/share/doc/papers/bufbio/bio.ms
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1.\" ---------------------------------------------------------------------------- 2.\" "THE BEER-WARE LICENSE" (Revision 42): 3.\" <phk@FreeBSD.ORG> wrote this file. As long as you retain this notice you 4.\" can do whatever you want with this stuff. If we meet some day, and you think 5.\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp 6.\" ---------------------------------------------------------------------------- 7.\" 8.\" $FreeBSD$ 9.\" 10.if n .ftr C R 11.nr PI 2n 12.TL 13The case for struct bio 14.br 15- or - 16.br 17A road map for a stackable BIO subsystem in FreeBSD 18.AU 19Poul-Henning Kamp <phk@FreeBSD.org> 20.AI 21The FreeBSD Project 22.AB 23Historically, the only translation performed on I/O requests after 24they they left the file-system layer were logical sub disk implementation 25done in the device driver. No universal standard for how sub disks are 26configured and implemented exists, in fact pretty much every single platform 27and operating system have done it their own way. As FreeBSD migrates to 28other platforms it needs to understand these local conventions to be 29able to co-exist with other operating systems on the same disk. 30.PP 31Recently a number of technologies like RAID have expanded the 32concept of "a disk" a fair bit and while these technologies initially 33were implemented in separate hardware they increasingly migrate into 34the operating systems as standard functionality. 35.PP 36Both of these factors indicate the need for a structured approach to 37systematic "geometry manipulation" facilities in FreeBSD. 38.PP 39This paper contains the road-map for a stackable "BIO" system in 40FreeBSD, which will support these facilities. 41.AE 42.NH 43The miseducation of \fCstruct buf\fP. 44.PP 45To fully appreciate the topic, I include a little historic overview 46of struct buf, it is a most enlightening case of not exactly bit-rot 47but more appropriately design-rot. 48.PP 49In the beginning, which for this purpose extends until virtual 50memory is was introduced into UNIX, all disk I/O were done from or 51to a struct buf. In the 6th edition sources, as printed in Lions 52Book, struct buf looks like this: 53.DS 54.ft C 55.ps -1 56struct buf 57{ 58 int b_flags; /* see defines below */ 59 struct buf *b_forw; /* headed by devtab of b_dev */ 60 struct buf *b_back; /* ' */ 61 struct buf *av_forw; /* position on free list, */ 62 struct buf *av_back; /* if not BUSY*/ 63 int b_dev; /* major+minor device name */ 64 int b_wcount; /* transfer count (usu. words) */ 65 char *b_addr; /* low order core address */ 66 char *b_xmem; /* high order core address */ 67 char *b_blkno; /* block # on device */ 68 char b_error; /* returned after I/O */ 69 char *b_resid; /* words not transferred after 70 error */ 71} buf[NBUF]; 72.ps +1 73.ft P 74.DE 75.PP 76At this point in time, struct buf had only two functions: 77To act as a cache 78and to transport I/O operations to device drivers. For the purpose of 79this document, the cache functionality is uninteresting and will be 80ignored. 81.PP 82The I/O operations functionality consists of three parts: 83.IP "" 5n 84\(bu Where in Ram/Core is the data located (b_addr, b_xmem, b_wcount). 85.IP 86\(bu Where on disk is the data located (b_dev, b_blkno) 87.IP 88\(bu Request and result information (b_flags, b_error, b_resid) 89.PP 90In addition to this, the av_forw and av_back elements are 91used by the disk device drivers to put requests on a linked list. 92All in all the majority of struct buf is involved with the I/O 93aspect and only a few fields relate exclusively to the cache aspect. 94.PP 95If we step forward to the BSD 4.4-Lite-2 release, struct buf has grown 96a bit here or there: 97.DS 98.ft C 99.ps -1 100struct buf { 101 LIST_ENTRY(buf) b_hash; /* Hash chain. */ 102 LIST_ENTRY(buf) b_vnbufs; /* Buffer's associated vnode. */ 103 TAILQ_ENTRY(buf) b_freelist; /* Free list position if not active. */ 104 struct buf *b_actf, **b_actb; /* Device driver queue when active. */ 105 struct proc *b_proc; /* Associated proc; NULL if kernel. */ 106 volatile long b_flags; /* B_* flags. */ 107 int b_error; /* Errno value. */ 108 long b_bufsize; /* Allocated buffer size. */ 109 long b_bcount; /* Valid bytes in buffer. */ 110 long b_resid; /* Remaining I/O. */ 111 dev_t b_dev; /* Device associated with buffer. */ 112 struct { 113 caddr_t b_addr; /* Memory, superblocks, indirect etc. */ 114 } b_un; 115 void *b_saveaddr; /* Original b_addr for physio. */ 116 daddr_t b_lblkno; /* Logical block number. */ 117 daddr_t b_blkno; /* Underlying physical block number. */ 118 /* Function to call upon completion. */ 119 void (*b_iodone) __P((struct buf *)); 120 struct vnode *b_vp; /* Device vnode. */ 121 long b_pfcent; /* Center page when swapping cluster. */ 122 /* XXX pfcent should be int; overld. */ 123 int b_dirtyoff; /* Offset in buffer of dirty region. */ 124 int b_dirtyend; /* Offset of end of dirty region. */ 125 struct ucred *b_rcred; /* Read credentials reference. */ 126 struct ucred *b_wcred; /* Write credentials reference. */ 127 int b_validoff; /* Offset in buffer of valid region. */ 128 int b_validend; /* Offset of end of valid region. */ 129}; 130.ps +1 131.ft P 132.DE 133.PP 134The main piece of action is the addition of vnodes, a VM system and a 135prototype LFS filesystem, all of which needed some handles on struct 136buf. Comparison will show that the I/O aspect of struct buf is in 137essence unchanged, the length field is now in bytes instead of words, 138the linked list the drivers can use has been renamed (b_actf, 139b_actb) and a b_iodone pointer for callback notification has been added 140but otherwise there is no change to the fields which 141represent the I/O aspect. All the new fields relate to the cache 142aspect, link buffers to the VM system, provide hacks for file-systems 143(b_lblkno) etc etc. 144.PP 145By the time we get to FreeBSD 3.0 more stuff has grown on struct buf: 146.DS 147.ft C 148.ps -1 149struct buf { 150 LIST_ENTRY(buf) b_hash; /* Hash chain. */ 151 LIST_ENTRY(buf) b_vnbufs; /* Buffer's associated vnode. */ 152 TAILQ_ENTRY(buf) b_freelist; /* Free list position if not active. */ 153 TAILQ_ENTRY(buf) b_act; /* Device driver queue when active. *new* */ 154 struct proc *b_proc; /* Associated proc; NULL if kernel. */ 155 long b_flags; /* B_* flags. */ 156 unsigned short b_qindex; /* buffer queue index */ 157 unsigned char b_usecount; /* buffer use count */ 158 int b_error; /* Errno value. */ 159 long b_bufsize; /* Allocated buffer size. */ 160 long b_bcount; /* Valid bytes in buffer. */ 161 long b_resid; /* Remaining I/O. */ 162 dev_t b_dev; /* Device associated with buffer. */ 163 caddr_t b_data; /* Memory, superblocks, indirect etc. */ 164 caddr_t b_kvabase; /* base kva for buffer */ 165 int b_kvasize; /* size of kva for buffer */ 166 daddr_t b_lblkno; /* Logical block number. */ 167 daddr_t b_blkno; /* Underlying physical block number. */ 168 off_t b_offset; /* Offset into file */ 169 /* Function to call upon completion. */ 170 void (*b_iodone) __P((struct buf *)); 171 /* For nested b_iodone's. */ 172 struct iodone_chain *b_iodone_chain; 173 struct vnode *b_vp; /* Device vnode. */ 174 int b_dirtyoff; /* Offset in buffer of dirty region. */ 175 int b_dirtyend; /* Offset of end of dirty region. */ 176 struct ucred *b_rcred; /* Read credentials reference. */ 177 struct ucred *b_wcred; /* Write credentials reference. */ 178 int b_validoff; /* Offset in buffer of valid region. */ 179 int b_validend; /* Offset of end of valid region. */ 180 daddr_t b_pblkno; /* physical block number */ 181 void *b_saveaddr; /* Original b_addr for physio. */ 182 caddr_t b_savekva; /* saved kva for transfer while bouncing */ 183 void *b_driver1; /* for private use by the driver */ 184 void *b_driver2; /* for private use by the driver */ 185 void *b_spc; 186 union cluster_info { 187 TAILQ_HEAD(cluster_list_head, buf) cluster_head; 188 TAILQ_ENTRY(buf) cluster_entry; 189 } b_cluster; 190 struct vm_page *b_pages[btoc(MAXPHYS)]; 191 int b_npages; 192 struct workhead b_dep; /* List of filesystem dependencies. */ 193}; 194.ps +1 195.ft P 196.DE 197.PP 198Still we find that the I/O aspect of struct buf is in essence unchanged. A couple of fields have been added which allows the driver to hang local data off the buf while working on it have been added (b_driver1, b_driver2) and a "physical block number" (b_pblkno) have been added. 199.PP 200This p_blkno is relevant, it has been added because the disklabel/slice 201code have been abstracted out of the device drivers, the filesystem 202ask for b_blkno, the slice/label code translates this into b_pblkno 203which the device driver operates on. 204.PP 205After this point some minor cleanups have happened, some unused fields 206have been removed etc but the I/O aspect of struct buf is still only 207a fraction of the entire structure: less than a quarter of the 208bytes in a struct buf are used for the I/O aspect and struct buf 209seems to continue to grow and grow. 210.PP 211Since version 6 as documented in Lions book, a three significant pieces 212of code have emerged which need to do non-trivial translations of 213the I/O request before it reaches the device drivers: CCD, slice/label 214and Vinum. They all basically do the same: they map I/O requests from 215a logical space to a physical space, and the mappings they perform 216can be 1:1 or 1:N. \** 217.FS 218It is interesting to note that Lions in his comments to the \fCrkaddr\fP 219routine (p. 16-2) writes \fIThe code in this procedure incorporates 220a special feature for files which extend over more than one disk 221drive. This feature is described in the UPM Section "RK(IV)". Its 222usefulness seems to be restricted.\fP This more than hints at the 223presence already then of various hacks to stripe/span multiple devices. 224.FE 225.PP 226The 1:1 mapping of the slice/label code is rather trivial, and the 227addition of the b_pblkno field catered for the majority of the issues 228this resulted in, leaving but one: Reads or writes to the magic "disklabel" 229or equally magic "MBR" sectors on a disk must be caught, examined and in 230some cases modified before being passed on to the device driver. This need 231resulted in the addition of the b_iodone_chain field which adds a limited 232ability to stack I/O operations; 233.PP 234The 1:N mapping of CCD and Vinum are far more interesting. These two 235subsystems look like a device driver, but rather than drive some piece 236of hardware, they allocate new struct buf data structures populates 237these and pass them on to other device drivers. 238.PP 239Apart from it being inefficient to lug about a 348 bytes data structure 240when 80 bytes would have done, it also leads to significant code rot 241when programmers don't know what to do about the remaining fields or 242even worse: "borrow" a field or two for their own uses. 243.PP 244.ID 245.if t .PSPIC bufsize.eps 246.if n [graph not available in this format] 247.DE 248.I 249Conclusions: 250.IP "" 5n 251\(bu Struct buf is victim of chronic bloat. 252.IP 253\(bu The I/O aspect of 254struct buf is practically constant and only about \(14 of the total bytes. 255.IP 256\(bu Struct buf currently have several users, vinum, ccd and to 257limited extent diskslice/label, which 258need only the I/O aspect, not the vnode, caching or VM linkage. 259.IP 260.I 261The I/O aspect of struct buf should be put in a separate \fCstruct bio\fP. 262.R 263.NH 1 264Implications for future struct buf improvements 265.PP 266Concerns have been raised about the implications this separation 267will have for future work on struct buf, I will try to address 268these concerns here. 269.PP 270As the existence and popularity of vinum and ccd proves, there is 271a legitimate and valid requirement to be able to do I/O operations 272which are not initiated by a vnode or filesystem operation. 273In other words, an I/O request is a fully valid entity in its own 274right and should be treated like that. 275.PP 276Without doubt, the I/O request has to be tuned to fit the needs 277of struct buf users in the best possible way, and consequently 278any future changes in struct buf are likely to affect the I/O request 279semantics. 280.PP 281One particular change which has been proposed is to drop the present 282requirement that a struct buf be mapped contiguously into kernel 283address space. The argument goes that since many modern drivers use 284physical address DMA to transfer the data maintaining such a mapping 285is needless overhead. 286.PP 287Of course some drivers will still need to be able to access the 288buffer in kernel address space and some kind of compatibility 289must be provided there. 290.PP 291The question is, if such a change is made impossible by the 292separation of the I/O aspect into its own data structure? 293.PP 294The answer to this is ``no''. 295Anything that could be added to or done with 296the I/O aspect of struct buf can also be added to or done 297with the I/O aspect if it lives in a new "struct bio". 298.NH 1 299Implementing a \fCstruct bio\fP 300.PP 301The first decision to be made was who got to use the name "struct buf", 302and considering the fact that it is the I/O aspect which gets separated 303out and that it only covers about \(14 of the bytes in struct buf, 304obviously the new structure for the I/O aspect gets a new name. 305Examining the naming in the kernel, the "bio" prefix seemed a given, 306for instance, the function to signal completion of an I/O request is 307already named "biodone()". 308.PP 309Making the transition smooth is obviously also a priority and after 310some prototyping \** 311.FS 312The software development technique previously known as "Trial & Error". 313.FE 314it was found that a totally transparent transition could be made by 315embedding a copy of the new "struct bio" as the first element of "struct buf" 316and by using cpp(1) macros to alias the fields to the legacy struct buf 317names. 318.NH 2 319The b_flags problem. 320.PP 321Struct bio was defined by examining all code existing in the driver tree 322and finding all the struct buf fields which were legitimately used (as 323opposed to "hi-jacked" fields). 324One field was found to have "dual-use": the b_flags field. 325This required special attention. 326Examination showed that b_flags were used for three things: 327.IP "" 5n 328\(bu Communication of the I/O command (READ, WRITE, FORMAT, DELETE) 329.IP 330\(bu Communication of ordering and error status 331.IP 332\(bu General status for non I/O aspect consumers of struct buf. 333.PP 334For historic reasons B_WRITE was defined to be zero, which lead to 335confusion and bugs, this pushed the decision to have a separate 336"b_iocmd" field in struct buf and struct bio for communicating 337only the action to be performed. 338.PP 339The ordering and error status bits were put in a new flag field "b_ioflag". 340This has left sufficiently many now unused bits in b_flags that the b_xflags element 341can now be merged back into b_flags. 342.NH 2 343Definition of struct bio 344.PP 345With the cleanup of b_flags in place, the definition of struct bio looks like this: 346.DS 347.ft C 348.ps -1 349struct bio { 350 u_int bio_cmd; /* I/O operation. */ 351 dev_t bio_dev; /* Device to do I/O on. */ 352 daddr_t bio_blkno; /* Underlying physical block number. */ 353 off_t bio_offset; /* Offset into file. */ 354 long bio_bcount; /* Valid bytes in buffer. */ 355 caddr_t bio_data; /* Memory, superblocks, indirect etc. */ 356 u_int bio_flags; /* BIO_ flags. */ 357 struct buf *_bio_buf; /* Parent buffer. */ 358 int bio_error; /* Errno for BIO_ERROR. */ 359 long bio_resid; /* Remaining I/O in bytes. */ 360 void (*bio_done) __P((struct buf *)); 361 void *bio_driver1; /* Private use by the callee. */ 362 void *bio_driver2; /* Private use by the callee. */ 363 void *bio_caller1; /* Private use by the caller. */ 364 void *bio_caller2; /* Private use by the caller. */ 365 TAILQ_ENTRY(bio) bio_queue; /* Disksort queue. */ 366 daddr_t bio_pblkno; /* physical block number */ 367 struct iodone_chain *bio_done_chain; 368}; 369.ps +1 370.ft P 371.DE 372.NH 2 373Definition of struct buf 374.PP 375After adding a struct bio to struct buf and the fields aliased into it 376struct buf looks like this: 377.DS 378.ft C 379.ps -1 380struct buf { 381 /* XXX: b_io must be the first element of struct buf for now /phk */ 382 struct bio b_io; /* "Builtin" I/O request. */ 383#define b_bcount b_io.bio_bcount 384#define b_blkno b_io.bio_blkno 385#define b_caller1 b_io.bio_caller1 386#define b_caller2 b_io.bio_caller2 387#define b_data b_io.bio_data 388#define b_dev b_io.bio_dev 389#define b_driver1 b_io.bio_driver1 390#define b_driver2 b_io.bio_driver2 391#define b_error b_io.bio_error 392#define b_iocmd b_io.bio_cmd 393#define b_iodone b_io.bio_done 394#define b_iodone_chain b_io.bio_done_chain 395#define b_ioflags b_io.bio_flags 396#define b_offset b_io.bio_offset 397#define b_pblkno b_io.bio_pblkno 398#define b_resid b_io.bio_resid 399 LIST_ENTRY(buf) b_hash; /* Hash chain. */ 400 TAILQ_ENTRY(buf) b_vnbufs; /* Buffer's associated vnode. */ 401 TAILQ_ENTRY(buf) b_freelist; /* Free list position if not active. */ 402 TAILQ_ENTRY(buf) b_act; /* Device driver queue when active. *new* */ 403 long b_flags; /* B_* flags. */ 404 unsigned short b_qindex; /* buffer queue index */ 405 unsigned char b_xflags; /* extra flags */ 406[...] 407.ps +1 408.ft P 409.DE 410.PP 411Putting the struct bio as the first element in struct buf during a transition 412period allows a pointer to either to be cast to a pointer of the other, 413which means that certain pieces of code can be left un-converted with the 414use of a couple of casts while the remaining pieces of code are tested. 415The ccd and vinum modules have been left un-converted like this for now. 416.PP 417This is basically where FreeBSD-current stands today. 418.PP 419The next step is to substitute struct bio for struct buf in all the code 420which only care about the I/O aspect: device drivers, diskslice/label. 421The patch to do this is up for review. \** 422.FS 423And can be found at http://phk.freebsd.dk/misc 424.FE 425and consists mainly of systematic substitutions like these 426.DS 427.ft C 428s/struct buf/struct bio/ 429s/b_flags/bio_flags/ 430s/b_bcount/bio_bcount/ 431&c &c 432.ft P 433.DE 434.NH 2 435Future work 436.PP 437It can be successfully argued that the cpp(1) macros used for aliasing 438above are ugly and should be expanded in place. It would certainly 439be trivial to do so, but not by definition worthwhile. 440.PP 441Retaining the aliasing for the b_* and bio_* name-spaces this way 442leaves us with considerable flexibility in modifying the future 443interaction between the two. The DEV_STRATEGY() macro is the single 444point where a struct buf is turned into a struct bio and launched 445into the drivers to full-fill the I/O request and this provides us 446with a single isolated location for performing non-trivial translations. 447.PP 448As an example of this flexibility: It has been proposed to essentially 449drop the b_blkno field and use the b_offset field to communicate the 450on-disk location of the data. b_blkno is a 32bit offset of B_DEVSIZE 451(512) bytes sectors which allows us to address two terabytes worth 452of data. Using b_offset as a 64 bit byte-address would not only allow 453us to address 8 million times larger disks, it would also make it 454possible to accommodate disks which use non-power-of-two sector-size, 455Audio CD-ROMs for instance. 456.PP 457The above mentioned flexibility makes an implementation almost trivial: 458.IP "" 5n 459\(bu Add code to DEV_STRATEGY() to populate b_offset from b_blkno in the 460cases where it is not valid. Today it is only valid for a struct buf 461marked B_PHYS. 462.IP 463\(bu Change diskslice/label, ccd, vinum and device drivers to use b_offset 464instead of b_blkno. 465.IP 466\(bu Remove the bio_blkno field from struct bio, add it to struct buf as 467b_blkno and remove the cpp(1) macro which aliased it into struct bio. 468.PP 469Another possible transition could be to not have a "built-in" struct bio 470in struct buf. If for some reason struct bio grows fields of no relevance 471to struct buf it might be cheaper to remove struct bio from struct buf, 472un-alias the fields and have DEV_STRATEGY() allocate a struct bio and populate 473the relevant fields from struct buf. 474This would also be entirely transparent to both users of struct buf and 475struct bio as long as we retain the aliasing mechanism and DEV_STRATEGY(). 476.bp 477.NH 1 478Towards a stackable BIO subsystem. 479.PP 480Considering that we now have three distinct pieces of code living 481in the nowhere between DEV_STRATEGY() and the device drivers: 482diskslice/label, ccd and vinum, it is not unreasonable to start 483to look for a more structured and powerful API for these pieces 484of code. 485.PP 486In traditional UNIX semantics a "disk" is a one-dimensional array of 487512 byte sectors which can be read or written. Support for sectors 488of multiple of 512 bytes were implemented with a sort of "don't ask-don't tell" policy where system administrator would specify a larger minimum sector-size 489to the filesystem, and things would "just work", but no formal communication about the size of the smallest transfer possible were exchanged between the disk driver and the filesystem. 490.PP 491A truly generalised concept of a disk needs to be more flexible and more 492expressive. For instance, a user of a disk will want to know: 493.IP "" 5n 494\(bu What is the sector size. Sector-size these days may not be a power 495of two, for instance Audio CDs have 2352 byte "sectors". 496.IP 497\(bu How many sectors are there. 498.IP 499\(bu Is writing of sectors supported. 500.IP 501\(bu Is freeing of sectors supported. This is important for flash based 502devices where a wear-distribution software or hardware function uses 503the information about which sectors are actually in use to optimise the 504usage of the slow erase function to a minimum. 505.IP 506\(bu Is opening this device in a specific mode, (read-only or read-write) 507allowed. The VM system and the file-systems generally assume that nobody 508writes to "their storage" under their feet, and therefore opens which 509would make that possible should be rejected. 510.IP 511\(bu What is the "native" geometry of this device (Sectors/Heads/Cylinders). 512This is useful for staying compatible with badly designed on-disk formats 513from other operating systems. 514.PP 515Obviously, all of these properties are dynamic in the sense that in 516these days disks are removable devices, and they may therefore change 517at any time. While some devices like CD-ROMs can lock the media in 518place with a special command, this cannot be done for all devices, 519in particular it cannot be done with normal floppy disk drives. 520.PP 521If we adopt such a model for disk, retain the existing "strategy/biodone" model of I/O scheduling and decide to use a modular or stackable approach to 522geometry translations we find that nearly endless flexibility emerges: 523Mirroring, RAID, striping, interleaving, disk-labels and sub-disks, all of 524these techniques would get a common framework to operate in. 525.PP 526In practice of course, such a scheme must not complicate the use of or 527installation of FreeBSD. The code will have to act and react exactly 528like the current code but fortunately the current behaviour is not at 529all hard to emulate so implementation-wise this is a non-issue. 530.PP 531But lets look at some drawings to see what this means in practice. 532.PP 533Today the plumbing might look like this on a machine: 534.DS 535.PS 536 Ad0: box "disk (ad0)" 537 arrow up from Ad0.n 538 SL0: box "slice/label" 539 Ad1: box "disk (ad1)" with .w at Ad0.e + (.2,0) 540 arrow up from Ad1.n 541 SL1: box "slice/label" 542 Ad2: box "disk (ad2)" with .w at Ad1.e + (.2,0) 543 arrow up from Ad2.n 544 SL2: box "slice/label" 545 Ad3: box "disk (ad3)" with .w at Ad2.e + (.2,0) 546 arrow up from Ad3.n 547 SL3: box "slice/label" 548 DML: box dashed width 4i height .9i with .sw at SL0.sw + (-.2,-.2) 549 "Disk-mini-layer" with .n at DML.s + (0, .1) 550 551 V: box "vinum" at 1/2 <SL1.n, SL2.n> + (0,1.2) 552 553 A0A: arrow up from 1/4 <SL0.nw, SL0.ne> 554 A0B: arrow up from 2/4 <SL0.nw, SL0.ne> 555 A0E: arrow up from 3/4 <SL0.nw, SL0.ne> 556 A1C: arrow up from 2/4 <SL1.nw, SL1.ne> 557 arrow to 1/3 <V.sw, V.se> 558 A2C: arrow up from 2/4 <SL2.nw, SL2.ne> 559 arrow to 2/3 <V.sw, V.se> 560 A3A: arrow up from 1/4 <SL3.nw, SL3.ne> 561 A3E: arrow up from 2/4 <SL3.nw, SL3.ne> 562 A3F: arrow up from 3/4 <SL3.nw, SL3.ne> 563 564 "ad0s1a" with .s at A0A.n + (0, .1) 565 "ad0s1b" with .s at A0B.n + (0, .3) 566 "ad0s1e" with .s at A0E.n + (0, .5) 567 "ad1s1c" with .s at A1C.n + (0, .1) 568 "ad2s1c" with .s at A2C.n + (0, .1) 569 "ad3s4a" with .s at A3A.n + (0, .1) 570 "ad3s4e" with .s at A3E.n + (0, .3) 571 "ad3s4f" with .s at A3F.n + (0, .5) 572 573 V1: arrow up from 1/4 <V.nw, V.ne> 574 V2: arrow up from 2/4 <V.nw, V.ne> 575 V3: arrow up from 3/4 <V.nw, V.ne> 576 "V1" with .s at V1.n + (0, .1) 577 "V2" with .s at V2.n + (0, .1) 578 "V3" with .s at V3.n + (0, .1) 579 580.PE 581.DE 582.PP 583And while this drawing looks nice and clean, the code underneat isn't. 584With a stackable BIO implementation, the picture would look like this: 585.DS 586.PS 587 Ad0: box "disk (ad0)" 588 arrow up from Ad0.n 589 M0: box "MBR" 590 arrow up 591 B0: box "BSD" 592 593 A0A: arrow up from 1/4 <B0.nw, B0.ne> 594 A0B: arrow up from 2/4 <B0.nw, B0.ne> 595 A0E: arrow up from 3/4 <B0.nw, B0.ne> 596 597 Ad1: box "disk (ad1)" with .w at Ad0.e + (.2,0) 598 Ad2: box "disk (ad2)" with .w at Ad1.e + (.2,0) 599 Ad3: box "disk (ad3)" with .w at Ad2.e + (.2,0) 600 arrow up from Ad3.n 601 SL3: box "MBR" 602 arrow up 603 B3: box "BSD" 604 605 V: box "vinum" at 1/2 <Ad1.n, Ad2.n> + (0,.8) 606 arrow from Ad1.n to 1/3 <V.sw, V.se> 607 arrow from Ad2.n to 2/3 <V.sw, V.se> 608 609 A3A: arrow from 1/4 <B3.nw, B3.ne> 610 A3E: arrow from 2/4 <B3.nw, B3.ne> 611 A3F: arrow from 3/4 <B3.nw, B3.ne> 612 613 "ad0s1a" with .s at A0A.n + (0, .1) 614 "ad0s1b" with .s at A0B.n + (0, .3) 615 "ad0s1e" with .s at A0E.n + (0, .5) 616 "ad3s4a" with .s at A3A.n + (0, .1) 617 "ad3s4e" with .s at A3E.n + (0, .3) 618 "ad3s4f" with .s at A3F.n + (0, .5) 619 620 V1: arrow up from 1/4 <V.nw, V.ne> 621 V2: arrow up from 2/4 <V.nw, V.ne> 622 V3: arrow up from 3/4 <V.nw, V.ne> 623 "V1" with .s at V1.n + (0, .1) 624 "V2" with .s at V2.n + (0, .1) 625 "V3" with .s at V3.n + (0, .1) 626 627.PE 628.DE 629.PP 630The first thing we notice is that the disk mini-layer is gone, instead 631separate modules for the Microsoft style MBR and the BSD style disklabel 632are now stacked over the disk. We can also see that Vinum no longer 633needs to go though the BSD/MBR layers if it wants access to the entire 634physical disk, it can be stacked right over the disk. 635.PP 636Now, imagine that a ZIP drive is connected to the machine, and the 637user loads a ZIP disk in it. First the device driver notices the 638new disk and instantiates a new disk: 639.DS 640.PS 641 box "disk (da0)" 642.PE 643.DE 644.PP 645A number of the geometry modules have registered as "auto-discovering" 646and will be polled sequentially to see if any of them recognise what 647is on this disk. The MBR module finds a MBR in sector 0 and attach 648an instance of itself to the disk: 649.DS 650.PS 651 D: box "disk (da0)" 652 arrow up from D.n 653 M: box "MBR" 654 M1: arrow up from 1/3 <M.nw, M.ne> 655 M2: arrow up from 2/3 <M.nw, M.ne> 656.PE 657.DE 658.PP 659It finds two "slices" in the MBR and creates two new "disks" one for 660each of these. The polling of modules is repeated and this time the 661BSD label module recognises a FreeBSD label on one of the slices and 662attach itself: 663.DS 664.PS 665 D: box "disk (da0)" 666 arrow "O" up from D.n 667 M: box "MBR" 668 M1: line up .3i from 1/3 <M.nw, M.ne> 669 arrow "O" left 670 M2: arrow "O" up from 2/3 <M.nw, M.ne> 671 B: box "BSD" 672 B1: arrow "O" up from 1/4 <B.nw, B.ne> 673 B2: arrow "O" up from 2/4 <B.nw, B.ne> 674 B3: arrow "O" up from 3/4 <B.nw, B.ne> 675 676.PE 677.DE 678.PP 679The BSD module finds three partitions, creates them as disks and the 680polling is repeated for each of these. No modules recognise these 681and the process ends. In theory one could have a module recognise 682the UFS superblock and extract from there the path to mount the disk 683on, but this is probably better implemented in a general "device-daemon" 684in user-land. 685.PP 686On this last drawing I have marked with "O" the "disks" which can be 687accessed from user-land or kernel. The VM and file-systems generally 688prefer to have exclusive write access to the disk sectors they use, 689so we need to enforce this policy. Since we cannot know what transformation 690a particular module implements, we need to ask the modules if the open 691is OK, and they may need to ask their neighbours before they can answer. 692.PP 693We decide to mount a filesystem on one of the BSD partitions at the very top. 694The open request is passed to the BSD module, which finds that none of 695the other open partitions (there are none) overlap this one, so far no 696objections. It then passes the open to the MBR module, which goes through 697basically the same procedure finds no objections and pass the request to 698the disk driver, which since it was not previously open approves of the 699open. 700.PP 701Next we mount a filesystem on the next BSD partition. The 702BSD module again checks for overlapping open partitions and find none. 703This time however, it finds that it has already opened the "downstream" 704in R/W mode so it does not need to ask for permission for that again 705so the open is OK. 706.PP 707Next we mount a msdos filesystem on the other MBR slice. This is the 708same case, the MBR finds no overlapping open slices and has already 709opened "downstream" so the open is OK. 710.PP 711If we now try to open the other slice for writing, the one which has the 712BSD module attached already. The open is passed to the MBR module which 713notes that the device is already opened for writing by a module (the BSD 714module) and consequently the open is refused. 715.PP 716While this sounds complicated it actually took less than 200 lines of 717code to implement in a prototype implementation. 718.PP 719Now, the user ejects the ZIP disk. If the hardware can give a notification 720of intent to eject, a call-up from the driver can try to get devices synchronised 721and closed, this is pretty trivial. If the hardware just disappears like 722a unplugged parallel zip drive, a floppy disk or a PC-card, we have no 723choice but to dismantle the setup. The device driver sends a "gone" notification to the MBR module, which replicates this upwards to the mounted msdosfs 724and the BSD module. The msdosfs unmounts forcefully, invalidates any blocks 725in the buf/vm system and returns. The BSD module replicates the "gone" to 726the two mounted file-systems which in turn unmounts forcefully, invalidates 727blocks and return, after which the BSD module releases any resources held 728and returns, the MBR module releases any resources held and returns and all 729traces of the device have been removed. 730.PP 731Now, let us get a bit more complicated. We add another disk and mirror 732two of the MBR slices: 733.DS 734.PS 735 D0: box "disk (da0)" 736 737 arrow "O" up from D0.n 738 M0: box "MBR" 739 M01: line up .3i from 1/3 <M0.nw, M0.ne> 740 arrow "O" left 741 M02: arrow "O" up from 2/3 <M0.nw, M0.ne> 742 743 D1: box "disk (da1)" with .w at D0.e + (.2,0) 744 arrow "O" up from D1.n 745 M1: box "MBR" 746 M11: line up .3i from 1/3 <M1.nw, M1.ne> 747 line "O" left 748 M11a: arrow up .2i 749 750 I: box "Mirror" with .s at 1/2 <M02.n, M11a.n> 751 arrow "O" up 752 BB: box "BSD" 753 BB1: arrow "O" up from 1/4 <BB.nw, BB.ne> 754 BB2: arrow "O" up from 2/4 <BB.nw, BB.ne> 755 BB3: arrow "O" up from 3/4 <BB.nw, BB.ne> 756 757 M12: arrow "O" up from 2/3 <M1.nw, M1.ne> 758 B: box "BSD" 759 B1: arrow "O" up from 1/4 <B.nw, B.ne> 760 B2: arrow "O" up from 2/4 <B.nw, B.ne> 761 B3: arrow "O" up from 3/4 <B.nw, B.ne> 762.PE 763.DE 764.PP 765Now assuming that we lose disk da0, the notification goes up like before 766but the mirror module still has a valid mirror from disk da1, so it 767doesn't propagate the "gone" notification further up and the three 768file-systems mounted are not affected. 769.PP 770It is possible to modify the graph while in action, as long as the 771modules know that they will not affect any I/O in progress. This is 772very handy for moving things around. At any of the arrows we can 773insert a mirroring module, since it has a 1:1 mapping from input 774to output. Next we can add another copy to the mirror, give the 775mirror time to sync the two copies. Detach the first mirror copy 776and remove the mirror module. We have now in essence moved a partition 777from one disk to another transparently. 778.NH 1 779Getting stackable BIO layers from where we are today. 780.PP 781Most of the infrastructure is in place now to implement stackable 782BIO layers: 783.IP "" 5n 784\(bu The dev_t change gave us a public structure where 785information about devices can be put. This enabled us to get rid 786of all the NFOO limits on the number of instances of a particular 787driver/device, and significantly cleaned up the vnode aliasing for 788device vnodes. 789.IP 790\(bu The disk-mini-layer has 791taken the knowledge about diskslice/labels out of the 792majority of the disk-drivers, saving on average 100 lines of code per 793driver. 794.IP 795\(bu The struct bio/buf divorce is giving us an IO request of manageable 796size which can be modified without affecting all the filesystem and 797VM system users of struct buf. 798.PP 799The missing bits are: 800.IP "" 5n 801\(bu changes to struct bio to make it more 802stackable. This mostly relates to the handling of the biodone() 803event, something which will be transparent to all current users 804of struct buf/bio. 805.IP 806\(bu code to stich modules together and to pass events and notifications 807between them. 808.NH 1 809An Implementation plan for stackable BIO layers 810.PP 811My plan for implementation stackable BIO layers is to first complete 812the struct bio/buf divorce with the already mentioned patch. 813.PP 814The next step is to re-implement the monolithic disk-mini-layer so 815that it becomes the stackable BIO system. Vinum and CCD and all 816other consumers should not be unable to tell the difference between 817the current and the new disk-mini-layer. The new implementation 818will initially use a static stacking to remain compatible with the 819current behaviour. This will be the next logical checkpoint commit. 820.PP 821The next step is to make the stackable layers configurable, 822to provide the means to initialise the stacking and to subsequently 823change it. This will be the next logical checkpoint commit. 824.PP 825At this point new functionality can be added inside the stackable 826BIO system: CCD can be re-implemented as a mirror module and a stripe 827module. Vinum can be integrated either as one "macro-module" or 828as separate functions in separate modules. Also modules for other 829purposes can be added, sub-disk handling for Solaris, MacOS, etc 830etc. These modules can be committed one at a time.