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197 lines
8.2 KiB
Text
197 lines
8.2 KiB
Text
The OSD Standard
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================
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OSD (Object-Based Storage Device) is a T10 SCSI command set that is designed
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to provide efficient operation of input/output logical units that manage the
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allocation, placement, and accessing of variable-size data-storage containers,
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called objects. Objects are intended to contain operating system and application
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constructs. Each object has associated attributes attached to it, which are
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integral part of the object and provide metadata about the object. The standard
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defines some common obligatory attributes, but user attributes can be added as
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needed.
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See: http://www.t10.org/ftp/t10/drafts/osd2/ for the latest draft for OSD 2
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or search the web for "OSD SCSI"
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OSD in the Linux Kernel
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=======================
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osd-initiator:
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The main component of OSD in Kernel is the osd-initiator library. Its main
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user is intended to be the pNFS-over-objects layout driver, which uses objects
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as its back-end data storage. Other clients are the other osd parts listed below.
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osd-uld:
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This is a SCSI ULD that registers for OSD type devices and provides a testing
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platform, both for the in-kernel initiator as well as connected targets. It
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currently has no useful user-mode API, though it could have if need be.
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exofs:
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Is an OSD based Linux file system. It uses the osd-initiator and osd-uld,
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to export a usable file system for users.
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See Documentation/filesystems/exofs.txt for more details
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osd target:
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There are no current plans for an OSD target implementation in kernel. For all
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needs, a user-mode target that is based on the scsi tgt target framework is
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available from Ohio Supercomputer Center (OSC) at:
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http://www.open-osd.org/bin/view/Main/OscOsdProject
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There are several other target implementations. See http://open-osd.org for more
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links.
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Files and Folders
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=================
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This is the complete list of files included in this work:
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include/scsi/
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osd_initiator.h Main API for the initiator library
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osd_types.h Common OSD types
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osd_sec.h Security Manager API
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osd_protocol.h Wire definitions of the OSD standard protocol
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osd_attributes.h Wire definitions of OSD attributes
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drivers/scsi/osd/
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osd_initiator.c OSD-Initiator library implementation
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osd_uld.c The OSD scsi ULD
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osd_ktest.{h,c} In-kernel test suite (called by osd_uld)
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osd_debug.h Some printk macros
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Makefile For both in-tree and out-of-tree compilation
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Kconfig Enables inclusion of the different pieces
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osd_test.c User-mode application to call the kernel tests
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The OSD-Initiator Library
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=========================
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osd_initiator is a low level implementation of an osd initiator encoder.
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But even though, it should be intuitive and easy to use. Perhaps over time an
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higher lever will form that automates some of the more common recipes.
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init/fini:
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- osd_dev_init() associates a scsi_device with an osd_dev structure
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and initializes some global pools. This should be done once per scsi_device
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(OSD LUN). The osd_dev structure is needed for calling osd_start_request().
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- osd_dev_fini() cleans up before a osd_dev/scsi_device destruction.
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OSD commands encoding, execution, and decoding of results:
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struct osd_request's is used to iteratively encode an OSD command and carry
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its state throughout execution. Each request goes through these stages:
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a. osd_start_request() allocates the request.
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b. Any of the osd_req_* methods is used to encode a request of the specified
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type.
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c. osd_req_add_{get,set}_attr_* may be called to add get/set attributes to the
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CDB. "List" or "Page" mode can be used exclusively. The attribute-list API
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can be called multiple times on the same request. However, only one
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attribute-page can be read, as mandated by the OSD standard.
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d. osd_finalize_request() computes offsets into the data-in and data-out buffers
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and signs the request using the provided capability key and integrity-
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check parameters.
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e. osd_execute_request() may be called to execute the request via the block
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layer and wait for its completion. The request can be executed
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asynchronously by calling the block layer API directly.
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f. After execution, osd_req_decode_sense() can be called to decode the request's
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sense information.
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g. osd_req_decode_get_attr() may be called to retrieve osd_add_get_attr_list()
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values.
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h. osd_end_request() must be called to deallocate the request and any resource
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associated with it. Note that osd_end_request cleans up the request at any
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stage and it must always be called after a successful osd_start_request().
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osd_request's structure:
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The OSD standard defines a complex structure of IO segments pointed to by
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members in the CDB. Up to 3 segments can be deployed in the IN-Buffer and up to
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4 in the OUT-Buffer. The ASCII illustration below depicts a secure-read with
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associated get+set of attributes-lists. Other combinations very on the same
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basic theme. From no-segments-used up to all-segments-used.
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|________OSD-CDB__________|
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|read_len (offset=0) -|---------\
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|get_attrs_list_length | |
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|get_attrs_list_offset -|----\ |
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|retrieved_attrs_alloc_len| | |
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|retrieved_attrs_offset -|----|----|-\
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|set_attrs_list_length | | | |
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|set_attrs_list_offset -|-\ | | |
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|in_data_integ_offset -|-|--|----|-|-\
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|out_data_integ_offset -|-|--|--\ | | |
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\_________________________/ | | | | | |
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|_______OUT-BUFFER________| | | | | | |
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| Set attr list |</ | | | | |
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|-------------------------| | | | | |
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| Get attr descriptors |<---/ | | | |
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|-------------------------| | | | |
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| Out-data integrity |<------/ | | |
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\_________________________/ | | |
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|________IN-BUFFER________| | | |
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| In-Data read |<--------/ | |
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|-------------------------| | |
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| Get attr list |<----------/ |
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|-------------------------| |
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| In-data integrity |<------------/
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\_________________________/
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A block device request can carry bidirectional payload by means of associating
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a bidi_read request with a main write-request. Each in/out request is described
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by a chain of BIOs associated with each request.
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The CDB is of a SCSI VARLEN CDB format, as described by OSD standard.
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The OSD standard also mandates alignment restrictions at start of each segment.
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In the code, in struct osd_request, there are two _osd_io_info structures to
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describe the IN/OUT buffers above, two BIOs for the data payload and up to five
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_osd_req_data_segment structures to hold the different segments allocation and
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information.
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Important: We have chosen to disregard the assumption that a BIO-chain (and
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the resulting sg-list) describes a linear memory buffer. Meaning only first and
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last scatter chain can be incomplete and all the middle chains are of PAGE_SIZE.
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For us, a scatter-gather-list, as its name implies and as used by the Networking
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layer, is to describe a vector of buffers that will be transferred to/from the
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wire. It works very well with current iSCSI transport. iSCSI is currently the
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only deployed OSD transport. In the future we anticipate SAS and FC attached OSD
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devices as well.
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The OSD Testing ULD
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===================
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TODO: More user-mode control on tests.
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Authors, Mailing list
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=====================
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Please communicate with us on any deployment of osd, whether using this code
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or not.
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Any problems, questions, bug reports, lonely OSD nights, please email:
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OSD Dev List <osd-dev@open-osd.org>
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More up-to-date information can be found on:
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http://open-osd.org
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Boaz Harrosh <ooo@electrozaur.com>
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References
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==========
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Weber, R., "SCSI Object-Based Storage Device Commands",
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T10/1355-D ANSI/INCITS 400-2004,
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http://www.t10.org/ftp/t10/drafts/osd/osd-r10.pdf
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Weber, R., "SCSI Object-Based Storage Device Commands -2 (OSD-2)"
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T10/1729-D, Working Draft, rev. 3
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http://www.t10.org/ftp/t10/drafts/osd2/osd2r03.pdf
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