Electrical computers and digital processing systems: memory – Storage accessing and control – Specific memory composition
Reexamination Certificate
1999-09-01
2002-12-10
Kim, Matthew (Department: 2186)
Electrical computers and digital processing systems: memory
Storage accessing and control
Specific memory composition
C710S022000, C709S213000
Reexamination Certificate
active
06493796
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a method and apparatus for maintaining consistency of data stored in a group of mirroring devices.
DESCRIPTION OF THE RELATED ART
A number of computer system configurations mirror information stored in source storage devices to target storage devices that can be disposed at locations remote from the source storage devices. An example of such a computer system is a data mirroring system
100
discussed below in connection with FIG.
1
.
In the data mirroring system
100
of
FIG. 1
, one or more host CPU's
102
are coupled (via one or more communication links
114
) to a source storage system
104
. In the example shown, the source storage system
104
includes a plurality of “source” storage devices
110
a-f
. The source storage devices
110
a-f
can be included in a single source storage system
104
as shown in
FIG. 1
, or multiple source storage systems
104
can be employed, and the source storage devices
110
a-f
can be distributed across the multiple source storage systems. Each source storage system
104
may, for example, comprise a storage system from the SYMMETRIX line of disc arrays available from EMC Corporation, Hopkinton, Mass.
The data stored in the source storage devices
110
a-f
may be crucial to the operation of the host CPU(s)
102
. Therefore, a contingency solution is desirable in the event that a problem is experienced with any of the source storage devices
110
a-f
to ensure that the data stored thereby is not lost, and to minimize the risk of the host CPU(s)
102
being down due to problems with the source storage devices
110
a-f
. Potential problems with the source storage devices
110
a-f
may include, for example, hardware or software errors that may make stored data unrecoverable. Additionally, catastrophic events such as an earthquake or other natural disaster could result in the destruction of one or more of the source storage devices
110
a-f.
One solution for protecting the data stored in the source storage devices
110
a-f
is to mirror the data in a corresponding set of “target” storage devices. The data mirroring system
100
of
FIG. 1
is an example of such a system, wherein the source storage system
104
is coupled (via links
112
a-f
) to a target storage system
106
. The target storage system
106
includes a plurality of target storage devices
116
a-f
corresponding, respectively, to the source storage devices
110
a-f
in the source storage system
104
. As data is written to any of the source storage devices
110
a-f
, it is also written to a corresponding one of the target storage devices
116
a-f
in the target storage system
106
. If one of the source storage devices
110
a-f
is destroyed or experiences an error that renders stored data unrecoverable, the data can be retrieved from the corresponding one of the target storage devices
116
a-f
. The target storage devices
116
a-f
can be included in a single target storage system
106
as shown in
FIG. 1
, or multiple target storage systems
106
can be employed, and the target storage devices
116
a-f
can be distributed across the multiple target storage systems
106
. As with the source storage system(s)
104
, each of the target storage systems
106
may, for example, be a storage system from the SYMMETRIX line of disc arrays available from EMC Corporation, Hopkinton, Mass.
When each of the source and target storage systems
104
and
106
is implemented using one or more of the SYMMETRIX line of disk arrays available from EMC Corporation, a feature called SYMMETRIX Remote Data Facility (SRDF) can be employed to implement the connection therebetween. SRDF is described in numerous publications available from EMC Corporation, including the SYMMETRIX Remote Data Facility Product Manual, P/N 200-999-554, rev. B, June 1995. SRDF is also described in U.S. Pat. No. 5,544,347 (Yanai).
In the illustrative data mirroring system
100
of
FIG. 1
, first, second, and third sets of source storage devices
110
a-b
,
110
c-d
, and
110
e-f
, respectively, in the source storage system
104
are in communication (via links
112
a-b
,
112
c-d
, and
112
e-f
, respectively) with corresponding first, second, and third sets of target storage devices
116
a-b
,
116
c-d
, and
116
e-f
in the target storage system
106
. The links
112
are referred to herein as “mirroring links,” and the term “mirroring communication” is used herein to refer to communication between source and target storage systems
104
and
106
over the mirroring links
112
that permits the target storage devices
116
a-f
to mirror data stored by the source storage devices
110
a-f
. As shown in
FIG. 1
, the source storage devices
110
a-b
, the mirroring links
112
a-b
, and the target storage devices
116
a-b
constitute a first mirroring group
108
a
; the source storage devices
110
c-d
, the mirroring links
112
c-d
, and the target storage devices
116
c-d
constitute a second mirroring group
108
b
; and the source storage devices
110
e-f
, the mirroring links
112
e-f
, and the target storage devices
116
e-f
constitute a third mirroring group
108
c
. A “mirroring group” is a group of storage devices in the source and target storage systems
104
and
106
between which data can be transferred via a common set of mirroring links
112
. Generally, storage devices
110
and
116
do not share any mirroring links
112
with storage devices
110
and
116
in other mirroring groups
108
.
To perform a write operation to the source storage system
104
, a host CPU
102
transmits an “I/O command chain” to the source storage system
104
. Each I/O command chain can include one or more commands directed to a particular logical volume stored by the source storage system
104
. A logical volume is a unit of information that the host CPU
102
perceives to correspond to a single physical storage device, but that may be mapped within the source storage system
104
to physical storage space on one or more source storage devices
110
. An I/O command chain for a write operation (“a write-requesting command chain”) includes various instructions for the source storage system
104
receiving the I/O command chain, as well as the data to be written during the operation. The host CPU
102
generally receives only a single message back from the source storage system
104
in response to its sending of a write-requesting command chain. This message generally indicates either: (1) that the I/O command chain has been “committed” (i.e., processed successfully) by the source storage system
104
, or (2) that an error occurred preventing the I/O command chain from being committed by the source storage system
104
. Typically, the source storage system
104
will not provide the “I/O command chain committed” message back to the host CPU
102
unless and until the data has been written successfully to one or more of the source storage devices
110
, or to a temporary storage space (such as cache) in the source storage system
104
.
One of the purposes for mirroring data in the target storage system
106
is that if the data on the source storage system
104
is rendered unrecoverable, the data can be retrieved from the target storage system
106
. Thus, it is desirable to ensure that the data stored in the target storage system
106
is internally consistent, and reflects an accurate mirror of the data stored in the source storage system
104
at some particular point in time. If the data in the target storage system
106
does not represent an accurate mirror of the data in the source storage system
104
for a given point in time, the data in the target storage system
106
cannot be reloaded onto the source storage system
104
to place the source storage system
104
back into a valid state. For example, if the data stored in the source storage system
104
and mirrored on the target storage system
106
is included in a database, but the information stored in target storage system
106
does not reflect an accurate picture of the database at a given point
Arnon Dan
LeCrone Douglas E.
Ofek Yuval
Chace Christian P.
EMC Corporation
Giunta Richard F.
Gunther John M.
Kim Matthew
LandOfFree
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