MAINTAINING CACHE SIZE PROPORTIONALTO POWER PACK
CHARGE
FIELD OF THE INVENTION
[0001] The present disclosure is directed generally towards caching of
data and more particularly to systems and methods for maintaining a
cache size proportional to power pack charge.
BACKGROUND
[0002] As cache memory is non-volatile, it requires a power pack such as
a battery in order to offload the contents of the cache to a non-volatile
memory during power loss. Failure to offload the contents of the controller
cache to a non-volatile memory results in the loss of the data in the cache.
[0003] Therefore, there exists a need for improved methods and systems
for maintaining cache size relative to power pack charge.
SUMMARY
[0004] The present disclosure is directed to a method for managing a
cache based on a charge of a power source. The method includes the
step of determining a charge of the power source at a first time instance.
The method also includes the step of designating for write back cache an.
amount of data in the cache which can be offloaded from the cache based
on the charge of the power source at the first time instance. The method
also includes the step of designating as write through cache an amount of
data remaining in the cache which was not designated as write back
cache.
[0005] Additional embodiments are described in the application including
the claims. It is to be understood that both the foregoing general
description and the following detailed description are exemplary and

explanatory only and are not restrictive. Other embodiments of the
invention will become apparent.
BRIEF DESCRIPTION OF THE FIGURES
[0006] Other embodiments of the invention will become apparent by
reference to the accompanying figures in which:
FIG. 1 shows a diagram of a system for managing a cache;
FIG. 2 shows a rechargeable power source and a cache including
data designated for write back mode;
FIG. 3 shows a rechargeable power source and a cache including
data designated for write through mode and write back mode;
FIG. 4 shows a rechargeable power source and a cache including
data designated for write through mode and write back mode;
FIG. 5 shows a rechargeable power source and a cache including
data designated for write back mode;
FIG. 6 shows a rechargeable power source and a cache including
data designated for write back mode and write through mode;
FIG. 7 shows a rechargeable power source and a cache including
data designated for write back mode and write through mode;
FIG. 8 shows a rechargeable power source and a cache including
data designated for write through mode;
FIG. 9 is a flow diagram of a method for managing a cache; and
FIG. 10 is a flow diagram of a method for managing a cache.
DETAILED DESCRIPTION
[0007] Reference will now be made in detail to the subject matter
disclosed, which is illustrated in the accompanying drawings. The scope
of embodiments of the invention is limited only by the claims; numerous
alternatives, modifications, and equivalents are encompassed. For the
purpose of clarity, technical material that is known in the technical fields
related to the embodiments has not been described in detail to avoid
unnecessarily obscuring the description.

[0008] As data stored in a cache memory is volatile, it requires a power
pack such as a battery in order to offload the contents of the controller
cache to a non-volatile memory during power loss. Failure to offload the
contents of the controller cache to a non-volatile memory will result in the
loss of the cache data which translates into data loss from the host system
perspective.
[0009] The amount of battery charge required to maintain the data in the
controller cache in write back mode depends on the time required to
offload the data. The time required to offload the data depends on the
amount of data in the controller cache. In some controllers, a fixed amount
of write back cache is provided in case the power pack charge is not
sufficient to offload the complete write back cache. Some systems set a
minimum amount of power pack charge in order to support data in the
cache being configured for write back cache. If the power pack is not
charged to this minimum amount of charge, the cache policy for all of the
data in the cache is designated as write through cache.
[0010] In general, this means the cache policy of all the data configured in
write back mode will require a change in policy to write through mode
when the power pack charge falls below the threshold. The cache policy is
changed to write back once the battery is charged to the threshold. As the
time required to charge the power pack is significant, overall system
performance is impacted when the data is placed in write through mode
during the charging period.
[0011] In one embodiment, the present disclosure provides a method
where the write back cache size is made proportional to the battery
capacity. The cache allocated for write back cache is flexible and
increases as the battery charge increases, and decreases with decreases
in battery charge.

[0012] FIG. 1 shows a system 100 including a power source 110, a cache
108 stored on a memory 106, a control module 104 and a designator
module 102. The system 100 is in communication with a host 103. The
cache 108 is configured for storing data. The data in the cache 108 may
be designated as either write back cache or write through cache
depending on the charge of power source 110. The control module 104 of
the system 100 is configured for determining a charge 116 of the power
source 110 at a first time instance and for calculating an amount of data
which can be offloaded from the cache 108 when the power source 110
has a predetermined amount of charge. The control module 104 is also
configured for dividing the data in the cache 108 into a plurality of data
segments 111 having a size equal to the amount of data which can be
offloaded from the cache 108 when the power source 110 has the
predetermined amount of charge. The system 100 also includes a
designator module 102 configured to designate for write back cache a
number of data segments 111 which can be offloaded from the cache 108
based on the charge of the power source 110 at the first time instance.
[0013] In one embodiment, the designator module 102 is further configured
for designating as write through cache a number of data segments 111
remaining in the cache 108 which are not designated as write back cache.
[0014] In one embodiment, the system 100 is configured to adjust the
amount of write back cache when there is a change in the charge of the
power source 110 exceeding a predetermined threshold. The control
module 104 detects a change in the charge of the power source 110
exceeding the predetermined threshold and determines a number of data
segments 111 which can be offloaded from the cache 108 based on the
change in the charge of the power source 110. The designator module 102
is configured to either remove write back cache or write through cache,
depending on whether the charge has increased or decreased. When the
charge has increased, the designator module 102 is configured to remove
the number of data segments 111 which can be offloaded from the cache

108 from the write through cache and add them to the write back cache.
When the charge has decreased, the designator module 102 is configured
to remove the number of data segments 111 which can be offloaded from
the cache 108 from the write back cache and add them to the write
through cache.
[0015] In one embodiment, the change exceeding a predetermined
threshold in the charge of the power source 110 is equal to the amount of
charge required to offload one data segment 111 from the cache 108. In
another embodiment, the change exceeding a predetermined threshold in
the charge of the power source 110 is either greater than or less than the
amount of charge required to offload one data segment 111 from the
cache 108.
[0016] In another embodiment, the system 100 is configured to adjust the
amount of write back cache after a predetermined amount of time has
elapsed. The control module 104 determines the charge of the power
source 110 at a second time instance following the first time instance and
determines a number of data segments 111 which can be offloaded from
the cache 108 when the charge of the power source 110 at the second
time instance has changed by an amount exceeding a predetermined
threshold. When the charge has increased from the first time instance to
the second time instance, the designator module 102 removes the number
of data segments 111 which can be offloaded from the cache 108 from the
write through cache and adds them to the write back cache. When the
charge has decreased, the designator module 102 removes the number of
data segments 111 which can be offloaded from the cache 108 from the
write back cache and adds them to the write through cache.
[0017] The power source 110 of the system 100 is a back-up or secondary
power source in one embodiment and is utilized when a primary power
source fails. The power source 110 includes any suitable rechargeable
battery or power pack in one embodiment.

[0018] FIGS. 2-4 show how the write back cache size varies according to
the battery charge in accordance with embodiments of the disclosure. For
example, FIG. 2 shows the power source 110 having a charge 116 of
70%. In the system shown in FIGS. 2-4, a 70% charge 116 is sufficient to
support the cache 108 having all data configured as write back cache 112.
[0019] FIG. 3 shows the adjustment of the cache 108 when the power
source 110 charge 116 has decreased. In FIG. 3, power source 110 has
been depleted to a 35% charge 116 which is sufficient to support
configuring 50% of the cache as write back cache 112. The data in the
cache 108 that is not configured for write back cache 112 is now allocated
for write through cache 114.
[0020] In FIG. 4, the charge 116 has decreased further to a level of 10%.
In this embodiment, a charge 116 of 10% is sufficient to support
configuring 14% of the cache 108 as write back cache 112 with the
remaining 86% configured for write through cache 114.
[0021] FIGS. 2-4 show the write back cache 112 decreasing as the charge
116 decreases, in accordance with embodiments of the invention. The
write back cache 112 also increases as the charge 116 increases. In
another embodiment, the write back cache 112 both increases and
decreases depending on the charge 116.
[0022] In addition, the example provided in FIGS. 2-4 shows the power
source 110 charge 116 at 70%, 35%, and 10%, with the corresponding
write back cache 112 sized at 100%, 50%, and 14%. It is to be understood
that the particular levels of charge 116 and the corresponding level of write
back cache 112 described herein are merely exemplary. The levels of
charge 116 and the corresponding level of write back cache 112 vary
depending on the system configuration and other factors.
[0023] Referring now to the embodiment provided in FIGS. 5-8, another
embodiment showing how the write back cache size varies according to

the battery charge is provided. In the embodiments shown in FIGS. 5-8,
the cache 108 is divided into a plurality of data segments 111. The size of
each data segment 111 is equal to the amount of data that can be
offloaded from the cache 108 based on a predetermined amount of battery
charge 116. For example, if 10% of charge is required to offload 50 MB of
data, all of the data in the cache 108 may be divided into data segments
111 having a size of 50 MB each. In the example shown in FIGS. 5-8; the
cache 108 holds 500 MB of data which is divided into 10 equally sized
data segments 111 of 50 MB each.
[0024] FIGS. 5-8 show the adjustment of each data segment 111 in the
cache 108 from either write back cache to write through cache, or write
through cache to write back cache, depending on changes to the amount
of charge 116. In FIG. 5, the charge 116 is 100% which is sufficient to
offload all of the 500 MB of data currently in the cache 108, so the policy of
all data segments 111 in the cache is write back.
[0025] In FIG. 6, the charge 116 of the power source 110 is 50%, which is
only sufficient to offload 250 MB of data from the cache 108. As 250 MB of
data is equal to five data segments 111, five of the data segments 111
previously designated as write back cache are now designated for write
through cache, with the other five data segments 111 remaining as write
back cache.
[0026] In FIG. 7, the charge 116 of the power source 110 is 15% which is
sufficient to offload 75 MB of data, or one full data segment 111. In this
embodiment, nine of the ten data segments 111 are designated as write
through cache with one of the data segments 111 designated for write
back cache.
[0027] In FIG. 8, the charge 116 of the power source 110 is 5% which is
sufficient to offload 25 MB of data. As the power source 110 lacks
sufficient charge 116 to offload a full 50 MB data segment 111, in this

embodiment all of the ten data segments 111 are designated as write
through cache with none of the data segments 111 designated for write
back cache.
[0028] In another embodiment, a method 900 for managing a cache is
provided, as shown in FIG. 9. The method 900 includes the step of
calculating an amount of data which can be offloaded from the cache
when a power source has a predetermined amount of charge 902. The
method 900 also includes the step of dividing the data in the cache into a
plurality of data segments, each data segment having a size equal to the
amount of data which can be offloaded from the cache when the power
source has the predetermined amount of charge 904. A further step of the
method 900 is to determine a charge of the power source at a first time
instance 906. The method also includes the step of determining a
number of data segments to be designated as write back cache which can
be offloaded from the cache based on the charge of the power source at
the first time instance 908.
[0029] In one embodiment, the method 900 also includes the step of
designating as write through cache a number of data segments remaining
in the cache which are not designated as write back cache.
[0030] In one embodiment, the method 900 also includes the step of
detecting a change in the charge of the power source exceeding a
predetermined threshold and determining a number of data segments
which can be offloaded from the cache based on the change in the charge
of the power source. A further step of the method 900 also includes
removing the number of data segments which can be offloaded from the
cache based on the change in the charge of the power source from the
write through cache and adding them to the write back cache when the
charge has increased. The method 900 also includes the step of removing
the number of data segments which can be offloaded from the cache
based on the change in the charge of the power source from the write

back cache and adding them to the write through cache when the charge
has decreased.
[0031] In another embodiment, the method 900 includes the steps of
determining a charge of the power source at a second time instance and
determining a number of data segments which can be offloaded from the
cache when the charge of the power source at the second time instance
has changed by an amount exceeding a predetermined threshold. When
the charge has increased from the first time instance to the second time
instance, the method 900 includes the step of removing the number of
data segments which can be offloaded from the cache based on the
change in the charge of the power source from the write through cache
and adding them to the write back cache. When the charge has decreased
from the first time instance to the second time instance, the method 900
includes the step of removing the number of data segments which can be
offloaded from the cache based on the change in the charge of the power
source from the write back cache and adding them to the write through
cache when the charge has decreased from the first time instance to the
second time instance.
[0032] In another embodiment, the present disclosure is directed to a
method 1000 for managing a cache based on a charge of a power source
as shown in FIG. 10. The method 1000 includes the step of determining a
charge of the power source at a first time instance 1002. The method 1000
also includes the step of designating for write back cache an amount of
data in the cache which can be offloaded from the cache based on the
charge of the power source at the first time instance 1004. The method
1000 also includes the step of designating as write through cache an
amount of data remaining in the cache which was not designated as write
back cache 1006.
[0033] In one embodiment, the method 1000 includes additional steps. For
instance, a further step of the method 1000 includes detecting a. change in

the charge of the power source exceeding a predetermined threshold and
determining an amount of data which can be offloaded from the cache
based on the change in the charge of the power source. Yet another step
of the method 1000 in one embodiment includes removing the amount of
data which can be offloaded from the cache based on the change in the
charge of the power source from the write through cache and adding the
amount of data which can be offloaded from the cache based on the
change in the charge of the power source to the write back cache when
the charge has increased. If the charge has decreased, the data is
removed from the write back cache and added to the write through cache.
[0034] The system and methods of the present disclosure support
Input/Output capabilities, including instances when the power pack
capacity is lower than the threshold to support complete DDR cache for
write back policy. The system and methods of the present disclosure also
assist with Input/output capabilities over time, including instances when
the battery degrades over a period of time. The system and methods of
the present disclosure support a flexible and adaptable cache memory
size. The system and methods of the present disclosure support
performance and assist with Input/output capabilities during times when
the battery source is variable, including non-transparent power pack learn
cycles in which the power unit gets discharged and charged again. The
system and methods of the present disclosure also support cache offload
even when partial DDR is used for write back cache.
[0035] In the present disclosure, the methods disclosed may be
implemented as sets of instructions or software readable by a device.
Further, it is understood that the specific order or hierarchy of steps in the
methods disclosed are examples of exemplary approaches. Based upon
design preferences, it is understood that the specific order or hierarchy of
steps in the method can be rearranged while remaining within the
disclosed subject matter. The accompanying method claims present

elements of the various steps in a sample order, and are not necessarily
meant to be limited to the specific order or hierarchy presented.
[0036] It is believed that the present disclosure and many of its attendant
advantages will be understood by the foregoing description, and it will be
apparent that various changes may be made in the form, construction and
arrangement of the components without departing from the disclosed
subject matter or without sacrificing all of its material advantages. The
form described is merely explanatory, and it is the intention of the following
claims to encompass and include such changes.

Claims
What is claimed is:
1. A method for managing a cache based on a charge of a
power source, the method comprising:
determining a charge of the power source at a first time
instance;
designating for write back cache an amount of data in the cache
which can be offloaded from the cache based on the charge of the power
source at the first time instance;
designating as write through cache an amount of data remaining in
the cache which was not designated as write back cache.
2. The method as claimed in claim 1, further comprising:
detecting a change in the charge of the power source exceeding a
predetermined threshold; and
determining an amount of data which can be offloaded from the
cache based on the change in the charge of the power source.
3. The method as claimed in claim 2, further comprising:
removing the amount of data which can be offloaded from the
cache based on the change in the charge of the power source from the
write through cache and adding the amount of data which can be offloaded
from the cache based on the change in the charge of the power source to
the write back cache when the charge has increased.
4. The method as claimed in claim 2, further comprising:
removing the amount of data which can be offloaded from the
cache based on the change in the charge of the power source from the
write back cache and adding the amount of data which can be offloaded

from the cache based on the change in the charge of the power source to
the write through cache when the charge has decreased.
5. A method for managing a cache, the method comprising:
calculating an amount of data which can be offloaded from the
cache when a power source has a predetermined amount of charge;
dividing the data in the cache into a plurality of data segments, each
data segment having a size equal to the amount of data which can be
offloaded from the cache when the power source has the predetermined
amount of charge;
determining a charge of the power source at a first time instance;
determining a number of data segments to be designated as write
back cache which can be offloaded from the cache based on the charge of
the power source at the first time instance.
6. The method as claimed in claim 5, further comprising:
designating as write through cache a number of data segments
remaining in the cache which are not designated as write back cache.
7. The method as claimed in claim 6, further comprising:
detecting a change in the charge of the power source exceeding a
predetermined threshold; and
determining a number of data segments which can be offloaded
from the cache based on the change in the charge of the power source.
8. The method as claimed in claim 7, further comprising:
removing the number of data segments which can be offloaded
from the cache based on the change in the charge of the power source
from the write through cache and adding them to the write back cache
when the charge has increased.

9. The method as claimed in claim 7, further comprising:
removing the number of data segments which can be offloaded
from the cache based on the change in the charge of the power source
from the write back cache and adding them to the write through cache
when the charge has decreased.
10. The method as claimed in claim 6, further comprising:
determining a charge of the power source at a second time
instance;
determining a number of data segments which can be offloaded
from the cache when the charge of the power source at the second time
instance has changed by an amount exceeding a predetermined
threshold.
11. The method as claimed in claim 10, further comprising:
removing the number of data segments which can be offloaded
from the cache based on the change in the charge of the power source
from the write through cache and adding them to the write back cache
when the charge has increased from the first time instance to the second
time instance.
12. The method as claimed in claim 10, further comprising:
removing the number of data segments which can be offloaded
from the cache based on the change in the charge of the power source
from the write back cache and adding them to the write through cache
when the charge has decreased from the first time instance to the second
time instance.

13. A system, comprising:
a power source;
a cache, the cache stored on a memory, the cache configured for
storing data;
a control module configured for determining a charge of the power
source at a first time instance, the control module further configured for
calculating an amount of data which can be offloaded from the cache
when the power source has a predetermined amount of charge, the control
module further configured for dividing the data in the cache into a plurality
of data segments, each data segment having a size equal to the amount of
data which can be offloaded from the cache when the power source has
the predetermined amount of charge; and
a designator module, the designator module configured to
designate for write back cache a number of data segments which can be
offloaded from the cache based on the charge of the battery at a first time
instance.
14. The system as claimed in claim 13, wherein the designator
module is further configured for designating as write through cache a
number of data segments remaining in the cache which are not designated
as write back cache.
15. The system as claimed in claim 14, wherein the control
module is further configured for detecting a change in the charge of the
power source exceeding a predetermined threshold and determining a
number of data segments which can be offloaded from the cache based
on the change in the charge of the power source.
16. The system as claimed in claim 15, wherein the designator
module is further configured for removing the number of data segments
which can be offloaded from the cache based on the change in the charge

of the power source from the write through cache and adding them to the
write back cache when the charge has increased.
17. The system as claimed in claim 15, wherein the designator
module is further configured for removing the number of data segments
which can be offloaded from the cache based on the change in the charge
of the power source from the write back cache and adding them to the
write through cache when the charge has decreased.
18. The system as claimed in claim 14, wherein the control
module is further configured for determining a charge of the power source
at a second time instance and determining a number of data segments
which can be offloaded from the cache when the charge of the power
source at the second time instance has changed by an amount exceeding
a predetermined threshold..
19. The system as claimed in claim 18, wherein the designator
module is further configured for removing the number of data segments
which can be offloaded from the cache based on the change in the charge
of the power source from the write through cache and adding them to the
write back cache when the charge has increased from the first time
instance to the second time instance.
20. The system as claimed in claim 18, wherein the designator
module is further configured for removing the number of data segments
which can be offloaded from the cache based on the change in the charge
of the power source from the write back cache and adding them to the
write through cache when the charge has decreased from the first time
instance to the second time instance.

ABSTRACT

[0037] The present disclosure is directed to a method for managing a
cache based on a charge of a power source. The method includes the
step of determining a charge of the power source at a first time instance.
The method also includes the step of designating for write back cache an
amount of data in the cache which can be offloaded from the cache based
on the charge of the power source at the first time instance. The method
also includes the step of designating as write through cache an amount of
data remaining in the cache which was not designated as write back
cache.