
  88 disadvantage. It means that the protection information must be in a
  89 format that can be understood by the disk.
  90
  91 Generally Linux/POSIX applications are agnostic to the intricacies of
  92 the storage devices they are accessing.  The virtual filesystem switch
  93 and the block layer make things like hardware sector size and
  94 transport protocols completely transparent to the application.
  95
  96 However, this level of detail is required when preparing the
  97 protection information to send to a disk.  Consequently, the very
  98 concept of an end-to-end protection scheme is a layering violation.
  99 It is completely unreasonable for an application to be aware whether
 100 it is accessing a SCSI or SATA disk.
 101
 102 The data integrity support implemented in Linux attempts to hide this
 103 from the application.  As far as the application (and to some extent
 104 the kernel) is concerned, the integrity metadata is opaque information
 105 that's attached to the I/O.
 106
 107 The current implementation allows the block layer to automatically
 108 generate the protection information for any I/O.  Eventually the
 109 intent is to move the integrity metadata calculation to userspace for
 110 user data.  Metadata and other I/O that originates within the kernel
 111 will still use the automatic generation interface.
 112
 113 Some storage devices allow each hardware sector to be tagged with a
 114 16-bit value.  The owner of this tag space is the owner of the block
 115 device.  I.e. the filesystem in most cases.  The filesystem can use
 116 this extra space to tag sectors as they see fit.  Because the tag
 117 space is limited, the block interface allows tagging bigger chunks by
 118 way of interleaving.  This way, 8*16 bits of information can be
 119 attached to a typical 4KB filesystem block.
 120
 121 This also means that applications such as fsck and mkfs will need
 122 access to manipulate the tags from user space.  A passthrough
 123 interface for this is being worked on.
 124
 125
 126 ----------------------------------------------------------------------
 127 4. BLOCK LAYER IMPLEMENTATION DETAILS
 128
 129 4.1 BIO
 130
 131 The data integrity patches add a new field to struct bio when
 132 CONFIG_BLK_DEV_INTEGRITY is enabled.  bio_integrity(bio) returns a
 133 pointer to a struct bip which contains the bio integrity payload.
 134 Essentially a bip is a trimmed down struct bio which holds a bio_vec
 135 containing the integrity metadata and the required housekeeping
 136 information (bvec pool, vector count, etc.)
 137
 138 A kernel subsystem can enable data integrity protection on a bio by
 139 calling bio_integrity_alloc(bio).  This will allocate and attach the
 140 bip to the bio.
 141
 142 Individual pages containing integrity metadata can subsequently be
 143 attached using bio_integrity_add_page().
 144
 145 bio_free() will automatically free the bip.
 146
 147
 148 4.2 BLOCK DEVICE
 149
 150 Because the format of the protection data is tied to the physical
 151 disk, each block device has been extended with a block integrity
 152 profile (struct blk_integrity).  This optional profile is registered
 153 with the block layer using blk_integrity_register().
 154
 155 The profile contains callback functions for generating and verifying
 156 the protection data, as well as getting and setting application tags.
 157 The profile also contains a few constants to aid in completing,
 158 merging and splitting the integrity metadata.
 159
 160 Layered block devices will need to pick a profile that's appropriate
 161 for all subdevices.  blk_integrity_compare() can help with that.  DM
 162 and MD linear, RAID0 and RAID1 are currently supported.  RAID4/5/6
 163 will require extra work due to the application tag.
 164
 165
 166 ----------------------------------------------------------------------
 167 5.0 BLOCK LAYER INTEGRITY API
 168
 169 5.1 NORMAL FILESYSTEM
 170
 171     The normal filesystem is unaware that the underlying block device
 172     is capable of sending/receiving integrity metadata.  The IMD will
 173     be automatically generated by the block layer at submit_bio() time
 174     in case of a WRITE.  A READ request will cause the I/O integrity
 175     to be verified upon completion.
 176
 177     IMD generation and verification can be toggled using the
 178
 179       /sys/block/<bdev>/integrity/write_generate
 180
 181     and
 182
 183       /sys/block/<bdev>/integrity/read_verify
 184
 185     flags.
 186
 187
 188 5.2 INTEGRITY-AWARE FILESYSTEM
 189
 190     A filesystem that is integrity-aware can prepare I/Os with IMD
 191     attached.  It can also use the application tag space if this is
 192     supported by the block device.
 193
 194
 195     int bio_integrity_prep(bio);
 196
 197       To generate IMD for WRITE and to set up buffers for READ, the
 198       filesystem must call bio_integrity_prep(bio).
 199
 200       Prior to calling this function, the bio data direction and start
 201       sector must be set, and the bio should have all data pages
 202       added.  It is up to the caller to ensure that the bio does not
 203       change while I/O is in progress.
 204
 205       bio_integrity_prep() should only be called if
 206       bio_integrity_enabled() returned 1.
 207
 208
 209     int bio_integrity_tag_size(bio);
 210
 211       If the filesystem wants to use the application tag space it will
 212       first have to find out how much storage space is available.
 213       Because tag space is generally limited (usually 2 bytes per
 214       sector regardless of sector size), the integrity framework
 215       supports interleaving the information between the sectors in an
 216       I/O.
 217
 218       Filesystems can call bio_integrity_tag_size(bio) to find out how
 219       many bytes of storage are available for that particular bio.
 220
 221       Another option is bdev_get_tag_size(block_device) which will
 222       return the number of available bytes per hardware sector.
 223
 224
 225     int bio_integrity_set_tag(bio, void *tag_buf, len);
 226
 227       After a successful return from bio_integrity_prep(),
 228       bio_integrity_set_tag() can be used to attach an opaque tag
 229       buffer to a bio.  Obviously this only makes sense if the I/O is
 230       a WRITE.
 231
 232
 233     int bio_integrity_get_tag(bio, void *tag_buf, len);
 234
 235       Similarly, at READ I/O completion time the filesystem can
 236       retrieve the tag buffer using bio_integrity_get_tag().
 237
 238
 239 5.3 PASSING EXISTING INTEGRITY METADATA
 240
 241     Filesystems that either generate their own integrity metadata or
 242     are capable of transferring IMD from user space can use the
 243     following calls:
 244
 245
 246     struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);
 247
 248       Allocates the bio integrity payload and hangs it off of the bio.
 249       nr_pages indicate how many pages of protection data need to be
 250       stored in the integrity bio_vec list (similar to bio_alloc()).
 251
 252       The integrity payload will be freed at bio_free() time.
 253
 254
 255     int bio_integrity_add_page(bio, page, len, offset);
 256
 257       Attaches a page containing integrity metadata to an existing
 258       bio.  The bio must have an existing bip,
 259       i.e. bio_integrity_alloc() must have been called.  For a WRITE,
 260       the integrity metadata in the pages must be in a format
 261       understood by the target device with the notable exception that
 262       the sector numbers will be remapped as the request traverses the
 263       I/O stack.  This implies that the pages added using this call
 264       will be modified during I/O!  The first reference tag in the
 265       integrity metadata must have a value of bip->bip_sector.
 266
 267       Pages can be added using bio_integrity_add_page() as long as
 268       there is room in the bip bio_vec array (nr_pages).
 269
 270       Upon completion of a READ operation, the attached pages will
 271       contain the integrity metadata received from the storage device.
 272       It is up to the receiver to process them and verify data
 273       integrity upon completion.
 274
 275
 276 5.4 REGISTERING A BLOCK DEVICE AS CAPABLE OF EXCHANGING INTEGRITY
 277     METADATA
 278
 279     To enable integrity exchange on a block device the gendisk must be
 280     registered as capable:
 281
 282     int blk_integrity_register(gendisk, blk_integrity);
 283
 284       The blk_integrity struct is a template and should contain the
 285       following:
 286
 287         static struct blk_integrity my_profile = {
 288             .name                   = "STANDARDSBODY-TYPE-VARIANT-CSUM",
 289             .generate_fn            = my_generate_fn,
 290             .verify_fn              = my_verify_fn,
 291             .get_tag_fn             = my_get_tag_fn,
 292             .set_tag_fn             = my_set_tag_fn,
 293             .tuple_size             = sizeof(struct my_tuple_size),
 294             .tag_size               = <tag bytes per hw sector>,
 295         };
 296
 297       'name' is a text string which will be visible in sysfs.  This is
 298       part of the userland API so chose it carefully and never change
 299       it.  The format is standards body-type-variant.
 300       E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC.
 301
 302       'generate_fn' generates appropriate integrity metadata (for WRITE).
 303
 304       'verify_fn' verifies that the data buffer matches the integrity
 305       metadata.
 306
 307       'tuple_size' must be set to match the size of the integrity
 308       metadata per sector.  I.e. 8 for DIF and EPP.
 309
 310       'tag_size' must be set to identify how many bytes of tag space
 311       are available per hardware sector.  For DIF this is either 2 or
 312       0 depending on the value of the Control Mode Page ATO bit.
 313
 314       See 6.2 for a description of get_tag_fn and set_tag_fn.
 315
 316 ----------------------------------------------------------------------
 317 2007-12-24 Martin K. Petersen <martin.petersen@oracle.com>