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<H1><A NAME="SECTION00940000000000000000"></A>
<A NAME="2249"></A>
<BR>
SD Data Structures
</H1>
<P>
In the Storage daemon, there is a Device resource (i.e. from conf file)
that describes each physical device. When the physical device is used it
is controled by the DEVICE structure (defined in dev.h), and typically
refered to as dev in the C++ code. Anyone writing or reading a physical
device must ultimately get a lock on the DEVICE structure - this controls
the device. However, multiple Jobs (defined by a JCR structure src/jcr.h)
can be writing a physical DEVICE at the same time (of course they are
sequenced by locking the DEVICE structure). There are a lot of job
dependent "device" variables that may be different for each Job such as
spooling (one job may spool and another may not, and when a job is
spooling, it must have an i/o packet open, each job has its own record and
block structures, ...), so there is a device control record or DCR that is
the primary way of interfacing to the physical device. The DCR contains
all the job specific data as well as a pointer to the Device resource
(DEVRES structure) and the physical DEVICE structure.
<P>
Now if a job is writing to two devices (it could be writing two separate
streams to the same device), it must have two DCRs. Today, the code only
permits one. This won't be hard to change, but it is new code.
<P>
Today three jobs (threads), two physical devices each job
writes to only one device:
<P>
<PRE>
Job1 -> DCR1 -> DEVICE1
Job2 -> DCR2 -> DEVICE1
Job3 -> DCR3 -> DEVICE2
</PRE>
<P>
To be implemented three jobs, three physical devices, but
job1 is writing simultaneously to three devices:
<P>
<PRE>
Job1 -> DCR1 -> DEVICE1
-> DCR4 -> DEVICE2
-> DCR5 -> DEVICE3
Job2 -> DCR2 -> DEVICE1
Job3 -> DCR3 -> DEVICE2
Job = job control record
DCR = Job contorl data for a specific device
DEVICE = Device only control data
</PRE>
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2012-01-24
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