Comments on SDA States
Doug, dtn 237-2145 Flames to NL: 07-Jan-1994 1100
hagerman at starch.enet.dec.com
Fri Jan 7 08:00:29 PST 1994
Date: January 5, 1994 X3T10/94-___ Rev 0
To: X3T10 Committee (SCSI)
From: Doug Hagerman (Digital)
Subject: Comments on SDA States (93-191)
This proposal is an expansion of the SDA States described by George
Penokie. Given the complex nature of RAID devices or any general
SCSI subsystem, it seems to me that the initiator used to configure
and control the subsystem must have considerable visibility into the
current state of the subsystem. The following example traces one
possible sequence of events in the use of a RAID subsystem, and shows
why the number of states needs to be expanded over George's proposal.
Event New State
----- ---------
Power on to subsystem Self-test in progress
Self-test completes Ready to handle INQUIRY command
Self-configuration completes Ready to handle any command,
P-addressing valid (only)
Define first extent At least one extent available
Define first redundancy group At least one redundancy group available,
R-addressing valid
Define first volume set At least one volume set available,
V-addressing valid
Enable RAID operation Normal quiescent operating state
Disk failure Degraded redundancy
Initiate rebuilding on spare Degraded redundancy, rebuild in progress
Rebuild complete Normal redundancy, degraded performance
(because of use of spare)
Failed disk replaced Normal redundancy, copyback in progress
Copyback complete Normal redundancy, normal performance,
erasing spare
Spare reconfiguration complete Nomal quiescent operating state
Many of the above states are volume set states, but P-extent,
P-LUI, redundancy group, spare, and overall SDA states are intermixed.
Considering that a RAID subsystem of moderate size might have several
volume sets, dozens of redundancy groups, and hundreds of P-LUIs,
it is clear that the state description needs to be more comprehensive.
Perhaps we need a separate table for each of these, as proposed below.
x.0 SCSI-3 Disk Array States
The following sections describe the possible states for each
of the addressable components of a RAID subsystem.
x.1 DACL States
An SDA consists of one or more addressable DACL devices at one
or more SCSI device addresses. Each DACL can report the following states.
State Condition
----- ---------
Self-test in progress Power is applied to system, but DACL is not
yet ready to respond to any command.
Ready to handle INQUIRY command Self-test has completed, DACL is ready
to respond to INQUIRY command.
Ready to handle any command Any self-configuration operations
(possibly requiring media access) are
complete, DACL is ready to respond to
any command.
Hung DACL is not responding to commands.
x.2 Volume Set States
A DACL implements one or more volume sets at one or more LUN addresses.
Each volume set can report the following states.
State Condition
----- ---------
Not available The volume set has been defined, but
is disabled.
Active The volume set is available for normal use.
Degraded performance Within the volume set some component
is in a state that causes degraded
performance. All data is still fully
protected.
Exposed Within the volume set some component
is in a state that causes the loss of
redundancy. All data is still valid.
Failed Within the volume set some component
is in a state that prevents a guarantee
that data will not be lost.
x.3 Redundancy Group States
A DACL implements one or more redundancy groups at one or more LUN addresses.
Each redundancy group can report the following states.
State Condition
----- ---------
Not available The redundancy group has been defined, but
is disabled.
Active The redundancy group is available for
normal use.
Degraded redundancy Something has failed, causing the protection
of the data to be lost, and the system
does not have a resource needed to provide
protection. No data has been lost.
Rebuild in progress Protection is being rebuilt onto a spare
or repaired disk. Protection has been lost,
but the system is working to rebuild it.
Degraded performance A component that affects performance has
failed, or a lower-performance spare
disk unit is in use. In this state all
data is still fully protected.
Copyback in progress Data is being copied back to a repaired
disk. All data fully protected. The system
is working to provide full performance.
x.4 P-LUI States
A P-LUI is an underlying device controlled by the DACL. P-LUIs include
the disks used to store the data, plus any other devices used in the SDA
that can be addressed with the physical addressing mechanism.
Since each P-LUI is an addressable physical device in the system,
its state is simply the state of the physical device. SCSI devices do
not report "state" as such, but the state can be inferred from the
data returned in response to commands such as INQUIRY, TEST UNIT READY,
and REQUEST SENSE.
[Not sure about this. Is the DACL capable of reporting how it is
using each P-LUI, e.g. "P-LUI is in use in R-LUI n"?]
Each redundancy group can report the following states.
State Condition
----- ---------
Not available The redundancy group has been defined, but
is disabled.
Active The redundancy group is available for
normal use.
x.5 Spare States
A spare is an addressable [something] that can be used as a spare.
Each spare can report the following states. Note that there is no
provision for reporting the history of prior use of a spare. It is
assumed that a spare is available for temporary use but that when the
original device is replaced, the data will be copied back to the
original device and the spare will be erased and made available again.
State Condition
----- ---------
Not available The spare has been defined, but is disabled.
Available The spare is available for use.
In use The spare has been allocated for use
as a spare.
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