Abstract: A system for efficient snapshot querying include: providing a first version value for each data in a history table, where the first version value indicates a beginning of a period in which the data is valid; providing at least a second version value for each data in the history table, where the second version value indicates an end of the period in which the data is valid; receiving a request for a version of a database; and retrieving the data with the first version value less than or equal to the requested version and the second version value greater than or equal to the requested version. By maintaining a history table with redundant version information, the system is able to provide efficient snapshot querying while also avoiding the overhead burden of conventional approaches. No aggregates, joins, or sub-queries are required to retrieve a snapshot.

Abstract: A system for synchronizing data on a first device with data at a second device, includes: determining a pre-image of the data on the first device according to a filter after a previous synchronization with the data at the second device; determining a current image of the data at the second device according to the filter; determining a delta of the current image, where the delta comprises data that exists in the current image and has changed since the previous synchronization; instructing the first device to delete data that exists in the pre-image but not in the current image; and instructing the first device to UPSERT a union of the data that exists in the delta and the data that exists in the current image but not the pre-image. This is efficient in time and bandwidth resources while also providing consistent data integrity.

Abstract: A method and system for efficient snapshot querying include: providing a first version value for each data in a history table, where the first version value indicates a beginning of a period in which the data is valid; providing at least a second version value for each data in the history table, where the second version value indicates an end of the period in which the data is valid; receiving a request for a version of a database; and retrieving the data with the first version value less than or equal to the requested version and the second version value greater than or equal to the requested version. By maintaining a history table with redundant version information, the method and system are able to provide efficient snapshot querying while also avoiding the overhead burden of conventional approaches. No aggregates, joins, or sub-queries are required to retrieve a snapshot.

Abstract: A system for efficient snapshot querying include: providing a first version value for each data in a history table, where the first version value indicates a beginning of a period in which the data is valid; providing at least a second version value for each data in the history table, where the second version value indicates an end of the period in which the data is valid; receiving a request for a version of a database; and retrieving the data with the first version value less than or equal to the requested version and the second version value greater than or equal to the requested version. By maintaining a history table with redundant version information, the system is able to provide efficient snapshot querying while also avoiding the overhead burden of conventional approaches. No aggregates, joins, or sub-queries are required to retrieve a snapshot.

Abstract: A system for synchronizing data on a first device with data at a second device, includes: determining a pre-image of the data on the first device according to a filter after a previous synchronization with the data at the second device; determining a current image of the data at the second device according to the filter; determining a delta of the current image, where the delta comprises data that exists in the current image and has changed since the previous synchronization; instructing the first device to delete data that exists in the pre-image but not in the current image; and instructing the first device to UPSERT a union of the data that exists in the delta and the data that exists in the current image but not the pre-image. This is efficient in time and bandwidth resources while also providing consistent data integrity.

Abstract: A method and system for synchronization-replication concurrency maintain a history table for a data table in a mirror, where the history table can include redundant version values that indicate a period in which data values corresponding to the version values are valid. Replication can thus be performed between a source and a mirror for a current version of the data table, while synchronization is simultaneously performed between the mirror and a client for an earlier version of the data table using the history table.

Abstract: A method for synchronizing data on a first device with data at a second device, includes: determining a pre-image of the data on the first device according to a filter after a previous synchronization with the data at the second device; determining a current image of the data at the second device according to the filter; determining a delta of the current image, where the delta comprises data that exists in the current image and has changed since the previous synchronization; instructing the first device to delete data that exists in the pre-image but not in the current image; and instructing the first device to UPSERT a union of the data that exists in the delta and the data that exists in the current image but not the pre-image. This is efficient in time and bandwidth resources while also providing consistent data integrity.

Abstract: A system and method for compensation for variation in a voice coil motor's torque factor due to changes in temperature to improve the seek performance of a disk drive. The method uses the numerical relationship between change in torque factor and change in temperature (G) for the voice coil motor, combined with torque factor KT and temperature at/near the voice coil motor measured at a first time (for example during the initial start-up of the disk drive during the automatic recalibrations that is performed during a start-up, (KT RECAL) and (TRECAL) respectively). These KRECAL and TRECAL can be stored in memory. At some later predetermined interval or event, the temperature at or near the voice coil motor is again measured (TEVENT). From these known temperature and torque factor parameters, in combination with the known relationship between temperature and torque factor for the voice coil motor, the KT at the future time/event (KT EVENT) can be calculated.

Abstract: A notch filter for a disk drive servo system is disclosed having selectable filtering capability. Different servo notch filters for different heads in the actuator head stack are provided. The notch filters may be selected to remove different resonant modes associated with the different disk drive heads. Each of the plurality of notch filters is tuned to remove a resonant frequency associated with at least one magnetic head of the disk drive. In operation, a notch filter is selected to remove a resonant frequency associated with a magnetic head commanded to write or read data to or from the disk drive. The notch filter system includes a multiplexor for controlling the selection of the notch filters. The multiplexor receives commands from a servo processor to control the selection of a notch filter. Different notch filters can be utilized, including series tapped notch filters or parallel notch filters.

Abstract: A computer disk drive unit having a control system that uses a separate velocity profile for read seeks and for write seeks is disclosed. Appropriate velocity profiles are provided according to whether a seek operation is a read seek operation or a write seek operation. The control system includes a memory for storing an initial velocity profile and a processor for determining whether a read seek operation or a write seek operation is to be performed. The processor obtains the initial velocity profile from memory to produce a read velocity profile or a write velocity profile. The processor multiplies the velocity profile obtain from the memory by a predetermined constant to derive the read velocity profile when the processor determines that a read operation is to be performed. Then the processor repositions the head according to the write velocity profile or the read velocity profile.

Abstract: Disclosed is a digital control servo system for read/write head positioning in data recording disk files of the type which use voice coil motor (VCM) actuators for positioning the head, with improved modelling of the servo system in saturated seek mode. The VCM has a characteristic torque factor which varies in accordance with a predetermined relationship to head position and with age and temperature, and the servo system drives the VCM in a saturated seek mode at a maximum level of current which has a low frequency variation due primarily to temperature change. The servo system computes an estimated VCM control representing the VCM current, computes estimates of the state of the VCM including head position and velocity, and computes from an algorithmic model, predictions of head position and velocity. The improved modelling changes the algorithmic model to reflect the actual torque factor of the VCM and actual maximum level of current in saturated seek mode rather than nominal torque factor and VCM current.