IBM 3850-Mass storage system

Transcription

1 BM 385-Mass storage system by CLAYTON JOHNSON BM Corporation Boulder, Colorado SUMMARY BM's 385, a hierarchical storage system, provides random access to stored data with capacity ranging from 35 X 1()9 to 472 X 1()9 bytes. The hierarchical architecture achieves access times varying from Direct Access Storage Device speeds to that of the Mass Storage Facility which can be as low as 1 seconds. The architecture of the Mass Storage System is examined to demonstrate its functional and performance capability. NTRODUCTON The goal of the Mass Storage System (MSS) is to provide capacities for handling all of an establishment's data. n addition, it provides the functions of Direct Access Storage Devices (DASD) at a cost which approximates that of today's half-inch tape systems, and with an architecture that allows users to migrate to MSS with minimal costs. A hierarchy of devices provides the flexibility needed to approach such a diverse set of goals. See Figure l. A hierarchical arrangement allows the MSS architecture to be hidden, or buffered, from main memory. This allows system designs that are specific to the problem of storage and retrieval of data, independent of how programs manipulate data. Utilization of an existing device eliminates the need to develop new Direct Access Devices (adding to the complexity of the development project) and provides a known, existing interface to data stored in the MSS. Actually, the intermediate devices in a storage hierarchy can be independent of the architectural interface to data. For example, in MSS the interface to data is architecturally that of an BM 3336 Model 1 DASD volume, regardless of whether a 333 Model 1, or Model 11 is being used. A new device or memory could be used in the intermediate levels of the hierarchy without changing the architectural interface to data itself. This allows for the incorporation of new technologies in the storage hierarchy with little or no disruptive impact to the users of the storage hierarchy. The ideal control of the external storage is to have on DASD that data required by the programs in the CPU when the programs need it, and to have space available on DASD to store new data. How close we come to this ideal is a measure of the trade-off of price versus performance. n this hierarchy, the best performance achievable comes from DASD. The time a program waits for data to appear on DASD or waits for space on DASD to store new data, is the visible impact of the storage hierarchy. A storage hierarchy also reduces the number of requests for data that must be resolved at the Mass Storage Facility. Recently active data sets can be kept on the DASD level of the hierarchy for a period of time after their last use. This is accomplished by providing storage capacity at the DASD level that is greater than the sum of the size of all concurrently active data sets. Reuse of data dramatically reduces the number of accesses to the Mass Storage Facility, and makes it possible to design a system that moves much less data between MSF and DASD than that which moves between Main Memory and DASD. MSS uses a Least Recently Used replacement algorithm to schedule which data on DASD should be destaged to the MSF when space is needed for new data, or when newly active data must be staged to DASD for access by the CPU(s). Only those DASD cylinders which have changed are actually destaged. The following exemplifies the work load an MSS might experience, and the advantages of this architecture. CONTROL AND DATA FLOW WTHN MSS MSS, as seen by the System Control Program, has nearly unlimited Direct Access Storage. This approach to Mass Storage furnishes the function of DASD, and it also provides a major new capability on BM CPU systems without the disruptive impact of a totally new architecture. MSS also follows the basic architecture of System/37 channels and /O addressing. The result avoids a major redesign of Operating Systems, UserApplication Programs and Channels. Today's systems gain addressability to data or storage space by mounting a storage volume (either half-inch tape or disk packs) on an auxiliary storage device. These devices have specific addresses on one or more CPU channels. An nput or Output operation is performed to a specific volume by addressing the specific device holding that volume. The total addressable data at anyone time is limited in this arrangement to the actual number of 59

2 51 National Computer Conference, J! nstructions ain Storage 1/ Operations DASD Buffer Staging/Destaging Requests Mass Storage Figure -Hierarchical store - Data movement j against the virtual device. f the specific cylinder required by the CPU (1/44th of a Mass Storage Volume) is already on DASD, an /O operation proceeds. f not, and data is being accessed, the MSC causes the cartridge containing the cylinder to be placed on a Data Recording Device (DRD), and the data contained in that cylinder to be transferred to the DASD staging buffer. The transfer path is to the Storage Controller, and from the Storage Controller to the Staging Buffer (DASD device) avoiding the extra load on channels and memory. The /O operation then proceeds as before. f the Operating System knows which cylinders will be accessed, it can cause the MSC to stage only those cylinders containing the data set; reducing the number of times cartridges need to be accessed. t should be apparent that the total amount of DASD Staging Buffer Space is related to the total size of the currently active data sets, and not to total number of active volumes. DASD is being used as a buffer to the MSF, and the actual device characteristics are architecturally unimportant. devices attached to a given system on which storage volumes are mounted. MSS resolves this limitation through a virtual device concept. Because DASD (called "staging buffers") is used in MSS as a buffer to the Mass Storage Facilities, the one volume per device relationship is no longer necessary. Only the active portion of a volume must be on DASD at a particular time; therefore, the remaining space of the DASD staging buffers can be used for portions of other active volumes. A virtual to real mapping capability is added to the Storage Controllers. Each Channel connection to a Storage Controller with MSS has the ability to address 64 unique volumes, independent of how many real devices are being used as staging buffers. A virtual device approach makes more data addressable at any time without the cost of a corresponding number of /O devices; this in turn allows larger data bases to appear to be on line, opening up new approaches to selected data base applications. On real DASD volumes (non-staging), a user must put data on volumes that tend to be active at the same time, even if they are unrelated. Many problems can result, such as backup, security, etc. A virtual device concept eases the problem of space management on DASD volumes. All space in MSS is managed in terms of DASD volumes. A pair of cartridges is assigned a name, and this name represents a specific DASD volume. This pair of cartridges is called a Mass Storage Volume. To gain address ability to a Mass Storage Volume, the operating system (as a result of the active user programs) requests a specific Mass Storage Volume be mounted at a specific virtual /O address. The mount command is directed to the Mass Storage Controller (MSC). The mount command allows MSS to translate an access to a specific Mass Storage Volume when /O access is made CONFGURNG MSS The objective of any configuration is to select the components required to do the work load of a specific customer. Excess performance capability, either as a result of the configuration or the design of the system, yields nothing useful to the customer and adds to the cost of the system. Systems must be configured to handle the highest work load case with the responsiveness required to meet work schedules or human factors requirements in interactive environments. Availability and security may also affect a configuration but are not the subject of this paper. n evaluating a Storage System, some terms must be established to describe performance. The basic objective of storage is the transfer of data to or from a CPU's Channels. The total amount of data transferred is a measure of throughput (total work) while the time to access data becomes a measure of responsiveness. Current systems can be measured using various techniques to determine the total amount of data transferred into or out of storage over given units of time. A basic difficulty in measuring a system is that, by definition, the measurement is based on past performance. Past performance provides an adequate measure only if the environment is static; which is seldom. A knowledge of existing applications growth and new application requirements must be acquired and factored into planned configurations. n a storage hierarchy such as MSS, the Direct Access Storage must have sufficient performance capability to supply all accesses required by the CPU's main memory. Direct Access Storage is also involved when data is staged from the Mass Storage Facility or destaged to the MSF. t must have sufficient capacity to hold all open data sets and additional capacity to maximize the reuse potential of data. MSF must have transmission capability to handle

3 BM 385-Mass Storage System 511 the data being staged or destaged, and the ability to handle the cartridge movement involved. t must have capacity to contain all the data, backup, archival and active. A work load analysis of the total system is required to choose the proper configuration. The following is a hypothetical work load compiled to illustrate the demand that might be placed on the major interfaces of a storage hierarchy such as MSS. (See Figures 2-6.) Figure 2 represents the /O work load over time, and the sustained data rate across the CPU's channels, with tape and DASD averaged over one-hour periods. Each vertical bar reflects the combined effect of operator mount delays, operating system overhead, device raw data rate, channell control unit configuration and demand for data by the programs that were active in that hour. At any point, the nput/ Output demand can be limited by the speed of the CPU and the number of /O bytes of data required per instruction executed, or by the number of programs that may run concurrently. Peak throughput of the entire system is always limited by full utilization of the CPU's processing cycles. f the /O is configured so it never limits the CPU to less than 1 percent utilization (and the cost of this /O is not prohibitive) an ideal throughput system can result. Full utilization also requires an unending supply of work and response time which may be exceptionally long. Figure 2 is more typical, showing peaks and valleys in the /O work performed by the system. Peaks represent 1 percent utilization and valleys represent the lack of work to do; or a system being limited by operations, number of available devices, or the mix of programs in the CPU. The total amount of data staged and destaged is determined by the capacity of DASD and by how the data is being used. The potential for reuse of data can be understood by making an assumption about DASD capacity, and observing the resultant staging rate between DASD and MSF. First, assume there is only enough DASD capacity to contain the total size of concurrently active data sets; but with no additional capacity to retain data on DASD, then observe the demand on the MSF for data sets Q.) +-' 2 C'l c C'l 15 co +-' (J) Retention Time (minutes) Figure 3-Staging data rate and DASD capacity relationship (72 hour example) and data rate. Next, assume there is enough DASD capacity to contain the total size of concurrently active data sets, and there is additional capacity to retain data on DASD after its active use for longer and longer periods of time. Now, if a data set is reused within that retention period, it does not have to be staged again nor is it necessary to handle the cartridges. Figure 3 represents the results of such an analysis. The plot shows the reduction in average MSF data rate as the retention period on DASD is increased. Once a retention period has been selected, it is more accurate to look at the required average capacity over smaller increments of time. Figure 4 shows the average capacity per hour that is required to contain all the active data sets during the time of their activity, plus the additional time included for optimum reuse. Given the average capacity to optimize the reuse of data, a closer analysis of 2 co o Q) > ';; u W E > (/) O'-----'-----'----'--'---'-----'----J'---'-----'------'-----L--'--L-..L----'----' Figure 2-Systems work load (72 hour example) o Total Data Rate Tape Data Rate!. c.... ( u'" () C mo (1):= = >:2: « o Total Capacity.Tape Contribution Figure 4-DASD capacity required (72 hour example)

4 512 National Computer Conference, 1975 V ---. CQ a: o Total Staging Required 18 Tape Contribution CQ 8 1:1 Q) 6 Cl CQ en Figure 5-Data staging rate required (72 hour example) MSF activity can be made. t must take two forms, the first is the total amount of data staged and destaged, the second is the total number of individual data sets handled. Both are necessary because they represent different work loads. Figure 5 shows the staged data rate in eah hour of te analysis. The Storage System must be confgured so ths demand can be met, and so it relates directly to the amount of DASD activity introduced by the storage hierarchy. The number of paths required for staging and destaging also result from this analysis. The number of data sets being handled at the MSF level of the storage hierarchy can be used to determine the work load in terms of cartridge moves. Figure 6 shows this work load stated in terms of cartridge moves per hour. The size of the data sets, and the Mass Storage Volume organization must be considered to translate data set accesses to cartridge moves. SYSTEMS CONTROL The preceding sections presented an analysis of the work load relationship that could be expected on the major interfaces of a storage hierarchy. f the system is designed with work load relationships in mind, then maximum reuse potential must be achieved. n addition, the control system must function in large establishments which typically have multiple CPU syste.'1s controlled by separate operating systems. t is necessary to have, therefore, a common point of intelligence to control both the concurrent use of data and the staging buffer replacement algorithms. The MSS has such a control system, called the Mass Storage Control or MSC. n MSS, regardless of how many Mass Storage Facilites exist, one and only one MSC controls the hierarchy. Some MSS models have two MSCs; however, the second serves only to improve availability. The MSC cannot perform the entire task without also having control over allocation of volumes to devices. Because of this, MSS is designed to allow the MSC to control the functions of space management, while the control of data and allocation is within the operating system. An interface between MSC and the operating system is a necessity. n BM System 37 architecture, a program gains access to data by having a volume mounted on an addressable device, as discussed previously. n a single CPU environment, the operating system is always aware of which volumes are currently mounted. n a multiple operating system environment, such as illustrated in Figure 7, the systems can take actions independent of each other. n addition, a storage hierarchy such as MSS is attempting to maximize the reuse of data. f any residual data remains on the staging buffer and the same using system or another system reuses that data, the MSC must know where that data resides to prevent unnecessary staging. Another condition which may exist is where one CPU is currently using a data set on a specific volume, and another CPU needs access to the same data set, or another data set also residing on that volume. The virtual device concept provides a convenient solu o Total Move Requirement Stage Up Contribution V'> Q) > ". u Data Flow Figure 6-Access rate (72 hour example) Figure 7-Multiple systems environment

5 BM 385-Mass Storage System 513 tion to the following requirements. First, assume that CPU 1 has access to a volume and it has allocated that volume to DASD POOL 1. Now, assume CPU 2 wants access to another data set on that same volume. f the operating system in CPU 2 were to independently allocate that volume, it would be pure chance if it selected a device address associated with POOL 1. n MSS, the CPU that is in the process of allocation, uses the MSC control interface to learn if the volume in question is currently mounted, or if any residual data from a previous usage of the volume exists anywhere on the staging buffers. f either condition exists, the operating system retrieves the information from the MSC and chooses a virtual address associated with the DASD POOL that contains the data. The MSC then conditions the Storage Controllers associated with that DASD POOL to access to the specific unit address maps to the desired data. This makes it possible for CPU 1 to access a specific volume on one virtual device address while at the same time, CPU 2 has access to the volume on a different virtual address. The converse is also true, CPU 1 and CPU 2 can be accessing different volumes on the same virtual address. The above described control makes it possible for multiple CPU's, which may be controlled by different operating systems (OS/VS1 and OS/VS2 for example), to concurrently share the DASD buffer space without having a common allocation system. As many as six different virtual addresses can be mapped to the same volume, or each may have different volumes associated with them. Each Storage Controller used in MSS can have up to three channel interfaces, and one or two Storage Controllers may access the same DASD POOL. Each interface can have up to 64 virtual addresses, independent of the actual number of DASD devices in the pool. This means that 64 volumes could be mounted in that pool with six alternate paths to each, or that up to 384 volumes could be mounted with no alternate paths. To control sharing within MSS, two mechanisms are used. A logical device reserve release function is provided and is functionally compatible on virtual devices with current BM 333 devices. A second mechanism is provided called exclusive Mass Storage Volumes. This function allows the volume to be defined to MSS so the MSC can allow only one CPU access at a time. Actually, if the data on the volume is read only, or only one user is in the update or write mode, there is usually no reason to prevent concurrent access. SPACE MANAGEMENT Managing space on DASD volumes is considerably more complex than managing a tape library. This is due to the capacity of DASD volumes and the function they provide. Typically tape volumes contain data from a single data set, while DASD volumes contain data from many data sets. f the data sets have' different life times (expiration dates), and are of a variety of sizes, the result can be frag-: mentation of the space. This is further complicated by dif- ferent security and backup requirements. t, therefore, is desirable to provide new functions to ease the task of managing space within MSS because of its capacity (equivalent to up to 4,72 Packs of a 3336, Modell). Again, it is necessary to divide the control between MSS and the attached CPU operating systems. The division is made to manage space within the MSC, and to manage the content of that space with the existing catalogue functions of the operating systems; also to complement the catalogue functions with new software functions called Mass Storage Volume Control (MSVC). The objectives of MSVC are to aid the users of MSS in space allocation and space reclamation and to provide a management reporting function so that use of space within MSS can be monitored. The main parameters of space management are accounting, data set size, life cycles, security and backup. Because these parameters vary widely between installations, a framework is implemented to give customers control with as little implementation effort as possible. MSVC maintains an inventory of Mass Storage Volumes. This inventory can be structurable into groups of volumes. The MSVC maintains information about ownership, space usage, space allocation defaults, backup volumes, retention dates, and location of the Mass Storage Volume if it is removed from the system. Volumes can be grouped by data set sizes, user group, security or any other parameter the customer chooses. Once a structure is chosen, attributes can be applied to the entire group or to individual volumes within the group. t should be possible in most cases to use MSVC so that the space management within MSS is automatic. MSVC also uses the control interface to MSC to manipulate the volumes physically or logically. For example, volumes may be removed from MSS for security purposes, or may be made inaccessible within the system. Volumes may be copied within MSS without data movement through the CPU's memory or channels, and the copy is inventoried by MSVC, and access to the copy is controlled by the MSC. CONCLUSON As stated earlier, the number of times and the amount of, this time, a program or task has to wait for data to appear on DASD or has to wait for space on DASD to store new data, is a measure of the visible impact of the storage hierarchy and the overhead it introduces into the 1/ system. The effect on total system performance can only be understood by comparing the projected overhead with the existing overhead. Again, averages are used to demonstrate the potential for improvement. Figure 8 is a comparison showing delay times in a half-inch tape environment where tape volumes are handled by operators; versus the delay times caused by MSS. Figure 8 assumes MSS has been configured to the optimum reuse DASD capacity and has a sufficient number of access paths to handle the peak work loads demanded by the job mix. t also assumes the events of data

6 514 National Computer Conference, 1975 GOO 962 o =MSS E = Operations delay % tape Average Delay Due to Staging =.1 Minutes 5 Average Delay Due to Manual :/ Tape Mounts = 1.3 Minutes C> 4 Q) u c: E3 ::J g o Task Delay Time (minutes) Figure 8-Estimated task delay time (comparison example) set usage are essentially the same in both environments. The potential for improvement in this specific case is approximately one order of magnitude. The preceding analyses allow the following observations to be made relative to the MSS storage hierarchy. 1. The best possible performance of the hierarchy is equal to that of the devices used in the top level of storage. For MSS, it is the DASD level. 2. The total cost of the Storage System is dependent on the size of data sets, their reuse characteristics, and the amount of data that must be transmitted to and from the CPU's main memory. 3. Mass Storage can be designed with considerably less data transmission capability than devices which must respond directly to the needs of an active program. 4. The measurable impact of the hierarchy is the length of time it takes to make data or space available to the CPU channels. The total amount of storage space at the DASD level of the hierarchy can have a greater effect on this response time than the transmission rates between the levels of the Mass Storage System. 5. A virtual device architecture allows the storage hierarchy to adapt new technologies without changing or impacting the user's interface to data. The hierarchy can be expanded to include additional levels of storage within this basic approach. RECOMMENDATONS Understanding the performance of a storage hierarchy is difficult because of its wide range of applications. t is relatively clear that properly configured, a storage hierarchy performance will be comparable to manually mounted tape/ disks in local and remote batch applications where data set sizes are within the 5 X 1()6 byte range and where reuse characteristics are present. Storage hierarchies are particularly interesting in remote compute applications. The use of storage hierarchies in interactive data base applications, however, needs considerably more understanding in most cases. There are a number of applications which require very large data bases but have a low rate of access. f this rate of access is less than delivery capability of cartridges in MSS, then the analysis is simple. f, however, the total work load on the storage system includes local and remote batch, interactive compute, interactive data base and utility type functions (backup, interchange, archive, etc.) understanding the total system is very complex. Usually this type of complexity can be understood by modeling. n the case of a storage system, however, proven modeling techniques are not available. Analytical models require that input parameters are some sort of distributions or averages. Simulation models require large quantities of trace data in order to cause sufficient activity at the lowest level of the storage hierarchy. The availability of trace data assumes the application has been done somewhere. Data organization must be included to relate data accesses to MSF accesses. Also, clustering and sequencing of requests must be related to data organization and understood relative to retention times in the higher levels of storage. The recommendation is to use caution in approaching data base applications where there is a dependency on "8 percent" of the activity versus "2 percent" of the data (or some other rule). Modeling techniques must be developed which allow total systems to be analyzed. Data must be collected and understood which can indicate the predictability of when data will be used and how it relates to other data: Lastly, it must be recognized that these applications will coexist within the same complex; therefore, the analysis must encompass the old as well as the more exciting, new applications.