Transcript Adding a Disk

UNIX System Administration
Adding a Disk
Chapter 8
Disk Interfaces
• Proprietary interfaces
– Big. Yellow. Different.
• SCSI
– Small Computer Standardized Interface
• IDE
– Integrated Drive Electronics
• Fibre Channel
– NKOTB
(IDE) ATA helping if you gave me one….
• IDE - The idea of incorporating the drive’s controller (NOT the bus
controller) onto the drive unit itself.
– PC IDE is attached to main system bus via only buffering/isolation
components - there is no IDE interface per-se
• More properly referred to as ATA - Advanced Technology Attachment
– Defined protocol for DISK DRIVES
– ATAPI - ATA Packet Interface - protocol encapsulated in ATA for other
“IDE” devices (CD-? Drives, misc removable storage devices)
ATA Performance
• Limited to two devices per channel, two channels per system
• Cheap, fast, single-user storage
– ATA protocols have less overhead than SCSI
– No error correction (until ATA-4)
– No behind-the-scenes management (e.g. integrity checks)
• Less extensible than SCSI
– Very simple protocol will not support more than two drives per channel
– Drives are not multitasking
– ATA bus is not “shared” with neighbor device - each drive is master of its
own domain while handling an I/O request
• Host CPU must manage drives
– 60%-100% CPU time of a PIII might be spent on a large transfer
SCSI
• SCSI - Small Computer Sstandardized Interface
– Began as Shugart Associates Standardized Interface in 1979, adopted as
ANSI standard (SCSI-1) in 1986 with a name change
• Separate, intelligent I/O bus attached to main bus. All devices on bus
are intelligent as well.
• Managing device - initiator - typically is the host interface [though this
is not required]. Peripherals are targets.
SCSI too
• Initiator controls whether a SCSI device is connected to the bus
(separate control lines facilitate initiator/target control
communications)
• Multiple I/O requests may be issued against multiple targets
• Controllers typically use DMA (bus mastering) to offload the work of
transferring data to memory from the host CPU
SCSI Buses
• Single Ended (SE)
– Oldest variety
– Most susceptible to noise due to low signal levels - required strict
adherence to cable specifications
– Uses common ground for all signal lines
• Differential varieties
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Developed to overcome SE limitations - noise immunity
Uses a + and - line set for each signal
High Voltage Differential (HVD) - 12 volts
Low Voltage Differential (LVD) - 5 volts
• Less expensive version of HVD
• LVD devices will detect connection to a SE bus and fall back to SCSI-2 speeds
• All are incompatible
• All require termination. SE - passive H/LVD - active
– Absorbs signal at ends of the bus to prevent reflections
Feeling a little...irregular? Add
some fibre to your diet.
• Fibre Channel - Member of a class of high-speed, serial interfaces for
disk storage (serial referring to the transmission order of data bytes)
– SSA (Serial Storage Architecture) is IBM’s proprietary attempt at a serial
interface for storage. As always, Big Blue sticks to that P-word.
Regardless, SSA is too little, too late. Revel in the irony.
• Fibre Channel media
– Coaxial copper cable
– Twisted Pair copper cable
– Multimode or Singlemode fiber (required for speeds over 1Gb/s
[100MB/s]) - two fibers required (Tx/Rx) in a P2P link configuration.
• Fibre channel protocols
– Provide capability to support FC-specific protocols as well as SCSI, IP,
and IEEE 802.2 (ethernet)
– Supports ‘class of service’ - datagram, fractional speed, connectionless,
etc.
FC Topo- and Termi- nology
• Point-to-point
– exactly two devices. Exclusive media access.
• Arbitrated loop
– up to 127 devices, chained in a loop. Shared media access.
• Fabric-switched
– up to 224 devices in a cross-point switched configuration. Requires a
switch, provides exclusive media access.
Drive Terminology
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Sector - atomic unit of storage, typically 512B
Block - abstracted sector free from track/surface designations
Track - sectors in a single groove on one surface of a platter
Cylinder - from a vertical perspective, set of tracks on all disk surfaces
at “equal distance from the center”
• Partition - Logical grouping of a set of consecutive cylinders or blocks
• LUN - Logical Unit Number - SCSI construct that permits dividing a
single target into up to 7 logical subdevices. All targets have LUN 0,
devices like multi-tray CD changers and drive arrays utilize other LUN
values as required.
Preparing your disk
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Connect the disk to the computer
Create device files so disk can be accessed
Format the disk ****
Partition the disk
Create filesystems on partitions
Configure filesystem mounting
Let there be light!
• Connect the disk
• Create device files to access the disk
– All devices have entries in the /dev structure
– Disks (and like storage media) have two:
• Character (or raw) - a “low-level” device used for maintenance
interaction (fsck, etc) - /dev/rdsk
• Block - conceptually “higher-level” device used for filesystem
interaction (backups, mounting, etc) - /dev/dsk
– Typically established by a reconfiguration sequence (though can be
established “manually” under certain circumstances)
– All /dev entries for LUN 0 will be created initially, extra LUNs
may require extra steps
• Format the disk ****
– In terms of “low-level” format, disks are pre-formatted by the
manufacturer. This procedure should not be necessary, and should
only be attempted with sufficient knowledge of the ramifications.
Click to add title
• Partition the disk
– Actual method is O/S dependent
– Logically groups cylinders (or blocks) into a usable portion which can be
referenced
– Partitions are associated with and referred to by a device entry
• CwTxDySz
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C = controller (w) - number of SCSI controller in the system
T = target (x) - SCSI ID of target on bus. 0-7 or 0-15
D = LUN (y) - 0 always exists, typically only one LUN. 0-7
S = slice (z) - partition ID. 0-7
• Slice 2 is the “backup” slice - a special partition that refers to the entire disk,
used for low-level disk manipulation
– Partitions should not overlap (but the customer is always right…)
– A disk’s label refers to its partition table map. Labeling the disk writes this
map onto the disk to be used by the system for interfacing with that drive.
• Remember to partition for O/S, swap, and other file storage needs (e.g.
home, log, application directories)
Click harder!
• Create a filesystem on new partitions
– Partitions (slices) need to be initialized to accept a filesystem.
– Actual procedure varies by O/S - typically the newfs command
• Requires the /dev/rdsk filename of the partition to initialize
• Writes out the required filesystem data structures to disk
(superblocks, disk block maps, inode tables, etc)
NOT THAT HARD!
• Configure filesystem mounting
– Procedure is (AGAIN) O/S dependent
– Filesystem must be mounted before can be accessible
– Add entries for automatic mounting
• /etc/vfstab
– Mount points are directories on an existing filesystem (or / if this is the
root disk), but once mounted, the new filesystem will overlay and hide
anything contained in that mount point directory.
– You must mount in order of dependence if mounting a filesystem on a
mounted filesystem other than /
When good disks go bad…
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fsck - Filesystem check and repair
Capable of repairing most soft errors
Runs automatically on boot (per settings in /etc/vfstab)
Required to be run against filesystems that were potentially made
inconsistent by an improper shutdown - mount will check a flag that
indicates the possible state of the filesystem and will usually refuse to
accept inconsistent filesystems
• A fsck should only be done on an unmounted filesystem especially if
changes will be made - O/S buffers will become inconsistent and the
system may panic…
• Certain repair operations may be destructive (but would be required
before disk could be mounted again)
• Running fsck on a journaled filesystem takes little time as the journal
log clears up inconsistencies (fsck will flush the log, which is very
small)
Solaris
• Connect the disk (power off, please) and set the SCSI Ids and place
terminators (or IDE master/slave jumper)
• Power on and enter PROM mode
– Issue probe-scsi (or probe-scsi-all) to verify disk presence
• Use probe-ide for IDE machines
– Issue boot -r for reconfig boot
• Use the format command to label (partition) the disk
– Ensure your slices do not overlap - no direct indication would be provided
of this oversight (the overlap exception is slice 2 - the backup slice - don’t
ever touch it). Zero-out any unused partitions to avoid accidental use.
Don’t forget to issue the label command before exiting.
• Use newfs to initialize the filesystem
• Mount the filesystem to verify
• Add entries in /etc/vfstab as required - enable journaling, but be
careful!!