Patent Publication Number: US-9405694-B2

Title: Caching data between a database server and a storage system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Benefit Claim 
     This application is a Continuation of U.S. patent application Ser. No. 12/631,985, filed Dec. 7, 2009, which claims benefit of Provisional Application 61/242,316, filed Sep. 14, 2009, under 35 U.S.C. §119(e). The entire contents of both of these applications are hereby incorporated by reference as if fully set forth herein. The applicant(s) hereby rescind any disclaimer of claim scope in the parent application(s) or the prosecution history thereof and advise the USPTO that the claims in this application may be broader than any claim in the parent application(s). 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to caching and, more specifically, caching data that is transferred between a database server and a storage system. 
     BACKGROUND 
     Database servers manage data for database clients. In a typical database system, database clients issue database commands to the database servers to cause the database server to create logical storage structures, such as tables, and to insert, update, read and delete items within those logical storage structures. The items that are managed by the database servers, as well as the logical structures used to contain the items, are typically stored on non-volatile memory, such as magnetic disks, provided by a storage system. 
     Before a database server can read or update data, the database server must retrieve the data from the storage system into volatile memory that is local to the database server. To prevent the same item from having to be retrieved every time it is needed, the database server typically stores the retrieved items in a buffer cache within the volatile memory that is local to the database server. Typically, the volatile memory used by the buffer cache is significantly faster, more expensive, and smaller than the non-volatile memory used by the storage system to persistently store the items. Consequently, even when the buffer cache is completely full, the buffer cache still only holds a fraction of the total data managed by the database server. 
     Because the buffer cache holds only a fraction of the total data managed by a database server, there is frequently a need to replace items in the buffer cache with newly requested items. Typically, the database server selects which items to replace based on how recently the items have been used, where least recently used items are replaced before more recently used items. 
       FIG. 1  is a block diagram illustrating a typical database system in which multiple clients (DB client A, DB client B and DB client C) issue commands to a database server  102 . Database server  102  resides in the volatile memory  106  of a computing device. That same volatile memory is typically used for both the buffer cache (shown as shared cache  108 ), and for private memory  110 A- 110 C. Private memory  110 A- 110 C differs from shared cache  108  in that all DB clients A-C are allowed read items that reside in shared cache  108 , but only the DB client A-C that corresponds to a particular private memory  110 A-C is allowed to see items that reside in that private memory  110 A-C. In the illustrated example, private memory  110 A is used for operations requested by DB client A, private memory  110 B is used for operations requested by DB client B, and private memory  110 C is used for operations requested by DB client C. 
     Typically, when a database client requests to read an item that does not currently reside in volatile memory  106 , database server  102  sends a request for the item to the storage system  104 . Storage system  104  responds to the request by reading the item from non-volatile memory (such as disk  112 ), and sending the item to the database server  102 . As data is written to or retrieved from disk  112 , the data is temporarily stored in volatile memory  106  within storage system  104 . 
     When the database server  102  receives the item from storage system  104 , database server  102  stores the item in shared cache  108 . If shared cache  108  is currently full, the database server  102  first makes room for the new item by selecting an existing item for replacement. If the item that is selected for replacement is “dirty”, then the item must be sent to the storage system  104  before replacing the item within the shared cache  108 . A dirty item is an item that has been updated after its retrieval from the storage system  104 , so that the version of the item that resides in shared cache  108  differs from and is more recent than the version of the item that resides in the storage system  104 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
         FIG. 1  is a block diagram of a typical database system; 
         FIG. 2  is a block diagram of a database system that employs an intermediate cache between the shared cache used by the database server and the non-volatile storage used by the storage system, according to an embodiment of the invention; 
         FIG. 3  is a block diagram of a database system in which the intermediate cache is local to the same server machine that is executing the database server, according to an embodiment of the invention; 
         FIG. 4  is a block diagram of a database system in which the intermediate cache resides in a storage system, according to an embodiment of the invention; 
         FIG. 5  is a block diagram of a database system that employs both a DB server-side intermediate cache, and a storage-side intermediate cache, according to an embodiment of the invention; and 
         FIG. 6  is a block diagram upon which the techniques described herein may be implemented. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid unnecessarily obscuring the present invention. 
     General Overview 
     Techniques are described hereafter for using an intermediate cache between the shared cache of a database server and the non-volatile storage of a storage system. The intermediate cache may be local to the machine upon which the database server is executing, or may be implemented within the storage system. In one embodiment, the database system includes both a DB server-side intermediate cache, and a storage-side intermediate cache. 
     According to one aspect of the invention, the intermediate cache is larger, slower, and less expensive than the shared cache used by the database server, but smaller, faster, and more expensive than the non-volatile storage used by the storage system. For example, in one embodiment, the shared cache is stored within DRAM, the intermediate cache is stored within FLASH memory, and the storage system uses magnetic disks to persistently store the items managed by the database server. However, this is merely one example of the types of memory devices that may be used to implement a database system according to an embodiment of the invention. Thus, the system may alternatively use DRAM for the shared cache, a smaller higher-speed magnetic disk for the intermediate cache, and larger lower-speed magnetic disks to persistently store the data. 
     The Intermediate Cache 
     As mentioned above, the techniques described herein involve making use of an intermediate cache that is logically positioned between the volatile memory of the database server and the non-volatile memory of the storage system.  FIG. 2  is a block diagram of the computer system of  FIG. 1  to which an intermediate cache  202  has been added. The nature of the intermediate cache  202  is such that database server  102  can retrieve items from the intermediate cache  202  faster than the database server  102  can retrieve items from disk  112 . For example, intermediate cache  202  may be implemented using FLASH memory, or a relatively fast magnetic disk. 
     When retrieving data, intermediate cache  202  operates as a conventional cache. Thus, when an item is needed by database server  102 , shared cache  108  is searched first. If the item is not in shared cache  108 , then intermediate cache  202  is searched. If the item is not in intermediate cache  202 , then the item is retrieved from non-volatile storage  112 . 
     While intermediate cache  202  is used in a conventional manner when retrieving data, the techniques used to determine what items to store within intermediate cache  202  are not conventional. Specifically, copies of items are not automatically stored in intermediate cache  202  in response to those items being retrieved from the disk  112 . Rather, in one embodiment, no items are stored in intermediate cache  202  at the time that they are retrieved from disk  112 . Instead, items are stored in intermediate cache  202  only in response to those items being replaced in shared cache  108 . Thus, intermediate cache  202  is used as an “overflow” area of shared cache  108 . 
     Further, caching policies are employed that treat items differently based on factors such as the database object to which the items belong, the item type of the item, a characteristic of the items, and the database operation in which the items are involved. Once cached, these same factors may be used to determine the conditions under which the items are replaced within the cache to make room for new items. For example, the caching policy may indicate that items from some tables may be replaced under one set of conditions, while items from other tables may be replaced under a different set of conditions. 
     Usage Example 
     As an example of how intermediate cache  202  operates as an overflow area for shared cache  108 , assume that an item A is needed for an operation that was requested by DB client A. To retrieve a copy of item A, the database server first checks shared cache  108  for item A. If shared cache  108  does not have item A, then intermediate cache  202  will be checked for item A. If intermediate cache  202  does not have item A, then item A is retrieved from one of disk  112 . The retrieved copy of item A is placed in shared cache  108 . However, if shared cache  108  is full, then an item in shared cache  108  is replaced with item A. 
     For the purpose of explanation, assume that database server  102  determines that an item B that resides in shared cache  108  should be replaced with item A. In response to item B being replaced in shared cache  108 , item B is written to intermediate cache  202 . 
     If item B is “dirty” at the time item B is replaced within shared cache  108 , item B is written to disk  112  before item B is written to intermediate cache  202 . Because dirty items are written to disk  112  before they are placed into intermediate cache  202 , the items in intermediate cache  202  will be the same versions as the copies that are on disk  112 . Thus, the items that are stored in the intermediate cache  202  are “clean”. 
     If the intermediate cache  202  is full, writing item B to intermediate cache  202  involves replacing an item C that currently resides in intermediate cache  202 . However, replacing item C in the intermediate cache  202  does not require writing item C back to disk  112 , because all of the items (including item C) that are stored in the intermediate cache  202  are clean. 
     After item B has been written to intermediate cache  202  (and to disk  112 , if item B was dirty), the database server  102  may be asked to perform an operation that involves item B. After determining that item B does not reside in shared cache  108 , intermediate cache  202  is searched for item B. Under these circumstances, the copy of item B will reside in intermediate cache  202 . Consequently, the database server  102  avoids the overhead associated with retrieving item B from disk  112  by retrieving item B directly from intermediate cache  202  into shared cache  108 . 
     Caching Policies 
     In the example given above, when item B was replaced in shared cache, item B was written to intermediate cache  202 . However, according to one embodiment, items are not always written to intermediate cache  202  when they are replaced in shared cache  108 . Rather, items are written to intermediate cache  202  only when they satisfy certain caching policies. The caching policies that govern intermediate cache  202  may take many forms. For example, the decision about whether to store an item into the intermediate cache may hinge on (a) the database object to which the item belongs, (b) the nature of the database operation in which the item was involved, (c) the item type of the item, and/or (d) an attribute of the item. These are merely examples of the types of factors that may be used to determine whether or not an item is to be cached in the intermediate cache. While examples of how each of these factors may be used by the caching policies that govern the intermediate cache  202 , these factors are merely exemplary. The caching policies that apply to intermediate cache  202  are not limited to any particular factors. 
     The Keep Directive 
     As mentioned above, the policies that govern intermediate cache  202  may include policies that take into account the database objects to which items belong. For example, according to one embodiment, database server  102  is configured to receive certain directives from users. In one embodiment, the directives include a KEEP directive by which users designate which objects within the database are “KEEP” objects. Items from KEEP objects are referred to herein as KEEP items. 
     Various mechanisms may be used to designate objects as KEEP objects. For example, in one embodiment, the database command that is submitted to create a database object may include a parameter value that indicates whether the object is a KEEP object. As another example, after an object has been created, a user may submit an ALTER command that changes a property of the object from non-KEEP to KEEP. The user may be motivated to do so, for example, by statistics that indicate that the object is accessed more frequently than other objects. There is no restriction on the type of objects that may be designated as KEEP objects. For example, KEEP objects may be tables, indexes, partitions of tables, tablespaces, etc. 
     As shall be explained hereafter, KEEP items receive preferential treatment once the KEEP items are stored in intermediate cache  202 . Specifically, in one embodiment, once stored in intermediate cache  202 , KEEP items are not replaced within intermediate cache  202  as long as there are non-KEEP items that can be replaced. For example, assume that a table X has been designated as a KEEP object, and that other tables Y and Z are non-KEEP objects. Further assume that intermediate cache  202  is full of items from tables X, Y and Z. Under these circumstances, if any new item needs to be placed in intermediate cache  202 , then the new item would replace either a non-KEEP item from table Y, or a non-KEEP item from table Z. Items within intermediate cache  202  that belong to table X would not be considered as candidates for replacement, regardless of how long it has been since the items have been accessed. 
     To select which non-KEEP item to replace with the new item, any one of a variety of replacement policies may be used For example, the non-KEEP item that is replaced by the new item may be selected using a least recently used replacement policy, or a least frequently used policy. 
     In an alternative embodiment, a new item is allowed to replace a KEEP item within intermediate cache  202  under certain circumstances. In one embodiment that allows KEEP items to fill the entire intermediate cache  202 , a new item is allowed to replace a KEEP item when the new item is a KEEP item and the entire intermediate cache is full of KEEP items. In an embodiment that shall be described hereafter, KEEP items are not allowed to fill the entire intermediate cache  202 . In an embodiment that does not allow KEEP items to fill the entire intermediate cache  202 , a new item is also allowed to replace a KEEP item when the new item is a KEEP item and the amount of intermediate cache  202  that is currently used by KEEP items has reached a predetermined maximum size. 
     Reserved and Unreserved Areas 
     According to one embodiment, the intermediate cache  202  is divided into two areas: a reserved area and an unreserved area. The reserved area is dedicated to storing KEEP items. The unreserved area stores non-KEEP items. 
     In one embodiment, the reserved area and the unreserved area have fixed sizes. For example, intermediate cache  202  may be a 100 gigabyte FLASH disk, where 70 gigabytes are used as the reserved area, and 30 gigabytes are used as the unreserved area. In this scenario, KEEP items are likely to remain in the intermediate cache  202  longer than non-keep items merely by virtue of the fact that there may be fewer KEEP items than non-KEEP items, and more storage within the FLASH disk is used to cache the KEEP items. 
     In an alternative embodiment, the sizes of the reserved and unreserved areas change dynamically based on the items that are stored in the intermediate cache  202 . For example, in one embodiment, the unreserved area is initially established to include 100% of the intermediate cache  202 . The unreserved area continues to include 100% of the intermediate cache until a KEEP item is stored in the intermediate cache  202 . As KEEP items are stored in the intermediate cache  202 , the size of the reserved area of the intermediate cache  202  automatically increases to accommodate the KEEP items. As the size of the reserved area increases, the size of the unreserved area decreases. For example, storing a 2 kilobyte KEEP item within in intermediate cache  202  increases the size of the reserved area by 2 kilobytes, and reduces the size of the unreserved area by 2 kilobytes. 
     If the size of the reserved area is allowed to increase unchecked, the reserved area may ultimately consume the entire intermediate cache  202 . Once the reserved area has taken over the entire intermediate cache  202 , the intermediate cache  202  will cease to cache any non-KEEP items. To avoid this situation, in one embodiment, a maximum size is established for the reserved area. In such an embodiment, the reserved area is allowed to expand until the reserved area has reached its maximum size. At that point, if additional KEEP items need to be stored in the intermediate cache  202 , the new KEEP items are stored in the reserved area over other KEEP items, thereby avoiding the need to further extend the reserved area. When a new KEEP item needs to be stored over an existing KEEP item, the KEEP item that is replaced may be selected using any one of a variety of replacement policies. For example, in one embodiment, the KEEP item that is written over is selected using a least recently used replacement policy. 
     The None Directive 
     In one embodiment, a NONE directive may be used to indicate that items from a particular object should not be placed in intermediate cache  202 . For example, if it is known that items from a particular table are rarely accessed with high frequency, the user may use the NONE directive to indicate that the table is a NONE object. Data items from NONE objects (“NONE items”) are not placed into intermediate cache when they would otherwise have been placed in intermediate cache  202  if they had not been NONE items. 
     Caching Policies Based on Operation Type 
     As mentioned above, the caching policies that govern intermediate cache  202  may include policies that take into account the type of database operation that is being performed on the items. According to one embodiment, when the nature of the database operation that causes the database server  102  to retrieve items into volatile memory  106  is such that it is unlikely that those items will be accessed again in the near future, intermediate cache  202  is not used to cache copies of the items. 
     For example, when database server  102  performs a table scan, there is a relatively low probability that the items that are retrieved during the table scan will be needed again soon after completion of the table scan. Thus, according to one embodiment, items that are retrieved by database server  102  during performance of a table scan are not cached in intermediate cache  202 . 
     Various mechanisms may be used to ensure that intermediate cache  202  is not used to cache items that are involved in certain types of operations. For example, in one embodiment, caching items in intermediate cache  202  is avoided by causing database server  102  to retrieve those items directly into the private memory of the DB client&#39;s for whom the operations are being performed. Because the items are loaded directly into private memory without first being placed in shared cache  108 , the items will never be replaced in shared cache  108 . In embodiments where the replacement of a dirty item in shared cache  108  is the trigger for storing the item in intermediate cache  202 , items never stored in shared cache  108  will also never be stored in intermediate cache  202 . 
     Caching Policies Based on Item Type 
     As mentioned above, the caching policies that govern intermediate cache  202  may include policies that take into account the item type of items that are being written to disk. For example, a typical database server  102  manages many types of disk blocks. Common block types include: an index block type, a data block type, an undo block type, a segment header block type, a transaction header block type, etc. The block type of a block generally corresponds to what type of information is contained within the block. For example, undo blocks contain information that allows database server  102  to undo changes that are made to other items. 
     According to one embodiment, when the type of an item is such that it is relatively unlikely that the item will be accessed again in the near future, the item is not stored in intermediate cache  202  in response to the item being written to disk  112 . For example, although database server  102  may need to undo changes to items in a variety of circumstances (such as when the transactions that performed the changes abort, or when a non-current version of the items is needed), those circumstances are relatively infrequent. Therefore, according to one embodiment, undo blocks are not stored in intermediate cache  202  in response to being written to disk  112 . 
     Caching Policies Based on Attributes of Items 
     As mentioned above, the caching policies that govern intermediate cache  202  may include policies that take into account the attributes of items that are being written to disk. The attributes reflect information about the item itself, rather than the type of the content of the item. For example, an attribute of an item may be whether the item is encrypted. In one embodiment, database server  102  supports both encrypted items and non-encrypted items. To reduce the likelihood that encrypted items will be compromised, the caching policy may be that encrypted items are not stored to intermediate cache  202 . 
     As another example, some storage systems support mirroring, where all changes that are made to a “primary” copy of an item are also made to a “secondary” copy of the same data item. In such systems, one attribute of an item is whether the item is a primary copy or a secondary copy. According to one embodiment, when an item is a primary copy, the item is stored to intermediate cache  202  in response to the primary copy being written to disk  112 . On the other hand, when an item is a secondary copy, the item is not stored to intermediate cache  202  when written to disk  112 . 
     Load-on-Read and Load-on-Replace 
     In the embodiments described above, copies of items are stored in intermediate cache  202  in response the items being replaced within shared cache  108 . Thus, those embodiments represent a load-on-replace policy for storing items in intermediate cache  202 . In alternative embodiment, load-on-read policies govern storage of items in intermediate cache  202 . 
     According to load-on-read policies, items are stored in intermediate cache  202  at the time they are retrieved into volatile memory  106 . Regardless of whether load-on-read or load-on-replace is used, the various caching policies described above may be applied to ensure that less-useful items are not stored in intermediate cache  202 , and that the most useful items are stored longer in intermediate cache  202  than other items. Specifically, similar to load-on-replace, an embodiment that uses load-on-read may also employ caching policies that take into account one or more of: the type of database operation in which the items are involved, the database objects to which the items belong, the item type of the items and attributes of the items. 
     DB Server-Side Intermediate Cache 
     In  FIG. 2 , intermediate cache  202  is located between volatile memory  106  and disk  112 . Thus, intermediate cache  202  may be local to the database server, local to the storage system, or deployed separate from both the database server and the storage system. According to one embodiment, the caching policy used by intermediate cache  202  depends on where the intermediate cache  202  is deployed.  FIGS. 3, 4 and 5  illustrate in greater detail various configurations of where intermediate cache  202  may be located, according to various embodiments of the invention. 
     Referring to  FIG. 3 , it illustrates an embodiment in which intermediate cache  202  is implemented as a DB server-side cache  304 . DB server-side cache  304  is local to the server machine  302  on which database server  102  is executing. Consequently, retrieving items from DB server-side cache  304  do not incur the transmission overhead that would otherwise exist if the intermediate cache  202  were remote to server machine  302 . 
     In some systems, the mirroring of data is handled by the storage system  104  in a manner that is transparent to the database server  102 . In such systems, the primary/secondary distinction does not exist outside of the storage system  104 . Therefore, when intermediate cache  202  is implemented as a DB server-side intermediate cache  304 , the caching policy does not make use of any primary/secondary distinctions. However, the DB server-side intermediate cache  304  may still take into account whether the database objects to which items belong are KEEP, non-KEEP or NONE, whether the items are encrypted, whether the items are undo blocks, whether the items are being access as part of a table scan, etc. 
     Storage-Side Intermediate Cache 
       FIG. 4  is a block diagram of an embodiment in which the intermediate cache  202  is implemented as a storage-side intermediate cache  406  that is local to the storage system  104 . Because the storage-side intermediate cache  406  is not local to the server machine  302 , retrieving items from storage-side intermediate cache  406  incurs more transmission overhead than would exist if the intermediate cache  202  were local to server machine  302 . However, retrieval of items from storage-side intermediate cache  406  is still faster than retrieval of items from disk  112 , since storage-side intermediate cache  406  is generally implemented on a faster storage device than disk  112 . For example, storage-side intermediate cache  406  may be implemented on a FLASH disk, or on a relatively fast magnetic disk, while disk  112  is implemented by a relatively slow magnetic disk. 
     According to one embodiment, when intermediate cache  202  is implemented on the storage side, the database server  102  communicates information to storage system  104  to enable storage system  104  to implement intelligent caching policies. For example, when storage system  104  receives an I/O request for an item from disk  112 , storage system  104  does not normally know the type of database operation in which the items are involved, the database objects to which the items belong, the item type of the items, nor the attributes of the items. Consequently, for storage system  104  to use caching policies that take into account these factors, database server  102  must provide additional information to storage system  104 . 
     According to one embodiment, the additional information provided by database server  102  to storage system with a particular I/O request may simply be a flag that indicates whether or not storage system  104  should store the item involved in the I/O request in storage-side intermediate cache  406 . For example, if the database server  102  is requesting items involved in a table scan, the database server  102  may set the flags that accompany the I/O requests to indicate that the items should not be stored in intermediate cache  406 . Similarly, database server  102  may set the flags to indicate that items should not be stored in storage-side intermediate cache  406  if: the database server  102  is requesting items that belong to a NONE object, the database server  102  is writing undo blocks, or the database server  102  knows that the I/O involves encrypted information. 
     In embodiments that support KEEP and non-KEEP items, the database server  102  may include a flag with each I/O request to indicate whether the item involved in the I/O request is a KEEP item or a non-KEEP item. Based on this information, the storage system  104  may maintain reserved and unreserved areas within storage-side intermediate cache  406 , as described above. 
     In an alternative embodiment, rather than merely passing flags, database server  102  passes information that enables storage system  104  to make the decisions relating to the caching policies. For example, database server  102  may pass storage system  104  information that indicates which database objects have been designated as KEEP objects or NONE objects. Then, with each I/O request, database server  102  may pass to storage system  104  information that indicates the database object involved in the I/O request. Based on this information, storage system  104  can make the decision about whether to store the item specified in the I/O request in storage-side intermediate cache  406 , and if the item is to be stored, whether to store the item in the reserved area or the unreserved area. 
     Similarly, database server  102  may pass to storage system  104  information that indicates the item type of the data item involved in the I/O request (e.g. whether the item is an undo block) and/or an attribute of the data item involved in the I/O request. Based on this information, the storage system  104  may make decisions about whether to store in storage-side intermediate cache  406  the items involved in the I/O requests that it receives. 
     Load-on-Read-or-Write 
     In one embodiment, the storage-side intermediate cache  406  employs both load-on-read and load-on-write techniques. Thus, based on the caching policy, storage system  104  may copy items into storage-side intermediate cache  406  both when the items are transferred from disk  112  to server machine  302 , and when the items are transferred from server machine  302  to disk  112 . 
     In systems where the storage system  104  performs mirroring, the storage system  104  may implement a caching policy that takes into account the primary/secondary distinction without any additional information from the database server  102 . Specifically, because storage system  104  is responsible for maintaining the primary and secondary copies, storage system  104  is aware of which copies are primary and which are secondary. Based on this information, the storage system  104  may implement a policy in which secondary copies of items are always treated as NONE items. 
     In one embodiment, the policy of treating secondary copies as NONE items has an exception that specifies that a secondary copy of an item is cached in storage-side intermediate cache  406  if involved in a read operation, but not when involved in a write operation. The secondary copy is cached during read operations because the secondary copy is typically read only when the primary copy is unavailable. Thus, when the subject of a read operation, the secondary copy effectively becomes the primary copy, and for the purpose of caching is treated as if the secondary copy were the primary copy. 
     Multiple-Server Systems 
     When implemented as a storage-side intermediate cache  406 , intermediate cache  202  may benefit many database servers. For example, server machine  302  may be merely one of a plurality of server machines that are running database servers that are accessing data stored on storage devices that are managed by storage system  104 . The same storage-side intermediate cache  406  may be used by storage system  104  to improve the performance of the I/O requests for all of those database servers. 
     Further, when multiple database servers are sharing access to the same database, it is possible that an item loaded into storage-side intermediate cache  406  in response to the request by one database server may be provided from the storage-side intermediate cache  406  to another database server that needs a copy of the same item. 
     Per-Object Cache-Hit Statistics 
     One benefit of having database server  102  provide storage system  104  information that identifies the database objects involved in the I/O requests is that this information enables storage system  104  to maintain statistics about I/O requests on a per-database-object basis. Such statistics may indicate that a particular database object is accessed frequently, while another database object is accessed rarely. This information may be helpful to a user who is deciding which database objects should be made KEEP objects and which database objects should be made NONE objects. 
     In one embodiment, these statistics are used to automatically transition objects between KEEP, non-KEEP, and NONE. For example, in response to determining, based on the per-object statistics, that a particular object is rarely accessed, the object may be automatically transitioned from a KEEP object to a non-KEEP object, or from a non-KEEP object to a NONE object. On the other hand, in response to determining that a particular object is frequently accessed, the object may be automatically transitioned from a NONE object to a non-KEEP object, or from a non-KEEP object to a KEEP object. 
     Dropped Objects 
     According to one embodiment, when a database object is dropped, all items that belong to the database object within intermediate cache  202  are invalidated, thereby making space for new objects. When KEEP items are dropped, the size of the reserved area shrinks, so that the memory occupied by the invalidated items can be used for either KEEP or non-KEEP items. 
     Preferably, the database server  102  sends a message to storage system  104  to inform storage system  104  when a data object has been dropped. In response, the storage system  104  uses the metadata about the database objects to which the cached items belong to identify and invalidate within the storage-side intermediate cache  506  all data items that belong to the dropped table. 
     Cache Expiration 
     According to one embodiment, items within intermediate cache  202  expire under certain conditions. When an item expires within cache  202 , the storage within intermediate cache  202  in which the item resides is freed to allow storage of other items. If the item was a KEEP item, then the reserved area of intermediate cache  202  shrinks, thereby making the space available for either KEEP items or non-KEEP items. 
     According to one embodiment, cache expiration can occur at both the item level and the object level. For example, the expiration policy that governs intermediate cache  202  may be that (1) if an item has not been accessed in a week, the item expires, and (2) if no item that belongs to a particular object has been accessed in three days, then all items that belong to that object expire. In one embodiment, expiration timers are only set for KEEP items and KEEP objects, because unused non-KEEP items will be replaced simply by using least-recently-used replacement techniques on items in the unreserved area of intermediate cache  202 . 
     Employing a cache expiration policy is particularly useful in embodiments that have a storage-side intermediate cache  406 . When a storage-side intermediate cache  406  is involved, it is possible for information to be lost between server machine  302  and storage system  104 . For example, assume that a particular KEEP object is dropped by database server  102 . If for any reason storage system  104  is not told that the object has been dropped, then the storage-side intermediate cache  406  may continue to store items from the KEEP object indefinitely. An expiration policy ensures that the items that will never be used again will not continue to occupy the storage-side intermediate cache  406 . 
     According to an alternative embodiment, the memory that stores KEEP items and KEEP objects that expire is not invalidated when in response to the expiration. Rather, the memory is logically moved from the reserved area to the unreserved area. As a result of this change, the reserved area shrinks, the unreserved area grows, and the expired KEEP items within the memory may be overwritten by non-KEEP items. If the expired KEEP items are accessed again before being replaced, then the memory is upgraded once again to be part of the reserved area, thereby removing those KEEP items from the unreserved area. 
     Combining Server-Side and Storage-Side Caches 
       FIG. 5  is a block diagram that employs both a DB server-side intermediate cache  504  and a storage-side intermediate cache  506 . In one embodiment, DB server-side intermediate cache  504  is local to and used exclusively by server machine  302 , while storage-side intermediate cache  506  is managed by storage system  104  and is used to speed up requests made by many database servers, some of which may not have their own DB server-side intermediate caches. 
     In a system that includes both a DB server-side intermediate cache  504  and a storage-side intermediate cache  506 , when database server  102  requires an item, shared cache  108  is first search for the item, and if the item is not in shared cache  108 , then DB server-side intermediate cache  504  is searched for the item. If the item does not exist in either shared cache  108  or DB server-side intermediate cache  504 , then the database server requests the item from storage system  104 . Storage system  104  first looks for the item in storage-side intermediate cache  506 , and if the item is not in storage-side intermediate cache  506 , then storage system  104  retrieves the item from disk  112 . 
     In a system that includes both a DB server-side intermediate cache  504  and a storage-side intermediate cache  506 , the caching policies need not be the same. For example, DB server-side intermediate cache  504  may allow caching of items retrieved during table scans but not allow caching of undo blocks, whereas storage-side intermediate cache  506  may allow caching of undo blocks, but may not allow caching of items retrieved during table scans. 
     Further, even in systems where both DB server-side intermediate cache  504  and a storage-side intermediate cache  506  allow the use of KEEP and NONE directives, the objects that are marked KEEP and NONE for DB server-side intermediate cache  504  may be different than the objects that are marked KEEP and NONE for the storage-side intermediate cache  506 . For example, the same table T may be marked KEEP for the purpose of DB server-side caching, and marked non-KEEP for the purpose of storage-side caching. Under these circumstances, items from the object will be stored in the reserved area of DB server side cache  504 , and stored in the unreserved area of storage-side intermediate cache  506 . 
     Cache-Copy Metadata 
     According to one embodiment, metadata is maintained for items that reside in intermediate cache  202 . However, in one embodiment, the amount of metadata maintained for items that reside in intermediate cache  202  is significantly smaller than the metadata maintained by database server  102  for items in shared cache  108 . For example, in an embodiment that uses a load-on-replace policy, the copies of items in shared cache  108  will never be out of sync with the copies of those same items that are on disk  112 . Therefore, the metadata for items in intermediate cache  202  need not include information about whether the items are dirty, and if dirty, when the times were modified. 
     As another example, in one embodiment the items in intermediate cache  202  are not owned exclusively by any transaction. Consequently, items within intermediate cache  202  need only support minimal lock state to indicate whether the items are subject to a shared lock or to no lock. In response to an item that resides in intermediate cache  202  being loaded into volatile memory  106 , the minimal lock structure required for the item in intermediate cache  202  is converted into a larger lock structure required to support numerous types of locks, dirty data items, etc. 
     Non-Database-Server Applications 
     While caching techniques have been described herein in the context of database-server-to-storage communications, the techniques are not limited to any particular context. For example, the caching techniques may be used with any application that stores data persistently on a storage device. Such applications may include, for example, word processing programs, spreadsheet programs, email servers, etc. 
     In one embodiment, any such application may make use of an intelligent intermediate cache by designating some objects as KEEP objects and/or by designating some objects as NONE objects. When deciding how to use the intermediate cache for a particular item, it is determined whether the item belongs to an object that has been given a particular designation by the application. In one embodiment, if the item belongs to a NONE object, then the item is not cached in the intermediate cache. If the item belongs to an object with no particular designation, then the item is stored in an unreserved section of the intermediate cache. Finally, if the item belongs to an object with a KEEP designation, the item is stored in a reserved area of the intermediate cache. The reserved area is extended, if necessary, to accommodate the KEEP item, unless the reserved area has already reached a maximum size. 
     The “objects” that are designated KEEP and NONE, as well as the items that belong to the objects, will vary based on the nature of the application. Such objects may be any form of data structure, file, or collections of data structures or files. For example, when the application is a database server, the objects may be database objects and the items may things such as rows, data blocks, or index entries. On the other hand, if the application is a word processing program, the objects may be documents, and the items may be parts of the documents. These are merely examples of the types of applications and objects to which the caching techniques described herein may be applied. The techniques are not limited to any particular type of objects or items. 
     Example Embodiments 
     According to an embodiment, a method comprises: receiving, from an application that performs operations on a plurality of objects, data that indicates which objects of the plurality of objects have a particular designation; caching, in an intermediate cache, data that is transferred between (a) a volatile memory used by the application and (b) a non-volatile storage device that is used to persistently store the data for the application; wherein caching the data includes using the intermediate cache, with respect to an item that is transferred between the volatile memory and the non-volatile storage, in a manner that is based, at least in part, on whether the item belongs to an object that has said particular designation; wherein the method is performed by one or more computing devices. 
     In an embodiment, the particular designation is a KEEP designation; and the step of using the intermediate cache includes: storing the item in a reserved area of the intermediate cache if the item belongs to an object with the KEEP designation; and storing the item in an unreserved area of the intermediate cache if the item belongs to an object that does not have the KEEP designation. In an embodiment, the item belongs to an object that has the KEEP designation; and storing the item in a reserved area includes: within the intermediate cache, replacing another item with the item, wherein the other item belongs to an object that does not have a KEEP designation; and in response to replacing the other item with the item, increasing the size of the reserved area and decreasing the size of the unreserved area. In an embodiment, the particular designation is a NONE designation; and the step of using the intermediate cache includes caching the item in the intermediate cache only if the item belongs to an object that does not have a NONE designation. In an embodiment, the application is a database server; and each of the plurality of objects is a database object. 
     According to an embodiment, a method comprises: caching, in an intermediate cache, data that is transferred between a volatile memory used by a database server and a non-volatile storage device that is used to persistently store the data; wherein the step of caching the data includes using the intermediate cache, with respect to an item that is transferred between the volatile memory and the non-volatile storage, in a manner that is based, at least in part, on one or more of: which database object, of a plurality of database objects managed by the database server, the item belongs to; an item type of the item; an attribute of the item; or a type of database operation in which the item is involved; wherein the method is performed by one or more computing devices. 
     In an embodiment, using the intermediate cache includes determining whether or not to store the data item in the intermediate cache based, at least in part, on which database object, of the plurality of objects managed by the database server, the item belongs to. In an embodiment, the method further comprises: receiving data that indicates that the particular object is a NONE object; and the step of determining includes determining not store the item in the intermediate cache because the item is from a NONE object. In an embodiment, the method further comprises: receiving data that indicates that the particular object is a KEEP object; storing the item in a reserved area of the intermediate cache because the particular object is a KEEP object; storing a second item in an unreserved area of the of the intermediate cache in response to determining that the second item belongs to a second database object that is not a KEEP object. In an embodiment, the method further comprises: preventing items that do not belong to KEEP objects from replacing, within the intermediate cache, items that belong to KEEP objects, while allowing items that belong to KEEP objects to replace, within the intermediate cache, items that do not belong to KEEP objects. 
     In an embodiment, using the intermediate cache includes determining whether or not to store the item in the intermediate cache based, at least in part, on an item type of the item. In an embodiment, the method further comprises: determining not to store the item in the intermediate cache based on the item being an undo block. In an embodiment, using the intermediate cache includes determining whether or not to store the data item in the intermediate cache based, at least in part, on an attribute of the item. In an embodiment, the method further comprises: determining not to store the item in the intermediate cache based on the item being a secondary copy of a mirrored item. In an embodiment, the method further comprises: determining not to store the item in the intermediate cache based on the item being encrypted. In an embodiment, using the intermediate cache includes determining whether or not to store the data item in the intermediate cache based, at least in part, on the type of database operation in which the item is involved. In an embodiment, the method further comprises: determining not to store the item in the intermediate cache based on the item being involved in a table scan operation. 
     In an embodiment, the method further comprises: the intermediate cache is implemented by FLASH memory. In an embodiment, the intermediate cache is local to a server machine on which the database server is executing. In an embodiment, the items are loaded into intermediate cache based on a load-on-replace policy. In an embodiment, the intermediate cache is managed by a storage system to which the non-volatile storage device belongs. In an embodiment, the database server is one of a plurality of database servers for which the storage system stores items in the intermediate cache. In an embodiment, items are loaded into the intermediate cache on a load-on-read and load-on-write policy. In an embodiment, the step of using the intermediate cache is performed by the storage system; and the method further comprises the storage system receiving, from the database server, information that indicates at least one of: which database object the item belongs to; the item type of the item; the attribute of the item; or the type of database operation in which the item is involved. 
     Hardware Overview 
     According to one embodiment, the techniques described herein are implemented by one or more special-purpose computing devices. The special-purpose computing devices may be hard-wired to perform the techniques, or may include digital electronic devices such as one or more application-specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs) that are persistently programmed to perform the techniques, or may include one or more general purpose hardware processors programmed to perform the techniques pursuant to program instructions in firmware, memory, other storage, or a combination. Such special-purpose computing devices may also combine custom hard-wired logic, ASICs, or FPGAs with custom programming to accomplish the techniques. The special-purpose computing devices may be desktop computer systems, portable computer systems, handheld devices, networking devices or any other device that incorporates hard-wired and/or program logic to implement the techniques. 
     For example,  FIG. 6  is a block diagram that illustrates a computer system  600  upon which an embodiment of the invention may be implemented. Computer system  600  includes a bus  602  or other communication mechanism for communicating information, and a hardware processor  604  coupled with bus  602  for processing information. Hardware processor  604  may be, for example, a general purpose microprocessor. 
     Computer system  600  also includes a main memory  606 , such as a random access memory (RAM) or other dynamic storage device, coupled to bus  602  for storing information and instructions to be executed by processor  604 . Main memory  606  also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor  604 . Such instructions, when stored in storage media accessible to processor  604 , render computer system  600  into a special-purpose machine that is customized to perform the operations specified in the instructions. 
     Computer system  600  further includes a read only memory (ROM)  608  or other static storage device coupled to bus  602  for storing static information and instructions for processor  604 . A storage device  610 , such as a magnetic disk or optical disk, is provided and coupled to bus  602  for storing information and instructions. 
     Computer system  600  may be coupled via bus  602  to a display  612 , such as a cathode ray tube (CRT), for displaying information to a computer user. An input device  614 , including alphanumeric and other keys, is coupled to bus  602  for communicating information and command selections to processor  604 . Another type of user input device is cursor control  616 , such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor  604  and for controlling cursor movement on display  612 . This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane. 
     Computer system  600  may implement the techniques described herein using customized hard-wired logic, one or more ASICs or FPGAs, firmware and/or program logic which in combination with the computer system causes or programs computer system  600  to be a special-purpose machine. According to one embodiment, the techniques herein are performed by computer system  600  in response to processor  604  executing one or more sequences of one or more instructions contained in main memory  606 . Such instructions may be read into main memory  606  from another storage medium, such as storage device  610 . Execution of the sequences of instructions contained in main memory  606  causes processor  604  to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions. 
     The term “storage media” as used herein refers to any media that store data and/or instructions that cause a machine to operation in a specific fashion. Such storage media may comprise non-volatile media and/or volatile media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device  610 . Volatile media includes dynamic memory, such as main memory  606 . Common forms of storage media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, NVRAM, any other memory chip or cartridge. 
     Storage media is distinct from but may be used in conjunction with transmission media. Transmission media participates in transferring information between storage media. For example, transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus  602 . Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications. 
     Various forms of media may be involved in carrying one or more sequences of one or more instructions to processor  604  for execution. For example, the instructions may initially be carried on a magnetic disk or solid state drive of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system  600  can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus  602 . Bus  602  carries the data to main memory  606 , from which processor  604  retrieves and executes the instructions. The instructions received by main memory  606  may optionally be stored on storage device  610  either before or after execution by processor  604 . 
     Computer system  600  also includes a communication interface  618  coupled to bus  602 . Communication interface  618  provides a two-way data communication coupling to a network link  620  that is connected to a local network  622 . For example, communication interface  618  may be an integrated services digital network (ISDN) card, cable modem, satellite modem, or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface  618  may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface  618  sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information. 
     Network link  620  typically provides data communication through one or more networks to other data devices. For example, network link  620  may provide a connection through local network  622  to a host computer  624  or to data equipment operated by an Internet Service Provider (ISP)  626 . ISP  626  in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet”  628 . Local network  622  and Internet  628  both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link  620  and through communication interface  618 , which carry the digital data to and from computer system  600 , are example forms of transmission media. 
     Computer system  600  can send messages and receive data, including program code, through the network(s), network link  620  and communication interface  618 . In the Internet example, a server  630  might transmit a requested code for an application program through Internet  628 , ISP  626 , local network  622  and communication interface  618 . 
     The received code may be executed by processor  604  as it is received, and/or stored in storage device  610 , or other non-volatile storage for later execution. 
     In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is the invention, and is intended by the applicants to be the invention, is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Any definitions expressly set forth herein for terms contained in such claims shall govern the meaning of such terms as used in the claims. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.