Note: Descriptions are shown in the official language in which they were submitted.
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lhe present invention relates to means for storing digital data,
and more particularly to an improved system and method for managing and
allocating data in a disk storage arrangement.
One of the main considerations in designing and operating a digital
data system is the provision of suitable means for storing information receiv-
ed from the central processing unit (CPU) of the computer and providing
appropriate information to the CPU when it is requested.
It is generally conceded that both magnetic tape and disk storage
systems have different and unique functional characteristics. Accordingly,
each type of storage medium lends itself particularly well to certain types
of applications, and not to others. In particular, data files which are
serially organized, and which are to be serially accessed, are most compatible
with tape drive technology~ inasmuch as information is necessarily written
upon and read from tape in serial fashion. Disk memory systems, on the other
hand, generally have the capability of having information written upon and
read from practically any portion of the disks almost instantaneously. As a
consequence, multiple data sets all stored on the same disk can be active at
once, which is not practical with tape. Therefore, it would always be pre-
ferred to use disk memory systems, except for the fact that disk memory is
much more expensive than tape.
As a result, while serially-organized data can conveniently be
continuously written upon and read from a tape; oftentimes writing or obtain-
ing data which is not serially organized presents a problem. If the data
is located upon a distant portion of the tape, the tape must be wound or un-
; wound to the point at which the data is located. A second far more serious
drawback to tape memory is the fact that, for most large-scale applications,
the number of reels of tape required to store information is far in excess
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of, perhaps hundreds of times greater than, the number of tape drives or
mounts available to read or write from the tapes. In fact, many large scale
data processing operations make use of thousands of reels of tape. In
general, scme 90% of all tapes contain but one data set (e.g. a customer list
or a payroll file) since as a rule the physical characteristics of tape make
it impractical to read or write from or to two or more data sets at once,
which is very often desirable. Clearly then, tape storage is very inefficient,
since only coincidentally will the length of a given data set approximate the
available memory area. In fact, studies show tha~ normally only about one
percent of tape is actually used.
Obviously, no user can provide a tape mount or drive for each reel
of tape; rather, individual reels are constantly brought from storage, mounted
for use, then demounted and replaced in storage. This is time-consuming and
inefficient, since it involves the intervention of a human operator.
; Efforts have been made to overcome this shortcoming of tape media,
for instance by providing a large number of small cartridges of tape and an
automatic mechanism to rapidly remove the cartridges from a storage rack and
place them on a tape mount. Automatic mounting devices have also been made
for use with conventional tape. To date, however, no completely successful
method exists for making large quantities of tape immediately available to
tape drives.
At first glance, disk storage systems would appear to overcome the
shortcomings of tape. Practically any portion of any disk can be accessed
very rapidly, so that a disk system does not have to scan through all of the
data recorded thereon in serial fashion. Accordingly, by responding to
address codes, a disk drive can access a given block of data almost instantly.
Thus, multiple data sets can be stored on a single disk and be concurrently
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processed. However, the cost of disk drives~ is comparatively great, and more-
over state-of-the-art disk drives do not make highly efficient use of the
potential storage area.
Studies have demonstrated that at any given time in a disk system,
only approximately 75% of potential storage space is actually available for
data storage purposes. The "overhead", or total storage space used to support
the read/write activity needed to maintain and log data, is thus approximately
20% to 25%. In addition, for most applications, only about two-thirds of
the available data space is actually used to store data, due to the space
requirements of the addressing systems commonly used. Hence, the total utili-
zation factor of most state-of-the-art disk systems is only about 50% or less.
erefore disk systems, although offering attractively high data access and
retrieval speeds, are relatively cost inefficient, and may therefore be
expensive in terms of the storage space which is actually used. By comparison,
the loss of space in tape storage due to provision of address marks is small,
since only a single address need be provided for a single sequentially organ-
i~ed record. The storage efficiency is thus increased to this extent.
It will therefore be seen that it would be highly advantageous to
provide a system which exhibits the addressing efficiensy of the tape medium
and the rapid-access characteristic of disk storage. Desirably, such a
memory system would not require any modification to the remainder of the com-
puting system involved; that is, it would "look" to the computer either as
tape or disk memories do in the prior art.
It is therefore an object of the present invention to provide an
improved storage system for a digital computer or the like.
It is another object of the invention to provide a system and
method for storing and accessing serially-arranged data more quickly than
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state~of-the-art storage systems.
Still another object is to provide a storage system which
responds to com~ands from the host system in the manner of a tape drive, but
makes use of the rapid access capability of disk storage units.
Yet another object of the present invention is to provide a
data storage management system which accepts and delivers data in serial
form, while recording and retrieving the information from disk storage units
in optimum piecemeal form.
Briefly stated, in accordance with one aspect of the invention
the foregoing objects are achieved by providing a "virtual storage system"
comprising a host interface stage responsive to commands from a host computer
for receiving and transmitting information in a serial, tape format, and a
first data buffer within said host interface for temporarily storing data
received from the host or directed thereto. A plurality of disk storage units
are also provided, along with a disk storage interface and an associated
second data buffer which temporarily stores data directed to and received from
the disk storage units. The interface stages are coupled together, and also
coupled in common to a large, main memory which receives and queues infor-
mation flowing from one interface to the other. Finally, a master control
processor is coupled to the interface units and to the main memory and
- directs the operation of the apparatus so that the host interface stages
respond to the host computer in the same manner as would a tape system, while
the disk interface stages read and write data upon disk memories in random
fashion in accordance with currently available memory space.
Broadly described, the present invention provides apparatus
for storage of a user defined data set, said apparatus adapted to be connected
to an input/output channel of a host computer, said channel of said host
outputting peripheral commands including specified addresses for storage of
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data sets on peripheral storage means, said apparatus for storage of data
comprising: long-term magnetic storage media for storing data; channel inter-
face means for mating with said channel of said host, device interface means
for connection to said long-term magnetic storage media; data path means con-
necting said channel interface means and said device interface means; and
controller means for maintaining records of available storage locations on
said long-term magnetic storage media, for assigning said locations to sub-
portions of a data to be stored, for dividing said data set by intelligence
external to the host computer into subportions sized to fit available storage
locations on said long-term magnetic storage media, for controlling flow of
data from said channel interface means over said data path means to said
device interface means for storage on said long-term magnetic storage media,
and for maintaining a record showing the correspondence between said addresses
for storage of data output by said host and the actual locations on said long-
term magnetic storage media at which the subportions of said data set are
stored.
According to another aspect of the invention, there is provided method
of improving the efficiency of utility of a data storage subsystem, said sub-
system being adapted to be connected to an input/output channel of a host com-
puter, said host being of the class which passes commands for data storage over
such channels, said commands comprising addresses for storage of data sets on
peripheral devices connected to said channel, the method comprising the steps
of receiving a data set from the host and storing the data set in short-term
solid state storage means connected to said host channel consulting a list of
available storage locations to determine what storage locations are available
for storage of subportions of said data set on long-term magnetic storage
media, dividing said data set by intelligence external to the host computer
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into subportions sized to fit such determined available storage locations on
said magnetic storage media, writing said subportions to said storage locations
on said magnetic storage media, deleting said storage locations from the list
of available storage locations, and maintaining a second list showing the
correspondence between the address for storage of said data output by said
host and the actual locations on storage media which said data set has been
stored.
The invention will now be described in greater detail with reference
to the accompanying drawings in which:
Figure 1 illustrates the overall organization of a virtual storage
system constructed in accordance with the invention;
Figure 2 is a schematic diagram illustrating the flow of control and
information signals within the system of Figure l; and
Figures 3 and 4 are illustrations of the flow of command signals
and information in "read" and "write" situations, respectively.
Figure 1 shows in simplified form the basic architecture of the
virtual storage system of the invention when coupled to a host computer 10.
The invention constitutes means for controlling one or more extremely fast and
accessible disk memory unit(s) to accept data and respond in precisely the
same fashion as a tape drive, while providing much better use of available
storage, thus in effect having vastly expanded storage capability and a far
more rapid response time than either tape or disks. The host computcr 10 may
be of any appropriate type, although it is anticipated that economical opera-
tion of the present invention will dictate its use with a large mainframe host
computer such as *IBM System/360, System/370 computers, and *IBM Models 3031,
* trade mark
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3032, and 3033 processor complexes. The virtual storage system of the inven-
tion can also be used with a plurality of host computers. The storage system
comprises a bank of host interface stages 12 through 12 , the number of inter-
face stages being dependent upon the amount of data to be transferred. Since
the host interface stages are identical, the actual number which is to be used
is for present purposes immaterial. The host interface stages are coupled
to the host computer lO by means of conventional channels 14 and receive and
transmit information to the computer 10 in the ordinary, serialized format
which is associated with tape drives.
One or more disk interface stages 16 through 16 are provided, and
are coupled to all of the host interface stages by means of a common informa-
tion bus 18. In this manner, information may be transferred between any given
host interface stage and any disk interface stage.
The disk interface stages are each coupled to respective ones of
disk memories 20a through 20n. Each disk memory typically comprises a plural-
ity of disks arranged on a common axis but other configurations are possible.
The combination of each disk memory with its associated disk interface stage
forms a disk storage unit. As will be discussed in further detail, each disk
interface stage comprises a memory interface, a data buffer, and a disk inter-
face, along with a local microprocessor controller for operating the various
elements of the interface stage.
Also connected to the main bus 18 is a main memory 22. Memory 22
forms a repository for information flowing from the host interface stages to
the disk interface stages, and may further serve as a memory for control pro-
cessor 24 also coupled to the main bus 18. The control processor 24, which
is the central processing unit or CPU of the virtual storage system, is
coupled to each of the host and disk interface stages 12 and 16 respectively,
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and directs the operation thereof so that information may be received, queued,organized, and stored in the proper manner. The control processor 24 further
instructs the various host interface stages-12 to cause them to respond to
the host computer 10 in a manner which simulates a tape drive.
Turning now to Figure 2, there is- shown in further detail the
structure of exemplary elements of the system. Processor 24 is coupled direct-
ly to host interface stage 12, and in particular to an internal interface 26
which simply facilitates the transfer of signals between the control processor
24 and a microprocessor 28 which forms the controller for the host interface
stage. Also forming part of the host interface stage are a data buffer 30,
a host interface 32 which makes the data buffer compatible with the host
computer 10, and a memory interface 34 through which the data buffer communi-
cates to data bus 18.
The function of the host interface stage 12 is to simulate a tape
drive system to the host computer; that is, to convert the host computer lO's
commands to tape drives into commands to the virtual storage system of the
invention, and to send the host computer 10 data in the form it would be sent
by a tape drive. ~ore particularly, host interface stage 12 accepts the sig-
nals from a host computer 10 which are of the type used to operate the tape
drives. Such signals are obtained by coupling the host interface stage to the
byte multiplex, or block multiplex, or selector channels of the host 10. It
will be appreciated that these signals include operator commands such as
"mount tape" and "demount tape" as well as machine commands such as "read",
"write", and "forward space file". The host interface stage responds to both
kinds of signals as if it were an operator and a tape drive, acknowledging the
signals, responding that "mounting" of an imaginary ~virtual) tape reel has
been accomplished, and the like.
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The data buffer 30 of each host interface stage accepts data from
the host computer 10 in serial form, usually nine bits in parallel, in the
precise manner that it would be reapplied to a tape system for writing upon a
tape. The buffer 30 in conjunction with the host interface 32 during a write
operation deserializes the information; that is, stores it in parallel fields
holding the individual bits until eight entire bytes are available. Typically
72 bits are transmitted at a time thus reducing transmission time. In this
manner, up to 90% compression of the time required to transmit the data on
the line 18 can be achieved. When data is to be exchanged between buffer 30
and the bus 18, the data flows through memory interface 34, again in eight
parallel bytes so that an extremely rapid exchange of data may take place.
The size of said buffer can vary greatly, depending on system requirements
but in a preferred embodiment is large enough, 64,000 bytes, to hold an entire
record, which expedites time sharing of bus 18.
Ultimately, data received from the host computer 10 is written onto
magnetic disks mounted on one or more disk drive units 20a to 20n. Disk
interface stage 16 couples data bus 18 to a disk drive 20 (not shown), and is
comprised of a local microprocessor controller 36 which operates the various
elements of the interface stage in accordance with instructions from control
processor 24. As was the case with the host interface stages 12, with disk
interface stages 16 instructions from the control processor are transmitted
through an interface 38 to the local controller 36. The latter then responds
by causing a data buffer 40J disk interface 42, and memory interface 44 to
transfer data to and from the disk drive.
In particularJ memory interface 44 serves to receive eight bytes of
data from bus 18 and serialize the data so that it is installed in buffer 40
in a single stream of bits. Arrangement and queueing of buffer storage is
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accomplished by the local controller 36, as is the operation of the disk in-
terface 42 which passes data from buffer 40 to the disk drive at the appropri-
ate times. Still further, the disk interface stages serve to seek and log
the locations of data upon the various disks of the disk drive associated
therewith, so that the information can be retrieved when needed.
Between the time data is in a host interface 12 and is received
in a disk interface 16, it is stored in a main memory 22. The high speed
main memory 22 thus acts as a "bank" or "cache" which holds the data until
one of the disk drive units 20 is ready to accept it. When this occurs, the
disk interface stage 42 associated with the disk drive 20 signals control
processor 24 of its availability. The control processor 24 then instructs
the main memory or cache 22 to discharge data to the disk interface stage 42
by way of bus 18.
In like manner, when the host computer 10 is seeking data, the
identity of the data sought is transferred through host interface stage 12 to
the control processor 24, which then determines if the requested data has
been previously moved into the main memory. If it has not, the control pro-
cessor 24 in turn will apply a "transmitter" signal to the disk interface
stage 16, and the local controller 36 therein will cause data to be read out
of the associated buffer 40, through the associated memory interface 44, and
onto the data bus 18, and thence to the main memory 22.
At the same time, the local controller 28 of the available
host interface stage 12 will enable the associated memory interface 34, so
that the newly-read data will be received from main memory 22 and gated into
the host interface data buffer 30. On the other hand, if it has, the data
will be immediately gated out of the buffer 30, serialized by the host inter-
face module 32, and transmitted to the host computer 10.
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In an alternative embodimentJ both host buffers 30 and disk
buffers 40 may be dispensed ~ith, their functions then being performed by
main memory 22.
In this fashion, segments of data which had been dispersed
throughout a number of disk drives are compiled, queued, and then re-assembled
automatically into serial form. Accordingly, the information flowing to the
host computer 10 from the various disk drives 20 appears in serial form,
precisely as if it were being read from a tape.
In the foregoing mannerJ the high speed main memory provides
a cache from which data can be selected and queued for subsequent reassembly
into serial form before being directed to the host computer.
As will be recognized by those skilled in the art upon learning
the teachings of the present invention, the various elements of the present
virtual storage system may be assembled from commercially-available elements,
and coupled together in any convenient fashion. For instance, while all of
the various elements of disk interface stage 16 are illustrated as being dis-
posed in one location, remote from control processor 24, in fact the elements
may be disposed at different locations, and coupled by means of appropriate
cables, buses, or the like. The local microprocessor controllers 24 used for
operating the components of each interface stage need only be of rather limit-
ed capability and may be comprised of any of the various high speed micro-
processors available on the market. An example of such microprocessors is
the LSI-ll marketed by DEC Incorporated of Boston, Mass.; alternatively, a
suitable unit can be assembled from the AMD Co.'s 2900 series of parts. In
like manner, the host and disk interfaces 12 and 16 which accomplish the
serializing and deserializing of information may be standard units, such as
the model 370 block multiplexor available from the IBM Corporation of Armonk,
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New York. In like manner, an IBM block multiplexor may be used for direct
memory access, the actual connection of the various units being well under-
stood by those skilled in the art.
Similarly, the buffers 30 and 40 used for temporary storage
of data in the host and disk interface stages 12 and 16 may be of any appro-
priate type, although in a presently preferred embodiment a memory of at
least 64K bytes is p~eferred. One commercially available buffer of this type
is manufactured by Fairchild Semiconductor, and comprises a N-MOS random ac-
cess memory having a speed of 200 nanoseconds.
The high speed memory 22, which serves as a data cache should
be of the type generally designated a fast access memory, i.e. one having a
cycle time of 400 nanoseconds or less. In a preferred embodiment, the high
speed cache has a capacity of 16 megabytes. One commercially-available memory
appropriate for use with the present invention is manufactured by Intersil
Corp., Sunnyvale, California, and is marketed by Storage Technology Corp. of
Louisville, Colorado, assignee of the present application, and designated
models 3758 and 3768.
Figure 3 is a functional illustration of the operation of the
inventive system in a "read" mode wherein the host computer 10 has requested
information from memory. Control signals are shown by solid lines, and data
flow by dashed lines. Accordingly~ the host 10 produces a command signal to
the human operator requesting certain information ~e.g. "Mount Tape N"), and
since it is assumed by the host 10 that the data is stored on tape in serial
form, only the initial identification or "label" need be specified. Other
elements of the virtual storage system, particularly the control processor 24,
respond to the information by accessing all data which corresponds to the
requested information and causing the data to be assembled in serial form.
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Thus, the host computer 10 simply calls for Tape N which, it
assumes, according to information supplied by the individual program being
run on the host computer 10, contains a particular data set. In fact, accor-
ding to the virtual storage system of the invention, this particular data
set may be spread over a number of different disks and/or may be in a number
of locations on a single disk. Therefore, the virtual storage system of the
invention responds to an operator command such as "Mount Tape N" by searching
through its own memory for data indicating where the various parts of the
record which the host computer has denominated "Tape N" have been stored.
Thus, the response of the system is not to direct an operator to mount Tape
N, but merely to recall the information as to the location(s) of the data set
identified as "Tape N" from its memory.
When the host interface stage 12 indicates that a "Mount Tape
N" instruction has been completed, which is practically instantaneous in com-
parison to the mounting of an actual tape, the host 10 outputs a command to
the interface 12 designating the specific identity of the block of information
to be read. This command is transmitted directly by the host interface stage
12 to the control processor 24 which identifies the various areas in which
the information is stored. This may be accomplished through the use of a
data log 46 which is coupled to the control processor. In a presently prefer-
red embodiment data log 46 comprises a random access memory ~RAM) (which may
be "backed-up" by a tape drive unit and associated tape in case of loss of
memory in the RAM) on which the location of the various sub-blocks of data
which together constitute the data requested by the host computer 10 are
recorded.
When the locations of the various sub-blocks of data have been
identified to the control processor 24, signals are supplied to the controller
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portions of the various disk interface stages 36, whereupon data is accessed
from the various disk storage units 20. The data is then read out into the
buffer 40 of the interface stage 16 and prepared for transmission to the main
memory cache 22 for temporary residence there until needed by the various
host interface stages 12.
When the buffer 40 is filled with data, a signal is outputted
from the disk interface stage 16 to the control processor 24 indicating this
fact, at which time the control processor 24 transmits the buffered informa-
tion directly to a previously-allocated space in the main, high speed memory
cache 22.
With prior art tape systems, the initial "Mount" operation
ordinarily occupies approximately 30 sec. - 5 min. The total time required
to execute a "read" command varies between 1-10 milliseconds depending on the
characteristics of the individual device and of course on the length of the
record being read. In a preferred embodiment ofthe illustrated virtual stor-
age system, a "Mount" operation typically occupies less than one second. The
actual "read" operation typically occupies 1-10 milliseconds as before, de-
pending on record length and the characteristics of the cache 22. At this
point, therefore, the information to have been read will have been very
rapidly placed in the cache 22 where it is stored until a signal from memory
interface 12 indicates to the cache that the host computer is ready to receive
the data. At this time, then, the data has been gathered from the various
points in the various disks 20 where it has been stored, has been put into
serial form in the proper order, and is ready to be read into the host com-
puter 10 with no delay for address time or for searching for the information
on the various disks. Clearly, therefore, the memory system of the invention
offers significant time advantages over disks; and as compared to tape systems,
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avoids the mounting time delays completely. In this way, the "anticipatory
buffering" provided by the use of the cache combines the advantages of disk
and tape in that it combines the lack of mounting required for disks with the
serial format of tapes, thus saving time to the host computer by having the
data completely organized in the cache. At the same time, owing to the
superior organization and cooperation of the various elements, very little of
the actual disk storage is dedicated to directing read/write activity or
maintaining or logging the data thereon, sometimes referred to as the "over-
head" factor in disk memory capacity. Accordingly, a much higher percentage
of actual disk memory is available for data. In this way, the storage
efficiency of disk memory can be upgraded to a level similar to that of tape.
In the host "write" mode illustrated in Figure 4, the procedure
is substantially the reverse of the "read" mode just described, As in Figure
3, control signals are shown by solid lines and the flow of data by dashed
lines. When the host computer 10 signals, through conventional tape drive
codes, that it desires to "write" or record data~ this command is transmitted
through interface stage 12 to the control processor 24. The latter instructs
interface 12 to acknowledge the command, and at the same time interrogates
the disk interface stages 16 in order to find a buffer 40 which will accept
some or all of the data. When such locations are determined, they are logged
in data log 46 by the control processor 24.
The control processor 24 then directs the host interface stage
12 to accept the data from the host computer 10 and direct it to appropriate
locations in the data buffer 30 of the host interface stage 12. Should the
buffer 30 become filled before corresponding space is available in the buffers
40 of the disk interface stages, an "allocate space" instruction is trans-
mitted by the control processor to the memory cache 22, after which data from
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the host interface stage buffer is applied to the memory cache 22. At some
subsequent time, when adequate buffer capacity is available in one or more
disk interface stages 16, the control processor instructs the cache memory
22 to supply the stored data to the appropriate disk interface stage buffers
16 for disposition upon the disks of the associated disk drives 20.
As each block of data is applied to a host interface 32 by
the host 10, the memory interface module 34 determines where in the associat-
ed buffer 30 the newly-received information will be stored; compresses the
data in serial fashion so as to produce, for instance, eight channels of
data; and adds an identification bit or bits to the information block
indicating the size of the block. In this manner, data is continually fed
to the buffers 30 until a given block of data is filled or until the host
computer generates a signal indicating the end of a given record. In accor-
dance with an important aspect of the present invention, such a signal is
interpreted by the local controller 28 as an end of message signal subsequent
to which no more buffer memory space need be allocated.
At this point, the control processor 24 directs the data now
stored in data buffer 30 to the high-speed cache 22. During the time the
host computer is storing data in data buffer 30 as described above, the
control processor 24 can be "looking for" sufficient space upon disk drives
20, and can allocate that space to the data then being stored in data buffer
30. Thus, when the end of record signal is received in the control processor,
data it can be serialized and fed into the high speed cache, as a temporary
storage space, and is then passed to disk drives 20 for storage.
Put differently, when the host computer produces an initial
"Mount Tape" command (i.e. directs the operator to provide a blank tape for
data storage purposes) this command is passed through the host interface
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stage 12 to control processor 24. The control processor 24 responds to the
"Mount Tape" instruction by seeking space upon one or more disks 20, and by
reserving adequate space in one or more of the disk interface stage buffers
30. This done, the control processor 24 instructs the host interface stage
12 to accept data from the host 10 and to deserialize it and store it in the
associated buffer 30. Once this buffer is full, if no more buffer space is
available, this data is passed to the memory cache 22. When sufficient disk
space is available, the next step is for the control processor 24 to instruct
the disk control interface 42 of the disk interface stage 16 to write the
data upon the associated disk device 20 after which the interface module 44
reads and serializes the information, placing it in its associated buffer 40,
whence it is written onto the associated disk 20. It will be understood that
another possibility is to have the data copied or stored onto a conventional
tape drive, if for example duplication be deemed desirable; that is, the data
storage system of the inventîon can be paralleled with a conventional prior
art tape system, if desired.
It will be apparent to those skilled in the art that the
association of a high speed cache with a control processor, along with host
interface and disk interface stages and associated control modules permits the
combination of the advantages of prior art tape and disk drive systems;that
is, the ready accessibility of disk drives is combined with the storage
efficiency of tape drives to yield a fast high efficiency "virtual storage
system" in which a disk system is made, through the intermediaries of the
invention, to look to the central processor like a tape drive, but without
the drawbacks thereof. Since the memory disks according to the invention
are permanently mounted, there is no need for an operator's intervention when
; a tape-type record (i.e. a sequential record) is sought. Moreover, prior art
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disk drives can be used with the virtual storage system of the invention
without modification; that is, the system of the invention can be viewed as
a unit or interface inserted between a prior art host computer and prior art
disk drives. Therefore, there is no need to modify the disk drives or the
host computer in order that they may be operated in conjunction with the
virtual storage system of the invention.
Furthermore, it will be appreciated that the virtual storage
system of the invention can be used to interface between any number of host
computers and any number of disk drives. Therefore, it will be appreciated
that should an operator have more than one host compute and a limited
number of disk drives, the virtual storage system of the invention can be
used to achieve maximum storage efficiency and eliminate undue multiplication
of disk or tape storage systems. The host interface units of the invention
may be coupled to more than one host computer. Moreover, it will be appreci-
ated that the provision of a high speed memory or cache is essential to the
"anticipatory buffering" which is a very important feature of the invention.
By using the high speed cache as an intermediate buffer between the host
computer interface and the various disk drive interface stages, buffering of
multiplex data can be accomp~shed; that is, according to the invention data
stored in a number of places upon a number of disk drives ~referred to as "in
random form") can be assembled, sequenced, and stored temporarily in the high
speed cache until it is called for by the host computer. In this way, there
;~ need be no delays in reading the information from memory into the host com-
puter. In a similar fashion, data output from the host computer can be
arranged, divided, and stored in the high speed cache until such time as
various disk storage areas are available to store the information.
A particularly significant possibility which is provided by the
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storage system of the invention is that of data compression. Heretofore a
common scheme of data CompTessiOn, which involves the replacement of a long
string of digital "l's" or "O's" with symbols indicating the length of the
string, had not been useful with disk drive units, since this scheme com-
pressed addressing information as well as data. However, since according to
the present application disk drives appear as tape, this difficulty of the
prior art is obviated, and data compression is made compatible with disk
storage. Preferably, data compression is implemented in the host interface
stage;that is during a write operation long continuous strings of "l's" or "O's"
may be detected and replaced with shorter symbols; during a read operation,
these are detected and replaced by the thus-defined data.
Finally, it will be appreciated that there are a number of
other modifications and refinements which can be made to the virtual storage
system of the invention and that the examples discussed above are exemplary
only, and that therefore thescope of the invention should be interpreted as
limited only by the following claims.
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