Abstract:
A system and method for storing infrequently accessed data with reduced power consumption. In one embodiment, a solid state drive (SSD) includes flash memory and environmental data logging circuitry. The SSD is shut off or operated in a sleep mode to reduce power consumption, and turned on or transitioned to an active mode as needed when data on the SSD is to be accessed, or when a calculation, based on a number of erase cycles previously performed in the flash memory and on a temperature history of the SSD indicates that a data refresh may be needed to prevent data corruption in the SSD, due to data retention limitation of the nonvolatile memory in the SSD.

Description:
FIELD 
     The following description relates to storage of infrequently accessed data and more particularly to a system and method of storing infrequently accessed data with low power consumption, e.g., with minimum power consumption. 
     BACKGROUND 
     Every day, several quintillion bytes of data may be created around the world. These data come from everywhere: posts to social media sites, digital pictures and videos, purchase transaction records, bank transactions, sensors used to gather data and intelligence, like weather information, cell phone GPS signal, and many others. This type of data and its vast accumulation is often referred to as “big data.” This vast amount of data eventually is stored and maintained in storage nodes, such as hard disk drives (HDDs), solid-state storage drives (SSDs), or the like, and these may reside on networks or on storage accessible via the Internet, which may be referred to as the “cloud.” In some cases the data is not accessed very frequently but it needs to be available at any time with minimal delay. For example, the data may be write once, read many (WORM) data, such as data posted to social media web sites, or video media posted by users on public video sharing sites. 
     Conventional storage solutions may not be well suited to this application. Hard disk drives, for example, may consume excessive power if kept spinning, and may take too long to start up if allowed to stop after each data access. Data corruption in solid state drives might be a consequence of very long periods of idle time. This limitation is sometimes referred to as a data retention limitation. Thus, there is a need for a system and method for storing large volumes of infrequently accessed data, providing rapid access, and in a power-efficient manner. 
     SUMMARY 
     In one embodiment of a system and method for storing infrequently accessed data with reduced power consumption, a solid state drive (SSD) includes flash memory and environmental data logging circuitry. The SSD is shut off or operated in a sleep mode to reduce power consumption, and turned on or transitioned to an active mode as needed when data on the SSD is to be accessed, or when a calculation, based on a number of erase cycles previously performed in the flash memory and on a temperature history of the SSD indicates that a data refresh may be needed to prevent data corruption in the SSD. 
     According to an embodiment of the present invention there is provided a method for operating a solid state drive (SSD) connected to a host, the SSD including nonvolatile memory and environmental data logging circuitry (EDLC) and a source of a battery power, the method including: requesting, by the host, a length of a first time interval from the SSD; providing, by the SSD, the length of the first time interval to the host; discontinuing, by the host, during the first time interval, a primary power supplied to the SSD; powering of the EDLC, by the SSD, from the source of the battery power during the first time interval; logging, by the EDLC, of environmental data; restoring, by the host, of the primary power supplied to the SSD; and refreshing of data stored in the SSD, by the SSD, when a module evaluated by the SSD indicates that, based on the logged environmental data, refreshing of the data is required. 
     In one embodiment, the environmental data includes a temperature of the SSD. 
     In one embodiment, the environmental data includes a time stamp. 
     In one embodiment, the method includes recording, by the SSD, a number of erase cycles performed on the nonvolatile memory. 
     In one embodiment, the providing, by the SSD, of the length of the first time interval to the host includes executing a remaining data life module. 
     In one embodiment, the remaining data life module is configured to accept as inputs a temperature history of the SSD since a most recent refresh, and a number of erase cycles previously performed on the nonvolatile memory. 
     According to an embodiment of the present invention there is provided a method for operating a solid state drive (SSD) connected to a host, the SSD comprising nonvolatile memory, the host including an environmental data logging system, the method including: requesting, by the host, a length of a first time interval from the SSD; providing, by the SSD, the length of the first time interval to the host; discontinuing, by the host, during the first time interval, a power supplied to the SSD; logging, by the environmental data logging system, of environmental data during the first interval; restoring, by the host, of the power supplied to the SSD; providing, by the host, logged environmental data to the SSD; and refreshing of data stored in the SSD, by the SSD, when a module evaluated by the SSD indicates that, based on the logged environmental data, refreshing of the data is required. 
     In one embodiment, the environmental data includes a temperature of the SSD. 
     In one embodiment, the environmental data includes a time stamp. 
     In one embodiment, the method includes recording, by the SSD, a number of erase cycles performed on the nonvolatile memory. 
     In one embodiment, the providing, by the SSD, the length of the first time interval to the host includes executing a remaining data life module. 
     In one embodiment, the remaining data life module is configured to accept as inputs a temperature history of the SSD since a most recent refresh, and a number of erase cycles previously performed on the nonvolatile memory. 
     According to an embodiment of the present invention there is provided a method for operating a solid state drive (SSD) connected to a host, the SSD including nonvolatile memory and environmental data logging circuitry (EDLC), the method including: instructing, by the host, the SSD to transition to a sleep mode; transitioning, by the SSD, to the sleep mode; logging, by the EDLC, of environmental data, while the SSD is in the sleep mode; instructing, by the host, the SSD to transition to an active mode; transitioning, by the SSD, to the active mode; refreshing of data stored in the SSD, by the SSD, when a module evaluated by the SSD indicates that, based on the logged environmental data, refreshing of the data is required; and sending, by the SSD, status information to the host. 
     In one embodiment, the environmental data includes a temperature of the SSD. 
     In one embodiment, the environmental data includes a time stamp. 
     In one embodiment, the method includes recording, by the SSD, a number of erase cycles performed on the nonvolatile memory. 
     In one embodiment, the module evaluated by the SSD is configured to accept as inputs a temperature history of the SSD since a most recent refresh, and a number of erase cycles previously performed on the nonvolatile memory. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will be appreciated and understood with reference to the specification, claims and appended drawings wherein: 
         FIG. 1  is a block diagram of a host in communication with a solid state drive (SSD) according to an embodiment of the present invention; 
         FIG. 2  is a block diagram of an SSD according to an embodiment of the present invention; 
         FIG. 3  is a block diagram of an SSD according to another embodiment of the present invention; 
         FIG. 4  is a block diagram of environmental data logging circuitry according to an embodiment of the present invention; 
         FIG. 5  is a flow chart of a method for operating a cold storage system according to an embodiment of the present invention; 
         FIG. 6  is a flow chart of another method for operating a cold storage system according to an embodiment of the present invention; 
         FIG. 7  is a flow chart of another method for operating a cold storage system according to an embodiment of the present invention; and 
         FIG. 8  is a flow chart of another method for operating a cold storage system according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of a power efficient method for cold storage data retention management provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features. 
     In one embodiment, infrequently accessed data may be stored at low power consumption in storage facilities or storage devices referred to as cold storage.  FIG. 1  is a block diagram of such a system according to one embodiment. The system of  FIG. 1  includes a host  110  in communication with a storage node such as an SSD  130 , which may include an SSD controller  140  and a non-volatile memory, e.g., a flash memory  150 . The host  110  and the SSD  130  may communicate using any suitable interface such as Peripheral Component Interconnect Express (PCIe), and using any suitable protocol.  FIG. 1  shows one host  110  in communication with an SSD  130 , but the invention is not limited to this configuration. The invention may be employed, for example, in a system in which a host  110  is in communication with several SSDs  130 , or with a combination of one or more SSDs  130  and one or more other storage devices such as HDDs, or the invention may be employed in a system with multiple hosts  110 , each in communication with one or more storage devices. 
     Referring to  FIG. 2 , an SSD suitable for cold storage may include an SSD controller  140  connected to flash memory  150 , and an interface connector  220 . The interface connector  220  may be used to connect the SSD  130  to a host  110 . The flash memory may include several flash devices  230 . Each flash device  230  may, for example, be an integrated circuit, and the flash devices  230  may be arranged in groups or in an array for purposes of addressing or for sharing communication channels with the SSD controller  140 . For example the SSD controller may have several flash channel interfaces for communicating with the flash memory  150 , and each flash channel interface may be shared by several flash devices  230 . Each flash device  230  may include a number of blocks, and each block may include a number of pages of memory. 
     Data in the flash devices  230  in the SSD  130  may have a certain data life. As used herein, a data item refers to a quantity of data with a common data life, or remaining data life. A data item may be the data in a page or in a block of a flash device  230 . When the age of a data item, which is measured from the time the data items is written to, or refreshed on, the flash device, exceeds the data life, the data item may be said to expire and the data may be corrupted or become unreliable. The data life may be different for different blocks or pages of the flash device  230 , and it may depend on the number of program and erase cycles that have been performed on the block, and on the temperature history of the flash device. In particular, the data life of a block of the flash device  230  may be shortened with each program and erase cycle performed on the page or block. As used herein, the remaining data life of a data item is the data life of the data item less the age of the data item. Moreover, higher temperatures may shorten the data life, so that the remaining data life of a data item may be shortened if the flash device  230  is exposed to high temperatures after the data item is written. The finite data life of a data item on a flash device  230  is of particular importance in cold storage applications because the likelihood of a data item expiring is greater than in other applications in which the data are frequently refreshed by being re-written or overwritten with fresh data. 
     In one embodiment, a power efficient method for cold storage data retention management employs one of several approaches to estimate the remaining data life of data items stored in the SSD  130 , and to refresh data on the SSD  130  before any of the data items expire. These approaches, may include configuring the SSD  130  to record the number of erase samples performed on each part of the flash memory  150 . These approaches may also include a mechanism for monitoring environmental conditions, e.g., temperature. For example, and as illustrated in  FIG. 2 , the SSD  130  may include environmental data logging circuitry (EDLC)  210  for this purpose. The EDLC  210  may be connected to a battery  240  which may be its principal source of power or a secondary source of power. 
     Referring to  FIG. 3 , in another embodiment, the EDLC battery  240  is absent, and the EDLC  210  is powered instead by external power, supplied, e.g. by the host  110 , which powers the entire SSD  130 . In one embodiment, the host  110  is configured to control the power to the SSD  130  and may shut off power to the SSD  130  during certain time intervals to reduce the power consumption of the system. 
     Referring to  FIG. 4 , the EDLC  210  may include a real-time clock  410 , a watchdog timer  420 , one or more sensors  430 , an EDLC controller  440 , and non-volatile memory (NVM)  450 . In operation, the EDLC controller  440  may periodically read the sensors  430  and the real-time clock  410  and collect, e.g., record, time-tagged sensor readings, such as temperature readings, in the NVM  450 . In other embodiments, the host  110  may contain an environmental data logging system, data logged by which may be used instead of, or in addition to, data logged by the EDLC  210 , to determine when data items are nearing expiration. In one embodiment, the EDLC  210  may be integrated into the SSD controller  140  provided that it is implemented in a dedicated power island, so that power may be provided to the EDLC  210  without providing power to the entire SSD  130 , which would result in excessive power consumption. 
     Referring to  FIG. 5 , in one embodiment in which the EDLC  210  has, and is able to operate on power supplied from, a battery  240 , avoiding expiration of data items is accomplished using the method illustrated, in which the host powers up the SSD  130  as needed to prevent expiration of data. In an act  510 , the host operates a host timer until a set interval, referred to as a shutdown interval, has expired. During this time, external power to the SSD  130  is shut off by the host  110 , and the EDLC  210 , operating on battery power, logs time-stamped environmental data, e.g., temperature data, in non-volatile memory  450 . In an act  520 , the host  110  then turns on the SSD  130 , i.e., turns on the power provided to the SSD  130 . The SSD  130  starts up, and, in an act  530 , the SSD  130  recovers time-stamped environmental data from the non-volatile memory  450  of the EDLC  210 , and uses it, in an act  540 , to identify the regions or portions of the flash devices  230  that need to be scanned and refreshed. If NAND flash is used, this process may be performed one flash block at a time. The SSD  130  then communicates, in an act  550 , to the host  110  through vendor-specific commands the result of the analysis, including the next shutdown interval, e.g., an estimate of the time interval until a refresh may next be needed. This time interval may, for example, be the shortest expected remaining data life for any data item, under conservative assumptions for the temperature during the next shutdown interval, reduced by a certain amount to provide a safety margin. Finally, in an act  560  the host  110  shuts the SSD  130  off again, i.e., turns off power to the SSD  130 , sets the host timer to the shutdown interval, and starts the host timer. If a need to access the data on the SSD  130  arises before the shutdown interval has ended, the host  110  may power up the SSD  130  briefly, to access the data, and then shut it down again. 
     Referring to  FIG. 6 , in another embodiment in which the EDLC  210  need not have a battery  240 , avoiding expiration of data items is accomplished using the method illustrated, in which the host powers up the SSD  130  as needed to prevent expiration of data. In an act  610 , the host operates a host timer until a set interval has expired. The host includes an environmental data logging system, and during the act  610 , the host  110  collects relevant environmental information, e.g., the host  110  records the temperature along with a time stamp for each temperature sample, while the SSD  130  is powered down. In act  620 , the host  110  then turns on the SSD  130 , i.e., turns on the power provided to the SSD  130 . The SSD  130  starts up, and, in an act  630 , the host  110  provides the collected environmental data to the SSD  130 . Any method may be used for sending the environmental data to the SSD  130 , including but not limited to side band protocols such as Management Component Transport Protocol (MCTP), and vendor-specific commands. The SSD  130  receives the environmental data and uses it, to decide, in an act  640 , which data items must be refreshed, e.g., moved. The SSD  130  then scans the flash devices  230  and refreshes the data as needed, based on the estimated remaining data life of the data items in the flash memory  150 . If the flash memory is composed of NAND flash, then each data item may be a block, i.e., this process may be performed in units of blocks. In an act  650 , the SSD  130  then sends to the host  110  a report including a new shutdown interval, and, in an act  660 , the host  110  determines, based on the length of the new shutdown interval, whether to shut the SSD  130  off or leave it powered up. 
     Referring to  FIG. 7 , in another embodiment avoiding expiration of data items is accomplished using the method illustrated. In an act  710 , the watchdog timer  420  in the EDLC  210  counts down, while the SSD  130  is in sleep mode, to the end of a certain time interval referred to herein as a sleep interval. During this time, the EDLC logs time-stamped environmental data, e.g., temperature data, in non-volatile memory  450 . In sleep mode the host  110  continues to provide power to the SSD  130 , but the SSD  130  disables all non-essential functions and operates in a reduced-functionality and reduced-power-consumption mode. In sleep mode the EDLC continues to operate, either from the power supplied by the host  110  or, if the EDLC  210  has, and is able to operate on power supplied from, a battery  240 , then in sleep mode the EDLC may be powered by the battery  240 . In an act  720 , when the watchdog timer  420  has expired, i.e., reached the end of the sleep interval, the SSD  130  wakes up, i.e., switches to an active mode. In an act  730 , the SSD  130  recovers time-stamped environmental data from the non-volatile memory  450  of the EDLC  210 , and uses it, in an act  740 , to decide which regions or portions of the flash devices  230  need to be scanned and refreshed, e.g., moved. If NAND flash is used, this process may be performed one flash block at a time. The SSD  130  then scans the flash devices  230  and refreshes the data as needed, based on the estimated remaining data life of the data items in the flash memory  150 . The SSD  130  then optionally communicates, in an act  750 , to the host  110 , through vendor-specific commands, the results of the analysis and a report containing, e.g., a list of blocks that were moved. Finally, in an act  760  the SSD  130  sets the watchdog timer  420  to a new sleep interval and enters sleep mode. This may be accomplished by firmware running in the SSD  130 , or by the operating system of the SSD  130 , if it is configured with one. The sleep time interval may, for example, be the shortest expected remaining data life for any data item, under conservative assumptions for the temperature during the next sleep interval, reduced by a certain amount to provide a safety margin. 
     Referring to  FIG. 8 , in another embodiment, avoiding expiration of data items is accomplished using the method illustrated. In an act  810 , a host timer counts down, while the SSD  130  is in sleep mode, to the end of a sleep interval. During this time, the EDLC logs time-stamped environmental data, e.g., temperature data, in non-volatile memory  450 . In sleep mode the host  110  continues to provide power to the SSD  130 , but the SSD  130  disables all non-essential functions and operates in a reduced-functionality and reduced-power-consumption mode. In sleep mode the EDLC continues to operate, either from the power supplied by the host  110  or, if the EDLC  210  has, and is able to operate on power supplied from, a battery  240 , then in sleep mode the EDLC may be powered by the battery  240 . In an act  820 , when the host timer has expired, i.e., reached the end of the sleep interval, the host wakes the SSD  130  up, i.e., causes the SSD  130  to switch to an active mode. In an act  830 , the SSD  130  recovers time-stamped environmental data from the non-volatile memory  450  of the EDLC  210 , and uses it, in an act  840 , to decide which regions or portions of the flash devices  230  need to be scanned and refreshed, e.g., moved. If NAND flash is used, this process may be performed one flash block at a time. The SSD  130  then scans the flash devices  230  and refreshes the data as needed, based on the estimated remaining data life of the data items in the flash memory  150 . The SSD  130  then sends to the host  110 , in an act  850 , a report including a new sleep interval. Finally, in an act  860  the host  110  determines, based on the length of the new sleep interval, whether to cause the SSD  130  to transition to sleep mode or to leave it in active mode. 
     In each of the methods of  FIGS. 5-8 , a software or firmware module may be executed to estimate the remaining data life of any data item, based on the number of erase cycles previously performed on the area of flash memory  150  in which the data item resides, on the temperature history the SSD  130  has experienced since the initial writing or most recent refresh of the data item, and the temperature the SSD is expected to experience during the remaining data life of the data item. This module may be referred to as a remaining data life module. Because the true remaining data life of a data item is not susceptible of precise calculation, the module may optionally reduce the estimated remaining data life so as to provide margin in the estimated remaining data life, and so that the likelihood of data corruption is acceptably low if the data item is refreshed prior to the expiration of the reduced estimated remaining data life. The remaining data life module may be executed on the host  110  or on the SSD  130 . 
     Although exemplary embodiments of a power efficient method for cold storage data retention management have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that a power efficient method for cold storage data retention management constructed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof.