GR-FB Block Cleaning Scheme in Flash Memory

ACEEE Int. J. on Network Security , Vol. 02, No. 04, Oct 2011

GR-FB Block Cleaning Scheme in Flash Memory
A.R. Rahiman1, N. Manshor1, A.A. Halin1, P. Sumari2, and H. Kamarulhaili2
1
Faculty of Computer Science and Information Technology, Universiti Putra Malaysia, 43400 Serdang, Malaysia
Email: {amir, ayu, alfian}@fsktm.upm.edu.my
2
School of Computer Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
Email: {putras, hailiza}@cs.usm.my

Abstract—Flash memory is a non-volatitle memory that offers block size) substantially impacts flash memory access
several superior advantages as a data storage device in many performance, energy consumption and block endurance [2].
electronic gadgets. However its two operational characteristics, Several studies focusing on identifying the victim block have
namely 1) Out-place updating and 2) Cleaning process will affect been proposed in the literature. The objectives of the studies
its performance as an efficient storage sub-system if not
are to minimize the cleaning cost and lengthen the memory
managed wisely. This paper proposes a new block cleaning
scheme, called Greedy and Frequency-based (GR-FB). The
block lifespan [1, 4, 6, 11, 13 – 15]. The cleaning cost refers to
scheme combines the victim block selection procedure and the total access time required in handling the cleaning
the valid-data re-organizing procedure to improve the block process. This paper proposes a new block cleaning scheme,
cleaning process performance. The trace-driven simulations called Greedy and Frequency-based (GR-FB) that combines
have shown the GR-FB scheme performs better than existing the victim block selection procedure with the valid data re-
cleaning process schemes where the cleaning cost is reduced organization process. According to the simulation results,
at least by 8% and the wear-leveling level for all blocks in the the GR-FB scheme perform better than existing block cleaning
memory array are nearly equal. schemes in terms of cleaning cost and valid data copying
amount, as well as guaranteeing memory block lifespan. The
Index Terms—flash memory, cleaning process, wear-leveling,
frequency, simulation, data
remainder of this paper is organized as follows. Section II
discusses about flash memory technology and its related
I. INTRODUCTION works. Then, the GR-FB scheme is presented in Section III.
The performance of the GR-FB is shown in Section IV. Finally,
A flash memory is a non-volatile semiconductor storage conclusions are presented in Section V.
device that provide faster data access, are small and light
weight, has zero noises, boasts solid-state reliability, consume II. BACKGROUNDS AND RELATED WORKS
lesser power and very resistant to shock [1 – 4]. Today, it has
become the demanding data storage device for storing and There are two types of flash memory with in the current
transferring data in most electronic gadgets, including MP3 market, 1) NOR-flash and 2) NAND-flash [12]. This paper
players, personal digital assistants (PDA’s), mobile phones, focuses on the NAND-flash and will throughout this paper,
digital cameras, etc. However, its two operational refer to it as flash memory.
characteristics namely, 1) Out-place updating and 2) A. Flash Memory Characteristics
Cleaning process have brought various technical challenges As shown in Fig. 1, flash memory is a blocks-and-pages-
in designing and implementing an efficient storage sub- based storage device. The page unit is the basic accessing
system. Updating existing data at the same location is strictly unit and a group of pages form the block unit. Each page unit
prohibited due to the time and the power consuming practice has two areas, data area and spare area. The data area stores
[3]. Therefore, the out-place updating policy has been the actual data while the spare area stores assisting
strongly suggested [3 – 5]. The updated data is stored into a information for the data area. Table I shows current flash
new location, while the original copy is set as garbage1. When memory specifications and attributes [16]. Next, data updating
the updating process frequently occurs, the amount of in flash memory is performed via the out-place-policy.
garbage hugely increases resulting in lesser device free space. Therefore, the page unit in flash memory can fall into three
Thus, the garbage needs to be cleaned to ensure the states namely the free, valid, and invalid. The free state
continuity of the data storing process. In the cleaning process, contains pages with no data and is ready for data storing.
the block containing large amounts of garbage (hereinafter The valid state contains pages with the current version of
referred to as victim block) is identified and being cleaned. data, whereas the invalid state contains pages with garbage.
As the cleaning process in flash memory is carried out on Moreover, the memory blocks containing valid pages are
block units, the block may contain valid data that are currently referred to as active block, while blocks containing free or
being used by particular applications. Therefore, before invalid pages are referred to as inactive block [7]. However,
initiating cleaning, all valid data residing in the block must be each memory block could be tolerant to a limited number of
identified and then being copied into available free spaces in erasure cycles. Excessive erasures will cause the block to
other blocks. Moreover, the cleaning process and the block become permanently spoiled. For instance, a multi-level cell
utilization level (the ratio between the valid data size and the (MLC) block can support approximately 10,000 erasure cycles.
1
The garbage refers to an obsolete or “dead” data and shall not be If the same block is erased and then re-programmed every
used any longer.
© 2011 ACEEE 37
DOI: 01.IJNS.02.04. 512


second, the block would exceed the 10,000 cycle limit in just scheme. The cost age time with age sort (CATA) [9] scheme
three hours. To avoid this limitation, a wear-leveling policy improves the CAT scheme by selects multiple victim blocks
that wears down all memory blocks as evenly as possible is that have maximum value according to the function (1-u/1+u)
necessary [4, 5, 8, 10]. x a x (1/e). Also, the scheme guarantees memory block lifespan.
Then, the S-Greedy (S-GR) [12] and the Endurant and Fast
Greedy (EF-GR) [13] schemes extend the existing GR scheme
by considering the wear-leveling policy. The victim blocks
are selected according to the block utilization level and both
schemes give focus on the valid data re-organization process.
The S-GR scheme re-organized the valid data into new blocks
according to the Swapped-Out Time (SOT) information while
the EF-GR scheme uses the predicted Inter Update (PIU) time.
Jang and Han [14] have propose an efficient cleaning scheme
by focusing in valid data re-organization. The valid data are
classified into three types according to modification
frequency- hot data, cold data, and warm data- and different
types of data are stored in different blocks. In this scheme,
Figure 1. Flash memory blocks and pages layout. they consider the current frequency of the data when the
TABLE I. FLASH MEMORY PROPERTIES
cleaning process is initiated. From the above studies, we
have found that the block cleaning schemes that combine
the victim block selection with the valid data re-organizing
procedures assists in achieving the cleaning process
objectives. Therefore, it has given us motivation to propose
a new block cleaning scheme that considers both procedures.

III. GR-FB BLOCK CLEANING SCHEME
The main focus of the GR-FB scheme is re-organizing the
valid data in the victim blocks into a new location by using
B. Cleaning Process in Flash Memory the decrement operation of the current appearance frequency
The cleaning process is realized in three stages. First, a for each valid data.
victim block is determined. Second, all valid data in the block
are identified and then copied into free pages in other active A. Victim Block Selection Procedure
or inactive blocks. Third, the victim block is erased upon The GR-FB scheme selects multiple victim blocks
completion of copying the valid data. Note that, when a according to the block utilization level. The amount of the
particular active block turns into inactive (all pages turn into active blocks is determined according to the predicted I/O
invalid), the block can be erased automatically without arrival rate that is similarly used in the CATA scheme. The
requiring the copying process. A number of studies focusing ˆ
predicted I/O arrival rate (  ) is the estimation of the average
on determining the victim block to be cleaned have been
length for the future arrival of the accessed data (either retrieve
proposed in [1, 3, 4, 10 – 13]. Two primary goals of these
or stored). It is estimated by using a time series analysis
studies are minimizing the cleaning cost and guaranteeing all
called autoregressive of order 1 AR(1) process [15]. To estimate
memory blocks are evenly wear. The Greedy (GR) [1] scheme
determines a victim block according to the block utilization ˆ
 , first, let Ai represent the time series sample of arrival request
level. The block with the lowest utilization level is selected ai in the estimated interval length I where the length of the
regardless of the block’s erasure cycle. Then, Kawaguchi et arrival is set to W. Thus, the sequence values of the arrival
al [3] have proposed the cost-benefit (CB) scheme which
request can be formed as a time series { a1 ...aim }. Then, by
i
selects the victim block that maximizes the function (a x (1–
u))/2u). Both parameters u and a refer to the block utilization using the AR(1) process, the number of future arrivals
level and the elapsed time from the last data invalidation. ˆ
(represented by ni ) is estimated using the previous sample
Similar to the GR, the wear-leveling policy is neglected in this value. The value of Ai is estimated as the following equation:
scheme. The combination between the victim block selection
scheme, called cost age time (CAT), and the valid data re- aij 1  ai  i (1)  (aij  ai )  eij
ˆ (1)
organizing process, namely dynamic dAta cluster (DAC), has Both  i and ai represent the autocorrelation function and the
been proposed by [4]. The scheme selects multiple victim
blocks according to the lower values from function (u/1-u) x mean of Ai while eij is a white noise component. In (1), is
(1/a) x e, where parameter e refers to block erasure counts. In assumed equal to 0 while is to be estimated values of
addition, the wear-leveling policy is guaranteed in this
aij for j  H  1 . Parameter H (History) represents a finite
ˆ
© 2011 ACEEE 38
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sequence of the previously recorded values. Then, as de- the same cluster range, the data is re-allocated to an available
fined by [9], the autocorrelation function  i is calculated block belonging to the current cluster. After all the valid data
in the victim blocks is copied out into the new blocks, the
using following equation:
victim blocks are erased. The decreased frequency is then

 i l  
 
E aij  ai  aij 1  ai ,0  l  H  1
2
 ai (2)
used for further data re-organization when the cleaning pro-
2 cess is initiated
Parameter  ai is the standard deviation of Ai while parameter
l is the lag between sample values for which the
ˆ
autocorrelation is computed. If the future arrival size ni is set
to M intervals, where M = W/I, then the arrival requests of
a iH 1 ,..., a iH  M are calculated using (1). Then, the estimated
ˆ ˆ
j
number of arrivals in W time units is given by ni   H M 1 a i
ˆ j H ˆ

nˆ
and the estimated arrival rate for the future arrival  is i .
ˆ
W
By using thevalue, the amount of victim blocks is specified.
If the value is high, then, only a single victim block is erased,
since erasing multiple blocks will interfere with the flash Figure 3. Valid data re-organization in GR-FB scheme.
memory’s current I/O operations.
C. Blocks Wearing Procedure
B. Valid Data Re-organization
In the GR-FB scheme, the wear leveling policy is controlled
First, the memory blocks are partitioned into three clusters using an efficient valid data re-organization strategy. First,
according to the appearance frequency in the I/O operational the erasure count for each block in each cluster is recorded.
behavior, see Fig. 2. The 1st cluster is used to store valid data After the block has been erased, its erasure count is increased
with irregular appearance. The 2nd cluster stores the average and the information regarding its erasure count and ID are
appearance while the 3rd cluster is allocated data with regular recorded by the data structure called cluster_list. Each cluster
appearance. Parameters p, k and s represent the data has an individual cluster_list and the block’s ID in the
appearance frequency and the maximum range of frequency cluster_list is arranged in ascending order, according to the
for the memory blocks in the 2nd and 3rd cluster. erasure count. When the data is allocated or re-organized
into a particular block, the block with minimum erasure count
is selected. When the block has been erased, it will be pushed
to the end of the list. Also, if the current active block in the
demoted cluster is available with a free page, the re-
organization will be stored in the first free page of the block.
This re-organization approach ensures all blocks in each
Figure 2. Clustering memory blocks with p frequency appearance cluster is used evenly. The decrement frequency operation
Each valid data in the victim blocks is re-organized into a helps the valid data to be re-organized into similar blocks
block in a particular cluster according to their decreased cur- each time the cleaning process is initiated.
rent appearance frequency. For example, if valid data pi’s cur-
rent frequency is k, then its frequency now becomes k – 1. IV. SIMULATION SETUP AND EVALUATION
The reason is, since the appearance frequency of the data is
The evaluation was performed using the trace-driven
solely determined by the user’s access behavior, the current
simulation of the real tracing data [16]. The file was collected
appearance frequency may show the current state of the data.
over a personal computer running general applications such
There are two conditions for the accessed data in the I/O
as Web browsers, PowerPoint, Word, and email applications.
access pattern. They are either last appearance or will re-
The details of the file are shown in Table II.
appear again and again. By decreasing their frequency, the
data to be re-allocated in the available blocks in the subse- TABLE II. T RACING FILE SPECIFICATIONS
quent cluster can be prevented. Fig. 3 shows the re-organiza-
tion of the valid data into new blocks according to the de-
creasing frequency. The valid data with a single appearance
are stored in a new block in the first cluster. If the current By employing the common cleaning process procedure, the
frequency is 2, the data is demoted from the second cluster GR-FB cleaning process is initiated when the size of the free
to the first cluster. Then, if the decreased frequency is within blocks is fewer than 15% of the total size. The cleaning will
© 2011 ACEEE 39
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be terminated when the free size exceed 25% of the device’s TABLE III. PERCENTAGE OF B LOCKS ERASURE C OUNTS
capacity. Three performance paramaters for the evaluations
are cleaning cost, valid data copying and block lifespan. The
cleaning cost measures the total access time required in
handling the cleaning process, while the valid data copying
records the amount of data copied or migrated during the
cleaning process.
A. Performance Evaluation Results
Fig. 4 illustrates the cleaning cost required for the trace
file in the simulation. Compared to exisiting schemes, the The GR-FB scheme records the lowest average value (39.5%)
cleaning cost required by GR-FB is the lowest. In the figure, compared to the other schemes. With the different k values,
the CB scheme consumes the highest cost. By decreasing the results show significant differences. For example, the
the current appearance frequency of the valid data, the GR- average increased from 39.5% to 40.17% when k was changed
FB scheme prevents the employment of a new active block from 2 to 3. From the minimum average values recorded, it is
during the valid data re-organization process. clear that the proposed scheme guarantees memory block
lifespan. Unlike the CB and GR schemes, for instance, the
erasure count is not the criterion in accessing the block for
allocating and re-organization of the access data. The
cluster_list in each cluster plays a significant role in selecting
the block to store the accessed data and the re-organization
of the valid data from the selected victim blocks.

V. CONCLUSIONS
The GR-FB scheme emphasizes on combining the victim
block selection procedure and the valid data re-organization
approach to minimize the cleaning cost and guaranteeing the
wear leveling policy. The victim blocks are selected according
to block utilization level and the valid data residing in the
Figure 4. Cleaning cost required for various cleaning schemes.
victim blocks are re-organized into new blocks by decreasing
their current appearance frequency. The decrement frequency
Fig. 5 shows the percentage amount of blocks involved in
approach is used because the accessed data is controlled
the copying process. The smallest percentage value shows
solely by the user’s access behavior and it prevents from
minimum block involvement during the copying process,
employing the new block in the next cluster during the re-
where the copying amount for the GR-FB scheme is not more
organization process. The GR-FB scheme performs better than
than 40%. The evaluation on the effectiveness of the wear-
the existing schemes in terms of the cleaning cost and valid
leveling policy in the simulation is illustrated in Table III. The
data copying percentage. The average percentage of blocks
performance was evaluated in terms of the average erasure
usage in each partitioned cluster is the metric used in
counts for each block in each partitioned cluster by using
evaluating the blocks endurance, and the GR-FB shows that
two levels of frequencies, k = 2 and k = 3. The similar average
the percentages for all clusters are nearly identical.
value for each cluster indicates that all blocks were evenly
accessed during the cleaning process.
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GR-FB Block Cleaning Scheme in Flash Memory

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