KR102062045B1 - Garbage Collection Method For Nonvolatile Memory Device - Google Patents

Abstract

The present invention relates to a garbage collection method of a nonvolatile memory device for improving data recovery performance of the nonvolatile memory device. The garbage collection method of a nonvolatile memory device comprises the steps of: storing valid data and invalid data in a plurality of pages in a memory area having a plurality of blocks including the plurality of pages; classifying the invalid data into first and second invalid data according to a possibility of a data recovery request; selecting a victim block among the plurality of blocks based on the valid data and the number of the first and second invalid data; transferring the valid data stored in the victim block to a free block among the plurality of blocks; and erasing the victim block.

Description

Garbage collection method for nonvolatile memory device

The present invention relates to a garbage collection method of a nonvolatile memory device, and more particularly, to a garbage collection method of a nonvolatile memory device capable of improving data recovery performance.

Storage devices are used to store data such as text, audio, and video, and semiconductor memory devices are used as such storage devices.

Semiconductor memory devices are classified into volatile memory devices and nonvolatile memory devices.

Here, data stored in the volatile memory device is destroyed when the power supply is interrupted, whereas data stored in the nonvolatile memory device is not destroyed even when the power supply is interrupted.

One type of nonvolatile memory device is a flash memory device. As a computer system moves to a mobile area, a demand for a flash memory device is increasing.

Such flash memory devices have the advantages of low power consumption, high integration, small size and light weight. In recent years, as the use of portable information devices such as mobile phones, PDAs, digital cameras, and the like proliferates, flash memory devices are widely used as storage devices instead of hard disks.

Flash memory, unlike hard disks, does not support data overwrite. Therefore, an erasing operation must be preceded in order to program data into the flash memory.

Such an erase operation of the flash memory generally takes a very long time compared to a program operation. In addition, since the erase unit (block) of the flash memory is much larger than the program unit (page), it can be erased together to a portion that does not need to be erased.

In addition, data stored in the flash memory can be deleted by a user's command. In general, erasing of data is performed to increase the storage capacity of the flash memory.

In some cases, however, there is a need to recover deleted data. For example, if data is deleted, overwritten or corrupted due to a user's mistake or damage to the storage device, the data should be recoverable.

On the other hand, newly written data in the flash memory area is stored in the free page and displayed as valid data, and deleted or overwritten data is displayed as invalid data. However, as such invalid data increases, free space for write operations in the flash memory area is reduced.

To solve this problem, the flash memory device performs garbage collection as an internal mechanism.

This garbage collection recycles the memory area where invalid data is stored to the space for write operations.

1 is a diagram illustrating a general garbage collection.

As shown in FIG. 1, the memory area includes a plurality of blocks, and the plurality of blocks includes a plurality of pages. Valid data and invalid data are stored in each page included in each block.

Here, a page in which valid data is stored is referred to as a valid page, a page in which invalid data is stored is referred to as an invalid page, and data is stored. An unsaved page will be referred to as a free page.

In general garbage collection, first, a victim block (Victim Block) to be erased is selected from among a plurality of blocks. Herein, a block 2 having the most invalid pages may be selected as the victim block.

Next, another block (Block 3) including only free pages of valid data stored in a valid page of a victim block, that is, a free block in which no data is stored, is included. ).

Next, the victim block is erased to free space for a write operation in the memory area.

Garbage collection like this causes memory performance to be lost due to the transfer of valid data. Accordingly, the previous operation of valid data is minimized by selecting the block having the most invalid pages as the victim block and erasing it.

However, as the victim block is erased, invalid data stored in the victim block becomes impossible to recover. As a result, a problem may occur that degrades the data recovery function of the nonvolatile memory device.

Patent Publication No. 10-2016-0075229

The present invention can prevent a block having a large number of valid data and invalid data having a high possibility of requesting data recovery from being selected and erased as a victim block, thereby extending the lifespan of invalid data having a high possibility of requesting data recovery. It is an object of the present invention to provide a garbage collection method of a nonvolatile memory device that can improve data recovery performance.

Technical problems to be achieved in the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned above may be clearly understood by those skilled in the art from the following description. There will be.

In order to solve this problem, the present invention provides a method for storing invalid data and invalid data in a plurality of pages in a memory area including a plurality of blocks including a plurality of pages, and according to a possibility of requesting data recovery. Classifying the first and second invalid data into a plurality of blocks; selecting a victim block among a plurality of blocks based on the valid data and the number of the first and second invalid data; and valid data stored in the victim block. A method of garbage collection of a nonvolatile memory device is provided. The method includes moving to a heavy free block and erasing a victim block.

Here, the second invalid data is data having a lower probability of a data recovery request than the first invalid data as previous version data of the first invalid data.

In addition, the selecting of the victim block includes selecting a block having the smallest number of valid data and first invalid data among the plurality of blocks as the victim block.

In addition, the step of selecting the victim block is selecting a block having the smallest significant block value among the plurality of blocks as the victim block.

Here, the critical block value is defined by <Equation> significant block value = (number of valid data / number of pages in a block) + (number of first invalid data / number of pages in a block) × weight.

Here, the weight is determined in the range of 0 to 1.

In addition, the selecting of the victim block is performed when the available memory area among the memory areas is less than or equal to the threshold value.

Further, the second invalid data is the previous version data of the first invalid data, and the first invalid data is the previous version data of the valid data.

The classifying into the first and second invalid data may be classified using a tracking table for tracking the relationship between the valid data, the first invalid data, and the second invalid data.

The method may further include transferring first invalid data stored in the victim block to a free block among the plurality of blocks.

According to the present invention, a block having a large number of valid data and invalid data having a high probability of requesting data recovery can be selected as a victim block and erased, thereby extending the lifespan of invalid data having a high possibility of requesting data recovery. This has the effect of improving data recovery performance.

The effects of the present invention are not limited to those mentioned above, and other effects not mentioned above will be clearly understood by those skilled in the art from the following description.

1 is a diagram illustrating a general garbage collection.
2 is a diagram for describing a garbage collection method of a nonvolatile memory device according to an exemplary embodiment of the present invention.
3 is a state transition diagram of a page.
4 is a flowchart of a garbage collection method of a nonvolatile memory device according to an embodiment of the present invention.
5 is a diagram illustrating weights and significant block values for selecting a victim block.
FIG. 6 is a table illustrating a tracking table applied to a garbage collection method of a nonvolatile memory device according to an embodiment of the present invention.

The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best explain their invention in the best way possible. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

Therefore, the embodiments shown in the specification and the configuration shown in the drawings are only the most preferred embodiments of the present invention and do not represent all of the technical idea of the present invention, various equivalents that may be substituted for them at the time of the present application It should be understood that there may be variations and examples.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention.

A nonvolatile memory device according to an embodiment of the present invention may have a software hierarchy in order of an application, a file system, a flash translation layer, and a flash memory.

The file system receives data store commands, data read commands, data move commands, data delete commands, and data recovery commands from the application, and the logical address of the data to be processed ( logical address) to the flash translation layer.

The flash translation layer receives a logical address from a file system and converts the logical address into a physical address by referring to an address mapping table.

Here, the address mapping table includes a plurality of mapping data, and each mapping data defines a correspondence relationship between a logical address and a physical address. The physical address is then provided to the flash memory.

The flash memory is divided into a meta data area and a user data area.

Here, user data such as text, video, and audio are stored in the user data area, and information regarding the user data, for example, location information of the user data, is stored in the meta data area. Accordingly, the file system can grasp the storage location of the user data by referring to the metadata area.

2 is a diagram for describing a garbage collection method of a nonvolatile memory device according to an embodiment of the present invention, FIG. 3 is a state transition diagram of a page, and FIG. 4 is a diagram of a nonvolatile memory device according to an embodiment of the present invention. 5 is a flowchart illustrating a garbage collection method, and FIG. 5 illustrates weights and significant block values for selecting a victim block.

As shown in FIG. 2, in a flash memory device according to an exemplary embodiment of the present invention, a memory area includes a plurality of blocks, and the plurality of blocks includes a plurality of pages. .

The garbage collection method of the flash memory device according to an embodiment of the present invention is preferably executed when the usable memory area of the flash memory area is less than or equal to a threshold.

According to an embodiment of the present invention, a garbage collection method of a flash memory device recovers data by predicting invalid data having a high probability of requesting data recovery, thereby extending the life expectancy of invalid data. It is characterized by improving the performance.

Here, valid data is data newly written to a free page in which no data is stored, and a page in which the valid data is stored is a valid page from a free page. Is changed to

The invalid data is data that is deleted or overwritten, and the page in which the invalid data is stored is changed from a valid page to an invalid page.

The garbage collection method of a flash memory device according to an embodiment of the present invention uses a version of invalid data to predict a possibility of a data recovery request.

That is, the invalid data may include several versions of invalid data, and in this case, it is highly likely that the most recent version of invalid data is used as the recovery data. Therefore, the data recovery performance is improved by extending the life of the most recent version of Invalid Data.

Here, the most recent version of Invalid Data is defined as first invalid data (sInvalid Data), and the previous version of Invalid Data is defined as second invalid data (dInvalid). Data).

That is, the second invalid data dInvalid Data is previous version data of the first invalid data sInvalid Data, and the first invalid data sInvalid Data is previous version data of valid data.

As shown in FIG. 3, each page transitions to a free state, a valid state, a first invalid state, a second invalid state, and the like. do.

The first transition (Transition 1) is performed when the first data is stored in a free page by a write operation, in which case the page state transitions from the free state to the valid state. . That is, the first data becomes valid data and the free page is changed to a valid page.

The second transition (Transition 2) is performed when the second data is stored in another free page by a delete or overwrite operation, in which case the page state is from the valid state to the first invalid state ( sInvalid State). That is, the valid data becomes first invalid data and the valid page is changed to a first invalid page.

Here, the second data becomes valid data and a free page in which the valid data is stored is changed to a valid page.

The third transition (Transition 3) is performed when the third data is stored in another free page by the next delete or overwrite operation, in which case the page state is removed from the first invalid state (sInvalid State). 2 transition to the invalid state (dInvalid State). That is, the first invalid data sInvalid Data becomes the second invalid data dInvalid Data, and the first invalid page sInvalid Page is changed to the second invalid page dInvalid Page.

Here, the second data becomes the first invalid data (sInvalid Data), the valid page in which the first invalid data (sInvalid Data) is stored is changed to the first invalid page (sInvalid Page), and the third data is The free page that becomes valid data and stores the valid data is changed to a valid page.

The fourth transition (Transition 4) is performed when the first invalid data (sInvalid Data) stored in the first invalid page (sInvalid Page) by the erase operation is erased, in which case the page state is the first invalid state (sInvalid State) Transitions to the free state. That is, the first invalid data sInvalid Data is erased, and the first invalid page sInvalid Page is changed to a free page.

Here, it is obvious that the first invalid data may not be a recovery target because the first invalid data has already been erased.

The fifth transition (Transition 5) is performed when the second invalid data (dInvalid Data) stored in the second invalid page (dInvalid Page) by the erase operation, in which case the page state is the second invalid state (dInvalid State) Transitions to the free state. That is, the second invalid data dInvalid Data is erased and the second invalid page dInvalid Page is changed to a free page.

Meanwhile, the flash memory area of the nonvolatile memory device according to an embodiment of the present invention includes a plurality of blocks, and a block to be erased among the blocks is defined as a victim block. A block in which no data is stored will be defined as a free block.

As described above, in the garbage collection method of the nonvolatile memory device according to the embodiment of the present invention, invalid data (Invalid Data) and second invalid data (dInvalid Data) are converted into invalid data through transitions between pages. ), And a block having the smallest number of valid data and first invalid data is selected as a victim block.

In other words, a block having a large number of valid data and first invalid data having a high probability of requesting data recovery is prevented from being selected as a victim block, thereby preventing the first invalid data ( sInvalid Data) can be extended, thereby improving data recovery performance.

In addition, valid data and first invalid data stored in the victim block may be transferred to a free block before the victim block is erased.

Such garbage collection can cause performance degradation of memory due to the transfer of valid data.

However, the garbage collection method of the nonvolatile memory device according to an embodiment of the present invention selects a block having the largest number of second invalid pages as a victim block and erases the valid data. The data recovery performance can be improved while minimizing the previous operations of the Valid Data and the first Invalid Data.

As shown in FIG. 4, the garbage collection method of the nonvolatile memory device according to an embodiment of the present invention includes storing valid data and invalid data in a plurality of pages. S10) and classifying invalid data into first invalid data and second invalid data according to the possibility of a data recovery request (S20), valid data (Valid Data), Selecting a victim block from among the plurality of blocks based on the number of the first invalid data and the second invalid data in operation S30, and the victim block; Transferring the valid data stored in the (Victim Block) to a free block of the plurality of blocks (S40) and the step of erasing the victim block (Victim Block) (S50).

Here, the second invalid data dInvalid Data is data of a previous version of the first invalid data sInvalid Data, and the data recovery request probability is lower than that of the first invalid data sInvalid Data.

In step S30 of selecting a victim block, the victim block may include a block having the smallest number of valid data and first invalid data among the plurality of blocks. Block) step.

In detail, in operation S30 of selecting a victim block, the block having the smallest significant block value among the plurality of blocks may be selected as the victim block.

Here, the important block value is defined by the following equation.

Equation

Critical Block Value = (Number of Valid Data / Number of Pages in Block) + (Number of First Invalid Data / Number of Pages in Block) x Weight ( Weight)

Here, the weight may be determined in a range of 0 to 1.

Referring to FIG. 5, the first block Block 1 and the second block Block 2 store two valid data and two invalid data. However, the first block (Block 1) stores two second invalid data (dInvalid Data), and the second block (Block 2) has one first invalid data (sInvalid Data) and one second invalid data. (dInvalid Data) is stored.

Herein, if the weight of the first invalid data is determined as 0, the important block values of the first block Block 1 and the second block Block 2 are equal to 0.5 according to the above equation. Both the first block Block 1 and the second block Block 2 may be selected as victim blocks.

In addition, if the weight of the first invalid data (sInvalid Data) is determined as 1, the critical block value of the first block Block 1 is 0.5 and the critical block of the second block Block 2 according to the above equation. Since the value is 0.75, the first block (Block 1) having a small important block value is selected as the victim block.

In addition, if the weight of the first invalid data (sInvalid Data) is determined as 0.5, the value of the significant block of the first block (Block 1) is 0.5 by the above equation and the significant block of the second block (Block 2). Since the value is 0.625, the first block (Block 1) having a small important block value is selected as the victim block.

Selecting a victim block (S30) is preferably performed when the available memory area among the memory areas is less than or equal to a threshold value.

Referring back to FIG. 2, in the garbage collection method of the nonvolatile memory device according to an embodiment of the present invention, first, a victim block is selected from among a plurality of blocks. In this case, a block having the smallest valid page and the first invalid page (Block 2), that is, a block having the largest number of the second invalid page (dInvalid page) is a victim block (Victim Block). Can be selected.

Next, the valid data stored in the valid page of the victim block and the first invalid data stored in the first invalid page are converted into free pages only. It is transferred to another block (Block 3) that is made, that is, a free block in which data is not stored.

Next, the victim block is erased to free space for a write operation in the memory area.

FIG. 6 is a table illustrating a tracking table applied to a garbage collection method of a nonvolatile memory device according to an embodiment of the present invention.

As shown in FIG. 6, a track table processes state information of a page for selecting a victim block, and includes a valid page, a first invalid page, and a second invalid page. Used to track the relationship between (dInvalid Page).

That is, in the step S20 of classifying invalid data into first invalid data and second invalid data, the valid data and the first invalid data are valid data. And a tracking table for tracking the relationship between the second invalid data (dInvalid Data).

Specifically, each entry in the tracking table represents an index, a physical page address (PPA), and a flag of a previous version page, where the index represents a physical page address (PPA) of the page. The flag indicates page status information. Each item in the tracking table changes according to the page state transition, and is used to select a victim block by recognizing this when recovering data.

For example, referring to FIG. 6, the page of PPA 400 is currently valid, and the page of PPA 300 was valid before the page of PPA 400 became valid. The page of is currently the first invalid state (sInvalid State). Since the page of the PPA 200 was valid before the page of the PPA 300 became valid, the page of the PPA 200 is currently in the second invalid state.

In a garbage collection method of a nonvolatile memory device according to an embodiment of the present invention, the step of transferring to a free block (S40) includes a plurality of first invalid data stored in a victim block. The method may include transferring to a free block among blocks of the block.

Through this, the lifespan of the first invalid data may be extended and data recovery performance may be improved.

The foregoing detailed description illustrates the present invention. In addition, the foregoing description merely shows and describes preferred embodiments of the present invention, and the present invention can be used in various other combinations, modifications, and environments. That is, changes or modifications may be made within the scope of the concept of the invention disclosed in this specification, the scope equivalent to the disclosed contents, and / or the skill or knowledge in the art. The above-described embodiments are for explaining the best state in carrying out the present invention, the use of other inventions such as the present invention in other state known in the art, and the specific fields of application and uses of the present invention. Various changes are also possible. Thus, the detailed description of the invention is not intended to limit the invention to the disclosed embodiments. Also, the appended claims should be construed to include other embodiments.

Valid Page: Valid Page
sInvalid Page: First Invalid Page
dInvalid Page: Second Invalid Page
Free Page: Free Page
Victim Block: Sacrifice Block

Claims (10)

In a memory area composed of a plurality of blocks including a plurality of pages,
Storing valid data and invalid data in the plurality of pages;
Classifying the invalid data into first and second invalid data according to a possibility of a data recovery request;
Selecting a victim block among the plurality of blocks based on the valid data and the number of the first and second invalid data;
Transferring the valid data stored in the victim block to a free block among the plurality of blocks; And
Erasing the sacrificial block;
The second invalid data is previous version data of the first invalid data, the first invalid data is previous version data of the valid data,
The classifying into the first and second invalid data may include:
Classifying using a tracking table for tracking the relationship between the valid data, the first invalid data and the second invalid data
Garbage collection method for nonvolatile memory devices.
The method of claim 1,
The second invalid data is
The previous version data of the first invalid data is data having a lower probability of the data recovery request than the first invalid data.
Garbage collection method for nonvolatile memory devices.
The method of claim 2,
Selecting the sacrificial block,
Selecting a block having the smallest number of the valid data and the first invalid data among the plurality of blocks as the victim block;
Garbage collection method for nonvolatile memory devices.
The method of claim 2,
Selecting the sacrificial block
Selecting a block having the smallest significant block value among the plurality of blocks as the victim block;
Garbage collection method for nonvolatile memory devices.
The method of claim 4, wherein
The critical block value is
Equation
Significant block value = (number of valid data / number of pages in block) + (number of first invalid data / number of pages in block) × weight
Defined by
Garbage collection method for nonvolatile memory devices.
The method of claim 5,
The weight is
Determined in the range 0 to 1.
Garbage collection method for nonvolatile memory devices.
The method of claim 1,
Selecting the sacrificial block,
Executed when an available memory area of the memory area is equal to or less than a threshold
Garbage collection method for nonvolatile memory devices.
delete delete The method of claim 1,
Transferring to the free block
Transferring the first invalid data stored in the victim block to the free block among the plurality of blocks;
Garbage collection method for nonvolatile memory devices.

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