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dmraid tool design document v1.0.0-rc5f Heinz Mauelshagen 2004.11.05
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----------------------------------------------------------------------------
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The dmraid tool supports RAID devices (RDs) such as ATARAID with
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device-mapper (dm) in Linux 2.6 avoiding the need to install a
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vendor specific (binary) driver to access them.
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It supports multiple on-disk RAID metadata formats and is open for
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extension with new ones.
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Initially dmraid aims to support activation of RDs and doesn't support
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*updates* of the ondisk metadata (eg, to record disk failures)
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It can optionally erase on disk metadata in case of multiple format
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signatures on a device.
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See future enhancements at the end.
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Functional requirements:
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------------------------
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1. dmraid must be able to read multiple vendor specific ondisk
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RAID metadata formats:
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- Silicon Image Medley
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2. dmraid shall be open to future extensions by other ondisk RAID formats,
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(http://www.snia.org/tech_activities/ddftwg/DDFTrial-UseDraft_0_45.pdf)
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3. dmraid shall generate the necessary dm table(s) defining
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the needed mappings to address the RAID data.
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4. Device discovery, activation, deactivation and
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property display shall be supported.
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5. Spanning of disks, RAID0, RAID1 and RAID10 shall be supported
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(in order to be able to support SNIA DDF, higher raid levels need
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implementing in form of respective dm targets; eg, RAID5);
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Some vendors do have support for RAID5 already which is outside the scope
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of dmraid because of the lag of a RAID5 target in device-mapper!
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6. Activation of MSDOS partitions in RAID sets shall be supported.
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Feature set definition:
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-----------------------
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Feature set summarizes as: Discover, Activate, Deactivate, Display.
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1 scan active disk devices identifying RD.
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2 try to find an RD signature and if recognized,
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add the device to the list of RDs found.
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3 Abstract the metadata describing the RD layout and translate the vendor
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specific format into it.
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1 group devices into abstracted RAID sets (RS) conforming to
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their respective layout (SPAN, RAID0, RAID1, RAID10).
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2 generate dm mapping tables for a/those RS(s) derived from the abstracted
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information about RS(s) and RD(s).
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3 create multiple/a dm device(s) for each RS to activate and
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load the generated table(s) into the device.
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4 Recusively discover and activate MSDOS partitions in active RAID sets.
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1 remove the dm device(s) making up an RS; can be a stacked hierachy of
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devices (eg, RAID10: RAID1 on top of n RAID0 devices).
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1 display RS and RD properties
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(eg, display information kept with RS(s) such as size and type)
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Technical specification:
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------------------------
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All functions returning int or a pointer return 0/NULL on failure.
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o RAID metadata format handler
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Tool calls the following function to register a vendor specific
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format handler; in case of success, a new instance with methods is
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accessible to the high level metadata handling functions (see below):
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- int register_format_handler(struct lib_context *lc,
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struct dmraid_format *fmt);
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x returns !0 on successfull format handler registration
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- Format handler methods:
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x struct raid_dev *(read)(struct lib_context *lc, struct dev_info* di);
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- returns 'struct raid_dev *' describing the RD (eg, offset, length)
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x int (*write)(struct lib_context *lc, struct raid_dev* rd, int erase);
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- returns != 0 successfully written the vendor specific metadata
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back to the respective RD rd (optionally erasing the
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metadata area(s) in case erase != 0)
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x struct raid_set (*group)(struct lib_context *lc,
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struct raid_dev *raid_dev)
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- returns pointer to RAID set structure on success
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x int (*check)(struct lib_context *lc, struct raid_set *raid_set)
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- returns != 0 in case raid set is consistent
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x void (*log)(struct lib_context *lc, struct raid_dev *rd)
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- display metadata in native format (ie. all vendor specific
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metadata fields) for RD rd
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1 retrieve block device information from sysfs for all disk
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devices by scanning /SYSFS_MOUNTPOINT/block/[sh]d*;
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keep information about the device path and size which is the base
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to find the RAID signature on the device in a linked list
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of type 'struct dev_info *'.
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2 walk the list and try to read RD metadata signature off the device
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trying vendor specific read methods (eg, Highpoint...) in turn;
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library exposes interface to register format handlers for vendor
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specific RAID formats in order to be open for future extensions
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(see register_format_handler() above).
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Tool calls the following high level function which hides
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the iteration through all the registered format handler methods:
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x void discover_devices(struct lib_context *lc)
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- returns != 0 in case it discovered all supported disks (IDE, SCSI)
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and added them to a list
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x void discover_raid_devices(struct lib_context *lc,
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- discovers all suported RDs using the list filled by
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discover_devices() and adds them to a list
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x void discover_partitions(struct lib_context *lc);
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- discovers all MSDOS partitions in active RAID sets and builds
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RAID sets with one linear device hanging off for activation.
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x int count_devices(struct lib_context *lc, enum dev_type type);
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- returns number of devices found by discover_devices() and
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discover_raid_devices() of specified type (eg, DEVICE, RAID, ...)
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x int perform(struct lib_context *lc, char **argv);
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- returns != 0 on success performing various actions
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(ie. activation/deacivation of RAID sets, displaying properties, ...)
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Tool calls the following high level function which hide
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the details of the RAID set assembly:
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x int group_set(struct lib_context *lc, char *name);
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- returns != 0 on successfully grouping an RS/RSs
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- don't activate non-RAID1 devices which have an invalid set check result
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- create the ASCII dm mapping table by iterating through the list
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of RD in a particular set, retrieving the layout (SPAN, ...)
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the device path, the offset into the device and the length to map
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and the stripe size in case of RAID
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- create a unique device_name
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- call device-mapper library to create the mapped device and load
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the mapping table using this function:
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x int activate_set(struct lib_context *lc, void *rs)
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- returns != 0 in case of successfull RAID set activation
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- activate MSDOS partitioned RAID sets as in 2+3
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- check if a partitioned RAID set is active and deactivate it first
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- check if the RAID set is active and call device-mapper library to
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remove the mapped device (recursively in case of a mapped-device hierarchy)
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x int deactivate_set(struct lib_context *lc, void *rs)
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- returns != 0 in case of successfull RAID set deactivation
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- list all block devices found
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- list all (in)active RD
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- display properties of a particular/all RD devices
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(eg, members of the set by block device name and offset/length mapped
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x void display_devices(struct lib_context *lc, enum dev_type type);
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- display devices of dev_type 'type' (eg, RAID, DEVICE, ...)
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x void display_set(struct lib_context *lc, void *rs,
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enum active_type type, int top);
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- display RS of active_type 'type' (ACTIVE, INACTIVE, ALL)
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| |-/format ---/ataraid
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o enhance write support to update ondisk metadata
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- to restore metadata backups
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- to record disk failures
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o support to log state (eg, sector failures) in standard/vendor ondisk logs;
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needs above write support
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o status daemon to monitor RAID set sanity
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(eg, disk failure, hot spare rebuild, ...) and
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o do we need to prioritize on device-mapper targets for higher RAID levels
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(in particular we'ld need RAID3+5 to support some ATARAID formats) ?
added
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1.0.0.rc14/doc/dmraid_design.txt
