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S ingapore Management UniversityI nstitutional Knowledge at Singapore Management UniversityR esearch Collection School Of Information Systems! ',22/2*1*240%6-21!:56)05 T ertiary Storage in Multimedia Systems: Staging orD irect Access?H wee Hwa PANGSi ngapore Management University, hhp ang@smu.edu.sgD OI:h tttps://doi.org/10.1007/s005300050070F ollow this and additional works at:, =35-1./-&4%4:507)(75+5-5$4)5)%4', %462*6,) %6%&%5)5%1(1*240%6-21!:56)05200215hThi

s Journal Article is brought to you for free and open access by the School of Information Systems at Institutional Knowledge at Singapore

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-& 507)(75+C itationP ANG, Hwee Hwa. Tertiary Storage in Multimedia Systems: Staging or Direct Access?. (1997).M ultimedia Systems. 5, (6), 386-399. vailable at:h

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Tertiary storage inmultimedia systems: staging or direct access?

HweeHwa Pang

Instituteof Systems Science, National University of Singapore, Heng Mui Keng Terrace, Kent Ridge, Singapore 119597,Republic of Singapore

e-mail: hhpang@iss.nus.sg Abstract.Multimedia applications that are required to ma- nipulate large collections of objects are becoming increas- ingly common. Moreover, the size of multimedia objects, which are already huge, are getting even bigger as the res- olution of output devices improve. As a result, many multi- media storage systems are not likely to be able to keep all of their objects disk-resident. Instead, a majority of the less popular objects have to be off-loaded to tertiary storage to keep costs down. The speed at which objects can be accessed from tertiary storage is thus an important consideration. In this paper, we propose an adaptive data retrieval algorithm that employs a combination of staging and direct access in servicing tertiary storage retrieval requests. At retrieval time, an object that resides in tertiary storage can either be staged to and then played back from disks, or the object can be accessed directly from the tertiary drives. We show that a simplistic policy that adheres strictly to staging or direct ac- cess does not exploit the full retrieval capacity of both the tertiary library and the secondary storage. To overcome the problem, we propose a data retrieval algorithm that dynam- ically chooses between staging and direct access, based on the relative load on the tertiary versus secondary devices. A series of simulation experiments conrms that the algorithm achieves good access times over a wide range of workloads and resource congurations. Moreover, the algorithm is very responsive to changing loadconditions. Key words:Multimedia server { Storage hierarchy { Ter- tiary library{ Datamigration { Feedback control

1 Introduction

In recent years, demand for multimedia applications that are capable of manipulating both continuous media (CM) data, such as video and audio, and non-CM data, e.g. text and images, has been growing rapidly. Many of these appli- cations, including interactive multimedia education [2] and news on demand [16], are expected to provide access to

tens of thousands of objects. Moreover, the size of thesemultimedia objects are likely to be large. For example, a 2-h MPEG-1 movie can easily occupy 1.5GB of storage [9].Consequently, it would be very costly to maintain all of theobjects on secondary storage devices like magnetic disks.A more economical alternative is to hold only the popularobjects on disks, and to keep the less frequently accessedobjects in a tertiary library that offers lower costs per byteof storage. Given that a small number of objects account fora majority of the accesses in most applications e.g., [15, 3],the bulk of the objects can reside in the tertiary library, thussignicantly reducing the number of disks that are required.

While off-loading objects that are less popular to a ter- tiary library reduces the disk space requirement of a mul- timedia storage system, this practice also complicates its data management function. Instead of dealing only with disk drives, the storage system now has to contend with both secondary and tertiary storages, which have very different access characteristics. One issue that arises is data retrieval from the slower, tertiary library. For non-CM objects, this is straightforward, as they can be transmitted directly to the users at the maximum transfer rate of the tertiary library. CM objects, however, require their components to be played back at a controlled rate, e.g., 30 frames per second. Since the playback rate is not likely to correspond to the maximum transfer rate of the tertiary library, there is a choice between staginganddirect access, which operate the tertiary drives at their maximum speed and the CM objects' playback rate, respectively. This paper focuses on the retrieval of CM ob- jects. In the staging mode, the cartridge containing a requested object is rst loaded into a tertiary drive. The object is then copied from the cartridge to a set of staging disks as fast as possible. Finally, the staging disks feed the data pages of the object to the user at the object's playback rate. The stag- ing mode has the advantage of minimizing turnaround time at the tertiary library, thus reducing its chances of becom- ing a system bottleneck. One drawback of staging is that it requires disk space for the requested object. Moreover, the disk scheduler must be capable of preventing the I/Os gen- erated by a staging activity from disrupting any active CM streams that the staging disks may be supporting. Another drawback is that staging delays could prolong access times. The direct access mode requires a tertiary drive to re- trieve a requested object at its playback rate, so that the retrieved data can be forwarded directly to the user. Thus, direct access is much simpler to implement than staging. However, direct access may not lead to effective tertiary li- brary utilization. This is because tertiary drives are typically capable of retrieving at higher rates than the playback rate 1 Due to the time-consuming nature of the search-forward and rewind operations, it is usually impractical to multiplex sev- eral retrievals from a drive, even if the required objects hap- pen to reside on the same cartridge. Moreover, the long car- tridge loading time of a tertiary library precludes each drive from exploiting any excess bandwidth to retrieve from mul- tiple cartridges concurrently. As a result of this ineffective resource utilization, the tertiary library is prone to develop into a system bottleneck that retards access times. To date, most work on handling tertiary storage devices was done in the context of mass storage systems [6, 7]. These systems include Lawrence Livermore Labs' LSS [8, 12], NASA's MSS-II [21], Los Alamos National Labs' CFS [4], the National Center for Atmospheric Research's MSS [20], and Epoch's InfiniteStorage Architecture [13]. All of these systems require data to be staged to disk before the data can be used. Recently, there has also been some work that specif- ically addresses tertiary storage support for multimedia ap- plications. In [10], Ghandeharizadeh and Shahabi proposed a pipelining mechanism to overlap the playing back of the front portion of a CM object from the disks with the staging of the object's remaining portion. Finally, Kienzle et al. [14] developed a cost model to compare the space and retrieval costs of direct access versus staging. Using this cost model, they concluded that an object should only be accessed di- rectly from a tertiary drive if its retrieval rate is similar to the playback rate of the object; staging is more appropriate when there is a considerable disparity between the two rates. While most of the reported studies have favored staging over direct access, there is no reason why a multimedia stor- age system has to operate only in the staging mode. Indeed, an interesting possibility that allows a storage system to en- joy the advantage of both modes is for it to perform stag- ing for some requests, and direct access for other requests. In this paper, we propose an adaptive staging-direct access algorithm, calledAsdac, that dynamically selects between staging and direct access in servicing a new object request. This choice is governed by feedback on the relative load on the staging disks and the tertiary library. Due to the feedback control mechanism, we could not model the performance of Asdacanalytically. Instead, the algorithm is evaluated using a multimedia storage system simulator that we developed. The evaluation shows thatAsdacconsistently outperforms static staging and direct access over a wide range of system configurations. Moreover,Asdacis able to quickly detect and adapt to changing load conditions. The remainder of this paper is organized as follows. Sec- tion2 describes the performance characteristics of a tertiary storage library. In Sect.3, we introduce a couple of algo- rithms to retrieve data from tertiary storage. A multimedia 1 The case where the retrieval rate of the tertiary drives is lower than the playback rate is not interesting to this work, as staging becomes the only feasible solution. Fig. 1.Schematic representation of a tertiary storage library storage system simulator, intended for studying the perfor- mance of the data retrieval algorithms, is presented in Sect.4. Section5 gives the results of a series of experiments high- lighting the gains thatAsdacbrings about. Finally, Sect.6 concludes the paper.

2 Tertiary storage libraries

Figure 1 is a schematic representation of a typical tertiary storage library. It consists of a large set of cartridges, a number of read/write drives, and a mechanism to automati- cally load/unload a required cartridge into/from a drive. The cartridge-handling mechanism services only one cartridge movement request at a time, completing a load or unload op- eration before entertaining the next request. Each cartridge is stored in one of several magazines. The storage library services an object retrieval request in the following steps:

1. A free read/write drive is assigned for this object re-

trieval.

2. The magazine rack spins until the cartridge containing

the requested object faces the cartridge-handling mech- anism.

3. The cartridge-handling mechanism extracts the target

cartridge from the magazine and slots the cartridge into the assigned drive.

4. The drive seeks to the starting location of the object.

5. The object is retrieved at a specified speed (the speed to

use will be discussed shortly).

6. If the cartridge is a magnetic tape, it is rewound.

7. The magazine rack spins until the magazine that is sup-

posed to hold the cartridge faces the cartridge-handling mechanism.

8. The cartridge-handling mechanism extracts the cartridge

from the drive and slots the cartridge into the magazine.

9. The drive is freed.

As discussed in the introduction, an object retrieval (Step 5) can be carried out in one of two modes. In the first mode, the read/write drive transfers data directly to the user terminal at the playback rate of the object, which is frequently lower than the maximum transfer rate of the drive. This mode is calleddirect access. The second mode,staging, transfers the object from the tertiary drive to a set of staging disks as fast as the two devices allow, before playing back the object from thestaging disks. The performance of a tertiary storage library depends on the technologies used for the read/write drives and the car- tridge autoloader, which comprises the magazine rack and

3Table 1.Magnetic tapeandmagneto-optical disk technologies

ProductCapacity Transfer rate Cost US$ Source

EXB-8505XL 7{14GB 0.5{1MB/s $3,000 [1]

DEC TZ87 20GB 2.5MB/s $8,000 [1]

IBM 1/2

00

3490E 0.8GB 3MB/s $20,000 [17]

Ampex DST 600 25GB 15MB/s $150,000 [17]

1 00

Metrum 12.5GB 22MB/s $160,000 [17]

Sony SMO-F521 1.3GB 1-2MB/s $2,200 [1]

EMO-1300 1-3GB (2 Sides) 3MB/s $3,300 [1]

RICOH RS-5060K 1.3GB 5MB/s (Sync) $4,290 [1]

cartridge-handling mechanism. Table1 lists the characteris- tics of some magnetic tape and magneto-optical disk drives that are on the market. As shown in the table, many dif- ferent types of tape drives are currently available, ranging from low-cost 8-mm drives that transfer at less than 1MB/s tohigh-end drivesboasting transfer rates of 22MB/s at a much higher cost. The choices for magneto-optical drives are more limited, both in terms of transfer rate and cost. As for the cartridge autoloader, this could involve a high-speed carousel that takes less than a second to load/unload a car- tridge, as in a StorageTek 9708 DataWheel. Alternatively, a magazine rack that holds a larger number of cartridges but incurs longer cartridge loading/unloading times could be used, as in a Box Hill Ice Box. These technologies enable a wide range of tertiary storage libraries representing different cost/performance trade-offs to be built. The focus of this paper is not on assembling a set of specic read/write drives and autoloader technologies into a tertiary storage library that best meets a customer's storage capacity, access speed, and budget requirements. Instead, our objective is primarily to optimize the effectiveness of a given tertiary storage library. Specically, we want to min- imize theaverage access timeof object retrievals from the tertiary library, defined as the elapsed time between the is- suance of a retrieval request and the instant when the first page of data arrives at the user terminal. To achieve this objective, we propose in the next section a couple of algo- rithms to retrieve objects from a tertiary library. We then study the behaviors of the algorithms over a wide range of tertiary library, staging disks, and workload configurations, paying particular attention to the interplay between tertiary library and staging disks. Of course, a thorough understand-quotesdbs_dbs14.pdfusesText_20