Electrical computers and digital processing systems: memory – Storage accessing and control – Specific memory composition
Reexamination Certificate
2001-01-05
2003-09-30
McLean-Mayo, Kimberly (Department: 2187)
Electrical computers and digital processing systems: memory
Storage accessing and control
Specific memory composition
C711S154000
Reexamination Certificate
active
06629203
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to data storage in a computerized storage unit, such as a storage array in a storage area network (SAN). More particularly, the present invention relates to improved management of stored data in the storage unit using alternating “shadow” directories for updating directories in a logical volume in which data accesses are performed primarily to add records to the database.
BACKGROUND OF THE INVENTION
Current computerized data storage systems typically contain data within logical volumes formed on one or more storage device or array of storage devices. For a logical volume, storage management software typically allocates and manages an amount of storage space that “logically” appears to be a single “volume,” file or database, but physically may be several files or “sub” volumes distributed across several storage devices and/or arrays of storage devices.
In addition to the data, the logical volumes also typically contain directories for the data. Typically, a directory hierarchy is used with a root directory and one or more subdirectories arranged in multiple levels. Each directory in the hierarchy is typically contained in a “block” of storage space and includes pointers either to other storage blocks containing additional directories (“directory storage blocks”) or to storage blocks containing the data (“data storage blocks”). Therefore, when a data storage block is accessed in the logical volume (e.g. to read data from or write data to the data storage block), it is the directories in the directory storage blocks that point to the location of the data storage block, so software can access the data storage block.
The “active” data and directories for the logical volumes with which the software is operating are typically kept in a main memory, or RAM, of the storage devices or storage arrays. Copies of the data and directories for the logical volumes are kept on the hard drives or other mass storage devices. Whenever data is needed that is not currently in the RAM, the data is copied to the RAM to be used. Periodically, the data and directories in the RAM are stored to the hard drives. Whenever a problem, such as a power failure, causes a loss of the data in the RAM, the data is copied from the hard drives to the RAM and operations resume at the point at which the data was last stored to the hard drives.
When data is added to or updated in a data storage block in the logical volume, one or more of the directories in the logical volume must be updated and/or new directories must be created to include pointers to the new data storage block. Subsequently, the updated and/or new directories and the new data storage block are stored to the hard drives. There is the potential of losing some data, or data coherency, in the logical volume if the data and/or directories are being updated or stored to the hard drives at the moment that a problem (e.g. a power failure) occurs. Therefore, updates and changes are typically not made directly to the existing directory and data storage blocks on the hard drives, so the existing information will not be lost or corrupted.
One technique to prevent loss of information involves allocating new directory storage blocks for the affected directories and storing the currently active directories from the RAM to the new directory storage blocks (“shadow directories”), instead of to the existing directory storage blocks. During the storing of the directories from the RAM to the shadow directories on the hard drives, the previously existing directories on the hard drives are still considered the most recently stored directories for the purpose of restoring the data and directories in the RAM in the case of loss of data in the RAM. Therefore, if a problem results in loss of data in the RAM while the shadow directories are being stored, the previously existing most recently stored directories are used to restore the data in the RAM without loss of data coherency.
The highest level directory, or “root” directory, in the directory hierarchy is typically stored last, after the data and lower level directories have been stored to the hard drives. The new root directory includes the time at which it was stored, so the software can determine which root directory is the most recently stored root directory for the purpose of restoring the data and directories, if needed. Therefore, the act of storing the new root directory effectively “activates” the new root directory and all of the lower level directories linked thereto and the new data in a transition that takes such a short time that the likelihood of the occurrence of a problem is very low.
An exemplary directory hierarchy
100
for a logical volume
102
that is updated with a shadow directory technique is shown in
FIGS. 1 and 2
. In
FIG. 1
, the state of the directory hierarchy
100
on the hard drives (not shown) is shown in a progression through five different states
104
,
106
,
108
,
110
and
112
as the logical volume
102
(
FIG. 2
) is stored from the RAM (not shown) to the hard drives each time that a data record
114
,
116
,
118
,
120
and
122
is added to the logical volume
102
. The data hierarchy
100
is shown as having three directory levels
124
,
126
and
128
. The logical volume
102
is shown as having 18 storage blocks
130
-
164
for directories or data.
The data record
114
is the first data record to be written to the logical volume
102
(FIG.
2
), resulting in the creation of initial root and level
2
and
3
directories
166
,
168
and
170
in the directory hierarchy
100
(see state
104
). The directories
166
,
168
and
170
and the data record
114
are stored from the RAM (not shown) to the hard drives (not shown) without shadow directories, since these directories
166
,
168
and
170
are the initial directories. For states
106
-
110
, the data records
116
,
118
and
120
are added to the logical volume
102
at different levels in the directory hierarchy
100
. The state
112
results from replacing one of the previously added data records (data record
118
) with data record
122
and “wrapping around” data storage from the last storage block
164
to the first available storage block
130
.
For each of the states
106
-
112
that follow the initial state
104
, one or more of the current directories on the RAM (not shown), including the current root directory, are stored from the RAM to shadow directories on the hard drives (not shown), so the updates due to each added data record
116
-
122
can occur to the shadow directories, while the previously existing directories are still considered the most recently stored directories on the hard drives. Additionally, in some cases, new directories are added to the logical volume
102
(FIG.
2
).
For example, for state
106
, the data record
116
is added to the logical volume
102
(
FIG. 2
) at the level
3
directory
170
in the RAM (not shown), so the initial root and level
2
and
3
directories
166
,
168
and
170
are updated in the RAM. When it is time to store the initial root and level
2
and
3
directories
166
,
168
and
170
to the hard drive (not shown), they are stored to shadow root and level
2
and
3
directories
172
,
174
and
176
, respectively, on the hard drive. Additionally, the data record
116
is added to the logical volume
102
on the hard drive. The shadow level
3
directory
176
includes a pointer to the data record
116
. The shadow level
2
directory
174
includes a pointer to the shadow level
3
directory
176
, and the shadow root directory
172
includes a pointer to the shadow level
2
directory
174
.
After the updated root directory is stored from the RAM (not shown) to the shadow root directory
172
on the hard drive (not shown), the shadow root directory
172
becomes the current most recently stored root directory, effectively “activating” the level
2
and
3
directories
174
and
176
and “removing” the initial root and level
2
and
3
directories
166
,
168
and
170
from the directory hierarch
John R. Ley, LLC
LSI Logic Corporation
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