From 7a8917a57110c7b820bca437644d32cf841eb6a2 Mon Sep 17 00:00:00 2001 From: Poul-Henning Kamp Date: Sun, 30 Jul 2000 08:57:03 +0000 Subject: [PATCH] Add the bio/buf paper --- share/doc/papers/bufbio/Makefile | 9 + share/doc/papers/bufbio/bio.ms | 829 ++++++++++++++++++++++++++++ share/doc/papers/bufbio/bufsize.eps | 479 ++++++++++++++++ 3 files changed, 1317 insertions(+) create mode 100644 share/doc/papers/bufbio/Makefile create mode 100644 share/doc/papers/bufbio/bio.ms create mode 100644 share/doc/papers/bufbio/bufsize.eps diff --git a/share/doc/papers/bufbio/Makefile b/share/doc/papers/bufbio/Makefile new file mode 100644 index 000000000000..cb221404af61 --- /dev/null +++ b/share/doc/papers/bufbio/Makefile @@ -0,0 +1,9 @@ +# $FreeBSD$ + +USE_PIC=1 +VOLUME= papers +DOC= bio +SRCS= bio.ms +MACROS= -ms -U + +.include diff --git a/share/doc/papers/bufbio/bio.ms b/share/doc/papers/bufbio/bio.ms new file mode 100644 index 000000000000..0ec96ba5c1a2 --- /dev/null +++ b/share/doc/papers/bufbio/bio.ms @@ -0,0 +1,829 @@ +.\" ---------------------------------------------------------------------------- +.\" "THE BEER-WARE LICENSE" (Revision 42): +.\" wrote this file. As long as you retain this notice you +.\" can do whatever you want with this stuff. If we meet some day, and you think +.\" this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp +.\" ---------------------------------------------------------------------------- +.\" +.\" $FreeBSD$ +.\" +.nr PI 2n +.TL +The case for struct bio +.br +- or - +.br +A road map for a stackable BIO subsystem in FreeBSD +.AU +Poul-Henning Kamp +.AI +The FreeBSD Project +.AB +Historically, the only translation performed on I/O requests after +they they left the file-system layer were logical sub disk implementation +done in the device driver. No universal standard for how sub disks are +configured and implemented exists, in fact pretty much every single platform +and operating system have done it their own way. As FreeBSD migrates to +other platforms it needs to understand these local conventions to be +able to co-exist with other operating systems on the same disk. +.PP +Recently a number of technologies like RAID have expanded the +concept of "a disk" a fair bit and while these technologies initially +were implemented in separate hardware they increasingly migrate into +the operating systems as standard functionality. +.PP +Both of these factors indicate the need for a structured approach to +systematic "geometry manipulation" facilities in FreeBSD. +.PP +This paper contains the road-map for a stackable "BIO" system in +FreeBSD, which will support these facilities. +.AE +.NH +The miseducation of \fCstruct buf\fP. +.PP +To fully appreciate the topic, I include a little historic overview +of struct buf, it is a most enlightening case of not exactly bit-rot +but more appropriately design-rot. +.PP +In the beginning, which for this purpose extends until virtual +memory is was introduced into UNIX, all disk I/O were done from or +to a struct buf. In the 6th edition sources, as printed in Lions +Book, struct buf looks like this: +.DS +.ft C +.ps -1 +struct buf +{ + int b_flags; /* see defines below */ + struct buf *b_forw; /* headed by devtab of b_dev */ + struct buf *b_back; /* ' */ + struct buf *av_forw; /* position on free list, */ + struct buf *av_back; /* if not BUSY*/ + int b_dev; /* major+minor device name */ + int b_wcount; /* transfer count (usu. words) */ + char *b_addr; /* low order core address */ + char *b_xmem; /* high order core address */ + char *b_blkno; /* block # on device */ + char b_error; /* returned after I/O */ + char *b_resid; /* words not transferred after + error */ +} buf[NBUF]; +.ps +1 +.ft P +.DE +.PP +At this point in time, struct buf had only two functions: +To act as a cache +and to transport I/O operations to device drivers. For the purpose of +this document, the cache functionality is uninteresting and will be +ignored. +.PP +The I/O operations functionality consists of three parts: +.IP "" 5n +\(bu Where in Ram/Core is the data located (b_addr, b_xmem, b_wcount). +.IP +\(bu Where on disk is the data located (b_dev, b_blkno) +.IP +\(bu Request and result information (b_flags, b_error, b_resid) +.PP +In addition to this, the av_forw and av_back elements are +used by the disk device drivers to put requests on a linked list. +All in all the majority of struct buf is involved with the I/O +aspect and only a few fields relate exclusively to the cache aspect. +.PP +If we step forward to the BSD 4.4-Lite-2 release, struct buf has grown +a bit here or there: +.DS +.ft C +.ps -1 +struct buf { + LIST_ENTRY(buf) b_hash; /* Hash chain. */ + LIST_ENTRY(buf) b_vnbufs; /* Buffer's associated vnode. */ + TAILQ_ENTRY(buf) b_freelist; /* Free list position if not active. */ + struct buf *b_actf, **b_actb; /* Device driver queue when active. */ + struct proc *b_proc; /* Associated proc; NULL if kernel. */ + volatile long b_flags; /* B_* flags. */ + int b_error; /* Errno value. */ + long b_bufsize; /* Allocated buffer size. */ + long b_bcount; /* Valid bytes in buffer. */ + long b_resid; /* Remaining I/O. */ + dev_t b_dev; /* Device associated with buffer. */ + struct { + caddr_t b_addr; /* Memory, superblocks, indirect etc. */ + } b_un; + void *b_saveaddr; /* Original b_addr for physio. */ + daddr_t b_lblkno; /* Logical block number. */ + daddr_t b_blkno; /* Underlying physical block number. */ + /* Function to call upon completion. */ + void (*b_iodone) __P((struct buf *)); + struct vnode *b_vp; /* Device vnode. */ + long b_pfcent; /* Center page when swapping cluster. */ + /* XXX pfcent should be int; overld. */ + int b_dirtyoff; /* Offset in buffer of dirty region. */ + int b_dirtyend; /* Offset of end of dirty region. */ + struct ucred *b_rcred; /* Read credentials reference. */ + struct ucred *b_wcred; /* Write credentials reference. */ + int b_validoff; /* Offset in buffer of valid region. */ + int b_validend; /* Offset of end of valid region. */ +}; +.ps +1 +.ft P +.DE +.PP +The main piece of action is the addition of vnodes, a VM system and a +prototype LFS filesystem, all of which needed some handles on struct +buf. Comparison will show that the I/O aspect of struct buf is in +essence unchanged, the length field is now in bytes instead of words, +the linked list the drivers can use has been renamed (b_actf, +b_actb) and a b_iodone pointer for callback notification has been added +but otherwise there is no change to the fields which +represent the I/O aspect. All the new fields relate to the cache +aspect, link buffers to the VM system, provide hacks for file-systems +(b_lblkno) etc etc. +.PP +By the time we get to FreeBSD 3.0 more stuff has grown on struct buf: +.DS +.ft C +.ps -1 +struct buf { + LIST_ENTRY(buf) b_hash; /* Hash chain. */ + LIST_ENTRY(buf) b_vnbufs; /* Buffer's associated vnode. */ + TAILQ_ENTRY(buf) b_freelist; /* Free list position if not active. */ + TAILQ_ENTRY(buf) b_act; /* Device driver queue when active. *new* */ + struct proc *b_proc; /* Associated proc; NULL if kernel. */ + long b_flags; /* B_* flags. */ + unsigned short b_qindex; /* buffer queue index */ + unsigned char b_usecount; /* buffer use count */ + int b_error; /* Errno value. */ + long b_bufsize; /* Allocated buffer size. */ + long b_bcount; /* Valid bytes in buffer. */ + long b_resid; /* Remaining I/O. */ + dev_t b_dev; /* Device associated with buffer. */ + caddr_t b_data; /* Memory, superblocks, indirect etc. */ + caddr_t b_kvabase; /* base kva for buffer */ + int b_kvasize; /* size of kva for buffer */ + daddr_t b_lblkno; /* Logical block number. */ + daddr_t b_blkno; /* Underlying physical block number. */ + off_t b_offset; /* Offset into file */ + /* Function to call upon completion. */ + void (*b_iodone) __P((struct buf *)); + /* For nested b_iodone's. */ + struct iodone_chain *b_iodone_chain; + struct vnode *b_vp; /* Device vnode. */ + int b_dirtyoff; /* Offset in buffer of dirty region. */ + int b_dirtyend; /* Offset of end of dirty region. */ + struct ucred *b_rcred; /* Read credentials reference. */ + struct ucred *b_wcred; /* Write credentials reference. */ + int b_validoff; /* Offset in buffer of valid region. */ + int b_validend; /* Offset of end of valid region. */ + daddr_t b_pblkno; /* physical block number */ + void *b_saveaddr; /* Original b_addr for physio. */ + caddr_t b_savekva; /* saved kva for transfer while bouncing */ + void *b_driver1; /* for private use by the driver */ + void *b_driver2; /* for private use by the driver */ + void *b_spc; + union cluster_info { + TAILQ_HEAD(cluster_list_head, buf) cluster_head; + TAILQ_ENTRY(buf) cluster_entry; + } b_cluster; + struct vm_page *b_pages[btoc(MAXPHYS)]; + int b_npages; + struct workhead b_dep; /* List of filesystem dependencies. */ +}; +.ps +1 +.ft P +.DE +.PP +Still 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. +.PP +This p_blkno is relevant, it has been added because the disklabel/slice +code have been abstracted out of the device drivers, the filesystem +ask for b_blkno, the slice/label code translates this into b_pblkno +which the device driver operates on. +.PP +After this point some minor cleanups have happened, some unused fields +have been removed etc but the I/O aspect of struct buf is still only +a fraction of the entire structure: less than a quarter of the +bytes in a struct buf are used for the I/O aspect and struct buf +seems to continue to grow and grow. +.PP +Since version 6 as documented in Lions book, a three significant pieces +of code have emerged which need to do non-trivial translations of +the I/O request before it reaches the device drivers: CCD, slice/label +and Vinum. They all basically do the same: they map I/O requests from +a logical space to a physical space, and the mappings they perform +can be 1:1 or 1:N. \** +.FS +It is interesting to note that Lions in his comments to the \fCrkaddr\fP +routine (p. 16-2) writes \fIThe code in this procedure incorporates +a special feature for files which extend over more than one disk +drive. This feature is described in the UPM Section "RK(IV)". Its +usefulness seems to be restricted.\fP This more than hints at the +presence already then of various hacks to stripe/span multiple devices. +.FE +.PP +The 1:1 mapping of the slice/label code is rather trivial, and the +addition of the b_pblkno field catered for the majority of the issues +this resulted in, leaving but one: Reads or writes to the magic "disklabel" +or equally magic "MBR" sectors on a disk must be caught, examined and in +some cases modified before being passed on to the device driver. This need +resulted in the addition of the b_iodone_chain field which adds a limited +ability to stack I/O operations; +.PP +The 1:N mapping of CCD and Vinum are far more interesting. These two +subsystems look like a device driver, but rather than drive some piece +of hardware, they allocate new struct buf data structures populates +these and pass them on to other device drivers. +.PP +Apart from it being inefficient to lug about a 348 bytes data structure +when 80 bytes would have done, it also leads to significant code rot +when programmers don't know what to do about the remaining fields or +even worse: "borrow" a field or two for their own uses. +.PP +.ID +.if t .PSPIC bufsize.eps +.if n [graph not available in this format] +.DE +.I +Conclusions: +.IP "" 5n +\(bu Struct buf is victim of chronic bloat. +.IP +\(bu The I/O aspect of +struct buf is practically constant and only about \(14 of the total bytes. +.IP +\(bu Struct buf currently have several users, vinum, ccd and to +limited extent diskslice/label, which +need only the I/O aspect, not the vnode, caching or VM linkage. +.IP +.I +The I/O aspect of struct buf should be put in a separate \fCstruct bio\fP. +.R +.NH 1 +Implications for future struct buf improvements +.PP +Concerns have been raised about the implications this separation +will have for future work on struct buf, I will try to address +these concerns here. +.PP +As the existence and popularity of vinum and ccd proves, there is +a legitimate and valid requirement to be able to do I/O operations +which are not initiated by a vnode or filesystem operation. +In other words, an I/O request is a fully valid entity in its own +right and should be treated like that. +.PP +Without doubt, the I/O request has to be tuned to fit the needs +of struct buf users in the best possible way, and consequently +any future changes in struct buf are likely to affect the I/O request +semantics. +.PP +One particular change which has been proposed is to drop the present +requirement that a struct buf be mapped contiguously into kernel +address space. The argument goes that since many modern drivers us +physical address DMA to transfer the data maintaining such a mapping +is needless overhead. +.PP +Of course some drivers will still need to be able to access the +buffer in kernel address space and some kind of compatibility +must be provided there. +.PP +The question is, if such a change is made impossible by the +separation of the I/O aspect into its own data structure ? +.PP +The answer to this is ``no''. +Anything that could be added to or done with +the I/O aspect of struct buf can also be added to or done +with the I/O aspect if it lives in a new "struct bio". +.NH 1 +Implementing a \fCstruct bio\fP +.PP +The first decision to be made was who got to use the name "struct buf", +and considering the fact that it is the I/O aspect which gets separated +out and that it only covers about \(14 of the bytes in struct buf, +obviously the new structure for the I/O aspect gets a new name. +Examining the naming in the kernel, the "bio" prefix seemed a given, +for instance, the function to signal completion of an I/O request is +already named "biodone()". +.PP +Making the transition smooth is obviously also a priority and after +some prototyping \** +.FS +The software development technique previously known as "Trial & Error". +.FE +it was found that a totally transparent transition could be made by +embedding a copy of the new "struct bio" as the first element of "struct buf" +and by using cpp(1) macros to alias the fields to the legacy struct buf +names. +.NH 2 +The b_flags problem. +.PP +Struct bio was defined by examining all code existing in the driver tree +and finding all the struct buf fields which were legitimately used (as +opposed to "hi-jacked" fields). +One field was found to have "dual-use": the b_flags field. +This required special attention. +Examination showed that b_flags were used for three things: +.IP "" 5n +\(bu Communication of the I/O command (READ, WRITE, FORMAT, DELETE) +.IP +\(bu Communication of ordering and error status +.IP +\(bu General status for non I/O aspect consumers of struct buf. +.PP +For historic reasons B_WRITE was defined to be zero, which lead to +confusion and bugs, this pushed the decision to have a separate +"b_iocmd" field in struct buf and struct bio for communicating +only the action to be performed. +.PP +The ordering and error status bits were put in a new flag field "b_ioflag". +This has left sufficiently many now unused bits in b_flags that the b_xflags element +can now be merged back into b_flags. +.NH 2 +Definition of struct bio +.PP +With the cleanup of b_flags in place, the definition of struct bio looks like this: +.DS +.ft C +.ps -1 +struct bio { + u_int bio_cmd; /* I/O operation. */ + dev_t bio_dev; /* Device to do I/O on. */ + daddr_t bio_blkno; /* Underlying physical block number. */ + off_t bio_offset; /* Offset into file. */ + long bio_bcount; /* Valid bytes in buffer. */ + caddr_t bio_data; /* Memory, superblocks, indirect etc. */ + u_int bio_flags; /* BIO_ flags. */ + struct buf *_bio_buf; /* Parent buffer. */ + int bio_error; /* Errno for BIO_ERROR. */ + long bio_resid; /* Remaining I/0 in bytes. */ + void (*bio_done) __P((struct buf *)); + void *bio_driver1; /* Private use by the callee. */ + void *bio_driver2; /* Private use by the callee. */ + void *bio_caller1; /* Private use by the caller. */ + void *bio_caller2; /* Private use by the caller. */ + TAILQ_ENTRY(bio) bio_queue; /* Disksort queue. */ + daddr_t bio_pblkno; /* physical block number */ + struct iodone_chain *bio_done_chain; +}; +.ps +1 +.ft P +.DE +.NH 2 +Definition of struct buf +.PP +After adding a struct bio to struct buf and the fields aliased into it +struct buf looks like this: +.DS +.ft C +.ps -1 +struct buf { + /* XXX: b_io must be the first element of struct buf for now /phk */ + struct bio b_io; /* "Builtin" I/O request. */ +#define b_bcount b_io.bio_bcount +#define b_blkno b_io.bio_blkno +#define b_caller1 b_io.bio_caller1 +#define b_caller2 b_io.bio_caller2 +#define b_data b_io.bio_data +#define b_dev b_io.bio_dev +#define b_driver1 b_io.bio_driver1 +#define b_driver2 b_io.bio_driver2 +#define b_error b_io.bio_error +#define b_iocmd b_io.bio_cmd +#define b_iodone b_io.bio_done +#define b_iodone_chain b_io.bio_done_chain +#define b_ioflags b_io.bio_flags +#define b_offset b_io.bio_offset +#define b_pblkno b_io.bio_pblkno +#define b_resid b_io.bio_resid + LIST_ENTRY(buf) b_hash; /* Hash chain. */ + TAILQ_ENTRY(buf) b_vnbufs; /* Buffer's associated vnode. */ + TAILQ_ENTRY(buf) b_freelist; /* Free list position if not active. */ + TAILQ_ENTRY(buf) b_act; /* Device driver queue when active. *new* */ + long b_flags; /* B_* flags. */ + unsigned short b_qindex; /* buffer queue index */ + unsigned char b_xflags; /* extra flags */ +[...] +.ps +1 +.ft P +.DE +.PP +Putting the struct bio as the first element in struct buf during a transition +period allows a pointer to either to be cast to a pointer of the other, +which means that certain pieces of code can be left un-converted with the +use of a couple of casts while the remaining pieces of code are tested. +The ccd and vinum modules have been left un-converted like this for now. +.PP +This is basically where FreeBSD-current stands today. +.PP +The next step is to substitute struct bio for struct buf in all the code +which only care about the I/O aspect: device drivers, diskslice/label. +The patch to do this is up for review. \** +.FS +And can be found at http://phk.freebsd.dk/misc +.FE +and consists mainly of systematic substitutions like these +.DS +.ft C +s/struct buf/struct bio/ +s/b_flags/bio_flags/ +s/b_bcount/bio_bcount/ +&c &c +.ft P +.DE +.NH 2 +Future work +.PP +It can be successfully argued that the cpp(1) macros used for aliasing +above are ugly and should be expanded in place. It would certainly +be trivial to do so, but not by definition worthwhile. +.PP +Retaining the aliasing for the b_* and bio_* name-spaces this way +leaves us with considerable flexibility in modifying the future +interaction between the two. The DEV_STRATEGY() macro is the single +point where a struct buf is turned into a struct bio and launched +into the drivers to full-fill the I/O request and this provides us +with a single isolated location for performing non-trivial translations. +.PP +As an example of this flexibility: It has been proposed to essentially +drop the b_blkno field and use the b_offset field to communicate the +on-disk location of the data. b_blkno is a 32bit offset of B_DEVSIZE +(512) bytes sectors which allows us to address two terabytes worth +of data. Using b_offset as a 64 bit byte-address would not only allow +us to address 8 million times larger disks, it would also make it +possible to accommodate disks which use non-power-of-two sector-size, +Audio CD-ROMs for instance. +.PP +The above mentioned flexibility makes an implementation almost trivial: +.IP "" 5n +\(bu Add code to DEV_STRATEGY() to populate b_offset from b_blkno in the +cases where it is not valid. Today it is only valid for a struct buf +marked B_PHYS. +.IP +\(bu Change diskslice/label, ccd, vinum and device drivers to use b_offset +instead of b_blkno. +.IP +\(bu Remove the bio_blkno field from struct bio, add it to struct buf as +b_blkno and remove the cpp(1) macro which aliased it into struct bio. +.PP +Another possible transition could be to not have a "built-in" struct bio +in struct buf. If for some reason struct bio grows fields of no relevance +to struct buf it might be cheaper to remove struct bio from struct buf, +un-alias the fields and have DEV_STRATEGY() allocate a struct bio and populate +the relevant fields from struct buf. +This would also be entirely transparent to both users of struct buf and +struct bio as long as we retain the aliasing mechanism and DEV_STRATEGY(). +.bp +.NH 1 +Towards a stackable BIO subsystem. +.PP +Considering that we now have three distinct pieces of code living +in the nowhere between DEV_STRATEGY() and the device drivers: +diskslice/label, ccd and vinum, it is not unreasonable to start +to look for a more structured and powerful API for these pieces +of code. +.PP +In traditional UNIX semantics a "disk" is a one-dimensional array of +512 byte sectors which can be read or written. Support for sectors +of 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 +to 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. +.PP +A truly generalised concept of a disk needs to be more flexible and more +expressive. For instance, a user of a disk will want to know: +.IP "" 5n +\(bu What is the sector size. Sector-size these days may not be a power +of two, for instance Audio CDs have 2352 byte "sectors". +.IP +\(bu How many sectors are there. +.IP +\(bu Is writing of sectors supported. +.IP +\(bu Is freeing of sectors supported. This is important for flash based +devices where a wear-distribution software or hardware function uses +the information about which sectors are actually in use to optimise the +usage of the slow erase function to a minimum. +.IP +\(bu Is opening this device in a specific mode, (read-only or read-write) +allowed. The VM system and the file-systems generally assume that nobody +writes to "their storage" under their feet, and therefore opens which +would make that possible should be rejected. +.IP +\(bu What is the "native" geometry of this device (Sectors/Heads/Cylinders). +This is useful for staying compatible with badly designed on-disk formats +from other operating systems. +.PP +Obviously, all of these properties are dynamic in the sense that in +these days disks are removable devices, and they may therefore change +at any time. While some devices like CD-ROMs can lock the media in +place with a special command, this cannot be done for all devices, +in particular it cannot be done with normal floppy disk drives. +.PP +If 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 +geometry translations we find that nearly endless flexibility emerges: +Mirroring, RAID, striping, interleaving, disk-labels and sub-disks, all of +these techniques would get a common framework to operate in. +.PP +In practice of course, such a scheme must not complicate the use of or +installation of FreeBSD. The code will have to act and react exactly +like the current code but fortunately the current behaviour is not at +all hard to emulate so implementation-wise this is a non-issue. +.PP +But lets look at some drawings to see what this means in practice. +.PP +Today the plumbing might look like this on a machine: +.DS +.PS + Ad0: box "disk (ad0)" + arrow up from Ad0.n + SL0: box "slice/label" + Ad1: box "disk (ad1)" with .w at Ad0.e + (.2,0) + arrow up from Ad1.n + SL1: box "slice/label" + Ad2: box "disk (ad2)" with .w at Ad1.e + (.2,0) + arrow up from Ad2.n + SL2: box "slice/label" + Ad3: box "disk (ad3)" with .w at Ad2.e + (.2,0) + arrow up from Ad3.n + SL3: box "slice/label" + DML: box dashed width 4i height .9i with .sw at SL0.sw + (-.2,-.2) + "Disk-mini-layer" with .n at DML.s + (0, .1) + + V: box "vinum" at 1/2 + (0,1.2) + + A0A: arrow up from 1/4 + A0B: arrow up from 2/4 + A0E: arrow up from 3/4 + A1C: arrow up from 2/4 + arrow to 1/3 + A2C: arrow up from 2/4 + arrow to 2/3 + A3A: arrow up from 1/4 + A3E: arrow up from 2/4 + A3F: arrow up from 3/4 + + "ad0s1a" with .s at A0A.n + (0, .1) + "ad0s1b" with .s at A0B.n + (0, .3) + "ad0s1e" with .s at A0E.n + (0, .5) + "ad1s1c" with .s at A1C.n + (0, .1) + "ad2s1c" with .s at A2C.n + (0, .1) + "ad3s4a" with .s at A3A.n + (0, .1) + "ad3s4e" with .s at A3E.n + (0, .3) + "ad3s4f" with .s at A3F.n + (0, .5) + + V1: arrow up from 1/4 + V2: arrow up from 2/4 + V3: arrow up from 3/4 + "V1" with .s at V1.n + (0, .1) + "V2" with .s at V2.n + (0, .1) + "V3" with .s at V3.n + (0, .1) + +.PE +.DE +.PP +And while this drawing looks nice and clean, the code underneat isn't. +With a stackable BIO implementation, the picture would look like this: +.DS +.PS + Ad0: box "disk (ad0)" + arrow up from Ad0.n + M0: box "MBR" + arrow up + B0: box "BSD" + + A0A: arrow up from 1/4 + A0B: arrow up from 2/4 + A0E: arrow up from 3/4 + + Ad1: box "disk (ad1)" with .w at Ad0.e + (.2,0) + Ad2: box "disk (ad2)" with .w at Ad1.e + (.2,0) + Ad3: box "disk (ad3)" with .w at Ad2.e + (.2,0) + arrow up from Ad3.n + SL3: box "MBR" + arrow up + B3: box "BSD" + + V: box "vinum" at 1/2 + (0,.8) + arrow from Ad1.n to 1/3 + arrow from Ad2.n to 2/3 + + A3A: arrow from 1/4 + A3E: arrow from 2/4 + A3F: arrow from 3/4 + + "ad0s1a" with .s at A0A.n + (0, .1) + "ad0s1b" with .s at A0B.n + (0, .3) + "ad0s1e" with .s at A0E.n + (0, .5) + "ad3s4a" with .s at A3A.n + (0, .1) + "ad3s4e" with .s at A3E.n + (0, .3) + "ad3s4f" with .s at A3F.n + (0, .5) + + V1: arrow up from 1/4 + V2: arrow up from 2/4 + V3: arrow up from 3/4 + "V1" with .s at V1.n + (0, .1) + "V2" with .s at V2.n + (0, .1) + "V3" with .s at V3.n + (0, .1) + +.PE +.DE +.PP +The first thing we notice is that the disk mini-layer is gone, instead +separate modules for the Microsoft style MBR and the BSD style disklabel +are now stacked over the disk. We can also see that Vinum no longer +needs to go though the BSD/MBR layers if it wants access to the entire +physical disk, it can be stacked right over the disk. +.PP +Now, imagine that a ZIP drive is connected to the machine, and the +user loads a ZIP disk in it. First the device driver notices the +new disk and instantiates a new disk: +.DS +.PS + box "disk (da0)" +.PE +.DE +.PP +A number of the geometry modules have registered as "auto-discovering" +and will be polled sequentially to see if any of them recognise what +is on this disk. The MBR module finds a MBR in sector 0 and attach +an instance of itself to the disk: +.DS +.PS + D: box "disk (da0)" + arrow up from D.n + M: box "MBR" + M1: arrow up from 1/3 + M2: arrow up from 2/3 +.PE +.DE +.PP +It finds two "slices" in the MBR and creates two new "disks" one for +each of these. The polling of modules is repeated and this time the +BSD label module recognises a FreeBSD label on one of the slices and +attach itself: +.DS +.PS + D: box "disk (da0)" + arrow "O" up from D.n + M: box "MBR" + M1: line up .3i from 1/3 + arrow "O" left + M2: arrow "O" up from 2/3 + B: box "BSD" + B1: arrow "O" up from 1/4 + B2: arrow "O" up from 2/4 + B3: arrow "O" up from 3/4 + +.PE +.DE +.PP +The BSD module finds three partitions, creates them as disks and the +polling is repeated for each of these. No modules recognise these +and the process ends. In theory one could have a module recognise +the UFS superblock and extract from there the path to mount the disk +on, but this is probably better implemented in a general "device-daemon" +in user-land. +.PP +On this last drawing I have marked with "O" the "disks" which can be +accessed from user-land or kernel. The VM and file-systems generally +prefer to have exclusive write access to the disk sectors they use, +so we need to enforce this policy. Since we cannot know what transformation +a particular module implements, we need to ask the modules if the open +is OK, and they may need to ask their neighbours before they can answer. +.PP +We decide to mount a filesystem on one of the BSD partitions at the very top. +The open request is passed to the BSD module, which finds that none of +the other open partitions (there are none) overlap this one, so far no +objections. It then passes the open to the MBR module, which goes through +basically the same procedure finds no objections and pass the request to +the disk driver, which since it was not previously open approves of the +open. +.PP +Next we mount a filesystem on the next BSD partition. The +BSD module again checks for overlapping open partitions and find none. +This time however, it finds that it has already opened the "downstream" +in R/W mode so it does not need to ask for permission for that again +so the open is OK. +.PP +Next we mount a msdos filesystem on the other MBR slice. This is the +same case, the MBR finds no overlapping open slices and has already +opened "downstream" so the open is OK. +.PP +If we now try to open the other slice for writing, the one which has the +BSD module attached already. The open is passed to the MBR module which +notes that the device is already opened for writing by a module (the BSD +module) and consequently the open is refused. +.PP +While this sounds complicated it actually took less than 200 lines of +code to implement in a prototype implementation. +.PP +Now, the user ejects the ZIP disk. If the hardware can give a notification +of intent to eject, a call-up from the driver can try to get devices synchronised +and closed, this is pretty trivial. If the hardware just disappears like +a unplugged parallel zip drive, a floppy disk or a PC-card, we have no +choice but to dismantle the setup. The device driver sends a "gone" notification to the MBR module, which replicates this upwards to the mounted msdosfs +and the BSD module. The msdosfs unmounts forcefully, invalidates any blocks +in the buf/vm system and returns. The BSD module replicates the "gone" to +the two mounted file-systems which in turn unmounts forcefully, invalidates +blocks and return, after which the BSD module releases any resources held +and returns, the MBR module releases any resources held and returns and all +traces of the device have been removed. +.PP +Now, let us get a bit more complicated. We add another disk and mirror +two of the MBR slices: +.DS +.PS + D0: box "disk (da0)" + + arrow "O" up from D0.n + M0: box "MBR" + M01: line up .3i from 1/3 + arrow "O" left + M02: arrow "O" up from 2/3 + + D1: box "disk (da1)" with .w at D0.e + (.2,0) + arrow "O" up from D1.n + M1: box "MBR" + M11: line up .3i from 1/3 + line "O" left + M11a: arrow up .2i + + I: box "Mirror" with .s at 1/2 + arrow "O" up + BB: box "BSD" + BB1: arrow "O" up from 1/4 + BB2: arrow "O" up from 2/4 + BB3: arrow "O" up from 3/4 + + M12: arrow "O" up from 2/3 + B: box "BSD" + B1: arrow "O" up from 1/4 + B2: arrow "O" up from 2/4 + B3: arrow "O" up from 3/4 +.PE +.DE +.PP +Now assuming that we lose disk da0, the notification goes up like before +but the mirror module still has a valid mirror from disk da1, so it +doesn't propagate the "gone" notification further up and the three +file-systems mounted are not affected. +.PP +It is possible to modify the graph while in action, as long as the +modules know that they will not affect any I/O in progress. This is +very handy for moving things around. At any of the arrows we can +insert a mirroring module, since it has a 1:1 mapping from input +to output. Next we can add another copy to the mirror, give the +mirror time to sync the two copies. Detach the first mirror copy +and remove the mirror module. We have now in essence moved a partition +from one disk to another transparently. +.NH 1 +Getting stackable BIO layers from where we are today. +.PP +Most of the infrastructure is in place now to implement stackable +BIO layers: +.IP "" 5n +\(bu The dev_t change gave us a public structure where +information about devices can be put. This enabled us to get rid +of all the NFOO limits on the number of instances of a particular +driver/device, and significantly cleaned up the vnode aliasing for +device vnodes. +.IP +\(bu The disk-mini-layer has +taken the knowledge about diskslice/labels out of the +majority of the disk-drivers, saving on average 100 lines of code per +driver. +.IP +\(bu The struct bio/buf divorce is giving us an IO request of manageable +size which can be modified without affecting all the filesystem and +VM system users of struct buf. +.PP +The missing bits are: +.IP "" 5n +\(bu changes to struct bio to make it more +stackable. This mostly relates to the handling of the biodone() +event, something which will be transparent to all current users +of struct buf/bio. +.IP +\(bu code to stich modules together and to pass events and notifications +between them. +.NH 1 +An Implementation plan for stackable BIO layers +.PP +My plan for implementation stackable BIO layers is to first complete +the struct bio/buf divorce with the already mentioned patch. +.PP +The next step is to re-implement the monolithic disk-mini-layer so +that it becomes the stackable BIO system. Vinum and CCD and all +other consumers should not be unable to tell the difference between +the current and the new disk-mini-layer. The new implementation +will initially use a static stacking to remain compatible with the +current behaviour. This will be the next logical checkpoint commit. +.PP +The next step is to make the stackable layers configurable, +to provide the means to initialise the stacking and to subsequently +change it. This will be the next logical checkpoint commit. +.PP +At this point new functionality can be added inside the stackable +BIO system: CCD can be re-implemented as a mirror module and a stripe +module. Vinum can be integrated either as one "macro-module" or +as separate functions in separate modules. Also modules for other +purposes can be added, sub-disk handling for Solaris, MacOS, etc +etc. These modules can be committed one at a time. diff --git a/share/doc/papers/bufbio/bufsize.eps b/share/doc/papers/bufbio/bufsize.eps new file mode 100644 index 000000000000..2396ac62aa40 --- /dev/null +++ b/share/doc/papers/bufbio/bufsize.eps @@ -0,0 +1,479 @@ +%!PS-Adobe-2.0 EPSF-2.0 +%%Title: a.ps +%%Creator: $FreeBSD$ +%%CreationDate: Sat Apr 8 08:32:58 2000 +%%DocumentFonts: (atend) +%%BoundingBox: 50 50 410 302 +%%Orientation: Portrait +%%EndComments +/gnudict 256 dict def +gnudict begin +/Color false def +/Solid false def +/gnulinewidth 5.000 def +/userlinewidth gnulinewidth def +/vshift -46 def +/dl {10 mul} def +/hpt_ 31.5 def +/vpt_ 31.5 def +/hpt hpt_ def +/vpt vpt_ def +/M {moveto} bind def +/L {lineto} bind def +/R {rmoveto} bind def +/V {rlineto} bind def +/vpt2 vpt 2 mul def +/hpt2 hpt 2 mul def +/Lshow { currentpoint stroke M + 0 vshift R show } def +/Rshow { currentpoint 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