1 ============== 2 Data Integrity 3 ============== 4 5 1. Introduction 6 =============== 7 8 Modern filesystems feature checksumming of data and metadata to 9 protect against data corruption. However, the detection of the 10 corruption is done at read time which could potentially be months 11 after the data was written. At that point the original data that the 12 application tried to write is most likely lost. 13 14 The solution is to ensure that the disk is actually storing what the 15 application meant it to. Recent additions to both the SCSI family 16 protocols (SBC Data Integrity Field, SCC protection proposal) as well 17 as SATA/T13 (External Path Protection) try to remedy this by adding 18 support for appending integrity metadata to an I/O. The integrity 19 metadata (or protection information in SCSI terminology) includes a 20 checksum for each sector as well as an incrementing counter that 21 ensures the individual sectors are written in the right order. And 22 for some protection schemes also that the I/O is written to the right 23 place on disk. 24 25 Current storage controllers and devices implement various protective 26 measures, for instance checksumming and scrubbing. But these 27 technologies are working in their own isolated domains or at best 28 between adjacent nodes in the I/O path. The interesting thing about 29 DIF and the other integrity extensions is that the protection format 30 is well defined and every node in the I/O path can verify the 31 integrity of the I/O and reject it if corruption is detected. This 32 allows not only corruption prevention but also isolation of the point 33 of failure. 34 35 2. The Data Integrity Extensions 36 ================================ 37 38 As written, the protocol extensions only protect the path between 39 controller and storage device. However, many controllers actually 40 allow the operating system to interact with the integrity metadata 41 (IMD). We have been working with several FC/SAS HBA vendors to enable 42 the protection information to be transferred to and from their 43 controllers. 44 45 The SCSI Data Integrity Field works by appending 8 bytes of protection 46 information to each sector. The data + integrity metadata is stored 47 in 520 byte sectors on disk. Data + IMD are interleaved when 48 transferred between the controller and target. The T13 proposal is 49 similar. 50 51 Because it is highly inconvenient for operating systems to deal with 52 520 (and 4104) byte sectors, we approached several HBA vendors and 53 encouraged them to allow separation of the data and integrity metadata 54 scatter-gather lists. 55 56 The controller will interleave the buffers on write and split them on 57 read. This means that Linux can DMA the data buffers to and from 58 host memory without changes to the page cache. 59 60 Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs 61 is somewhat heavy to compute in software. Benchmarks found that 62 calculating this checksum had a significant impact on system 63 performance for a number of workloads. Some controllers allow a 64 lighter-weight checksum to be used when interfacing with the operating 65 system. Emulex, for instance, supports the TCP/IP checksum instead. 66 The IP checksum received from the OS is converted to the 16-bit CRC 67 when writing and vice versa. This allows the integrity metadata to be 68 generated by Linux or the application at very low cost (comparable to 69 software RAID5). 70 71 The IP checksum is weaker than the CRC in terms of detecting bit 72 errors. However, the strength is really in the separation of the data 73 buffers and the integrity metadata. These two distinct buffers must 74 match up for an I/O to complete. 75 76 The separation of the data and integrity metadata buffers as well as 77 the choice in checksums is referred to as the Data Integrity 78 Extensions. As these extensions are outside the scope of the protocol 79 bodies (T10, T13), Oracle and its partners are trying to standardize 80 them within the Storage Networking Industry Association. 81 82 3. Kernel Changes 83 ================= 84 85 The data integrity framework in Linux enables protection information 86 to be pinned to I/Os and sent to/received from controllers that 87 support it. 88 89 The advantage to the integrity extensions in SCSI and SATA is that 90 they enable us to protect the entire path from application to storage 91 device. However, at the same time this is also the biggest 92 disadvantage. It means that the protection information must be in a 93 format that can be understood by the disk. 94 95 Generally Linux/POSIX applications are agnostic to the intricacies of 96 the storage devices they are accessing. The virtual filesystem switch 97 and the block layer make things like hardware sector size and 98 transport protocols completely transparent to the application. 99 100 However, this level of detail is required when preparing the 101 protection information to send to a disk. Consequently, the very 102 concept of an end-to-end protection scheme is a layering violation. 103 It is completely unreasonable for an application to be aware whether 104 it is accessing a SCSI or SATA disk. 105 106 The data integrity support implemented in Linux attempts to hide this 107 from the application. As far as the application (and to some extent 108 the kernel) is concerned, the integrity metadata is opaque information 109 that's attached to the I/O. 110 111 The current implementation allows the block layer to automatically 112 generate the protection information for any I/O. Eventually the 113 intent is to move the integrity metadata calculation to userspace for 114 user data. Metadata and other I/O that originates within the kernel 115 will still use the automatic generation interface. 116 117 Some storage devices allow each hardware sector to be tagged with a 118 16-bit value. The owner of this tag space is the owner of the block 119 device. I.e. the filesystem in most cases. The filesystem can use 120 this extra space to tag sectors as they see fit. Because the tag 121 space is limited, the block interface allows tagging bigger chunks by 122 way of interleaving. This way, 8*16 bits of information can be 123 attached to a typical 4KB filesystem block. 124 125 This also means that applications such as fsck and mkfs will need 126 access to manipulate the tags from user space. A passthrough 127 interface for this is being worked on. 128 129 130 4. Block Layer Implementation Details 131 ===================================== 132 133 4.1 Bio 134 ------- 135 136 The data integrity patches add a new field to struct bio when 137 CONFIG_BLK_DEV_INTEGRITY is enabled. bio_integrity(bio) returns a 138 pointer to a struct bip which contains the bio integrity payload. 139 Essentially a bip is a trimmed down struct bio which holds a bio_vec 140 containing the integrity metadata and the required housekeeping 141 information (bvec pool, vector count, etc.) 142 143 A kernel subsystem can enable data integrity protection on a bio by 144 calling bio_integrity_alloc(bio). This will allocate and attach the 145 bip to the bio. 146 147 Individual pages containing integrity metadata can subsequently be 148 attached using bio_integrity_add_page(). 149 150 bio_free() will automatically free the bip. 151 152 153 4.2 Block Device 154 ---------------- 155 156 Block devices can set up the integrity information in the integrity 157 sub-struture of the queue_limits structure. 158 159 Layered block devices will need to pick a profile that's appropriate 160 for all subdevices. queue_limits_stack_integrity() can help with that. DM 161 and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6 162 will require extra work due to the application tag. 163 164 165 5.0 Block Layer Integrity API 166 ============================= 167 168 5.1 Normal Filesystem 169 --------------------- 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 ------------------------------ 190 191 A filesystem that is integrity-aware can prepare I/Os with IMD 192 attached. It can also use the application tag space if this is 193 supported by the block device. 194 195 196 `bool bio_integrity_prep(bio);` 197 198 To generate IMD for WRITE and to set up buffers for READ, the 199 filesystem must call bio_integrity_prep(bio). 200 201 Prior to calling this function, the bio data direction and start 202 sector must be set, and the bio should have all data pages 203 added. It is up to the caller to ensure that the bio does not 204 change while I/O is in progress. 205 Complete bio with error if prepare failed for some reason. 206 207 208 5.3 Passing Existing Integrity Metadata 209 --------------------------------------- 210 211 Filesystems that either generate their own integrity metadata or 212 are capable of transferring IMD from user space can use the 213 following calls: 214 215 216 `struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);` 217 218 Allocates the bio integrity payload and hangs it off of the bio. 219 nr_pages indicate how many pages of protection data need to be 220 stored in the integrity bio_vec list (similar to bio_alloc()). 221 222 The integrity payload will be freed at bio_free() time. 223 224 225 `int bio_integrity_add_page(bio, page, len, offset);` 226 227 Attaches a page containing integrity metadata to an existing 228 bio. The bio must have an existing bip, 229 i.e. bio_integrity_alloc() must have been called. For a WRITE, 230 the integrity metadata in the pages must be in a format 231 understood by the target device with the notable exception that 232 the sector numbers will be remapped as the request traverses the 233 I/O stack. This implies that the pages added using this call 234 will be modified during I/O! The first reference tag in the 235 integrity metadata must have a value of bip->bip_sector. 236 237 Pages can be added using bio_integrity_add_page() as long as 238 there is room in the bip bio_vec array (nr_pages). 239 240 Upon completion of a READ operation, the attached pages will 241 contain the integrity metadata received from the storage device. 242 It is up to the receiver to process them and verify data 243 integrity upon completion. 244 245 246 ---------------------------------------------------------------------- 247 248 2007-12-24 Martin K. Petersen <martin.petersen@oracle.com>
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