Disk Drive Decisions: Digital cinema creates greater storage demands

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Technology

Digital cinema has become the dominant theatrical display technology worldwide, displacing older film-based display. Most modern distributed media content is done in a digital format. This content is ingested into a server system in a digital theatre complex. This server system then distributes this content to integrated media blocks (IMBs) located in individual projectors. The integrated media block converts encrypted high-resolution content from the server into a format that the projector can display. Putting the IMB inside the projector prevents raw high-resolution content from ready access and copying. Additional security measures within the projector make this raw content even more secure.

These integrated media blocks (IMBs) may have some onboard digital storage for the projected content, or they may use external storage for this content. With onboard, content streaming is done directly from the IMB internal storage. With the second option, the encrypted content is streamed to the IMB from the external storage using eSATA (Direct Attached Storage, or DAS) or via Ethernet (Network Attached Storage, or NAS). Control in both configurations is via an Ethernet line connected to the theatre network.

Because of the size of the media files used in modern theatres, typically 90 to 300 GB in size, hard disk drives (HDDs) are the most common storage technology used. Hard disk drives can provide fast sequential data flows important in video streaming applications and they are also more cost-effective than other technologies, such as flash memory.

Because digital-cinema files can be quite large, it is more cost-effective to put this content on hard disk drives (HDDs) than on solid-state storage. The two most common sizes of HDDs are 2.5-inch form factor and 3.5-inch form factor. In addition to the physical size of the HDD, they can also differ in the data interfaces used, as well as other characteristics such as the rotations per minute (RPM). Common HDD interfaces are SATA (Serial ATA) and SAS (Serial Attached SCSI).

Hard disk drives write and read information on rotating rigid disks in concentric tracks using read/write heads attached to stainless-steel suspensions. These suspensions are mounted to an e-block that is moved by a rotary motor to bring the heads across the disk surfaces. Each side of the disk can have these recorded tracks. These tracks are broken up into annular regions called sectors. There is servo information written in the sector headers that the read/write head can detect as it passes over the disk surface.

This servo information is used by the HDD electronics to determine the location of the head and to guide it where it needs to go using the rotary motor to write or read information. Because the HDD is very sensitive to even very small amounts of contamination, it is kept in a very clean enclosure that prevents tiny particles and other contamination from entering the drive casing.

3.5-inch HDDs have larger disks inside them than 2.5-inch disk drives. In general, 3.5-inch HDDs can have a larger capacity than 2.5-inch HDDs because the disk surface where the data is stored is greater for the 3.5-inch HDD and also because common HDD case heights for 3.5-inch HDDs are higher than 2.5-inch HDDs and thus they can contain more disks.

As a consequence of having a larger surface area for storing data per disk and with a larger number of disks, 3.5-inch HDDs can have a greater storage capacity than a 2.5-inch HDD. As a result, the cost of digital storage in dollars/TB of storage capacity is about two times less for a 3.5-inch HDD than a 2.5-inch HDD. The 2.5-inch HDD thus has more than a 50% higher $/TB price than a 3.5-inch HDD.

HDDs are electromechanical devices and there are components in an HDD that can wear out and fail with use and time. HDDs are often designed for a five-year service life (even if the warranty period is less), so the incidence of failure of the majority of HDDs used for their intended purpose should be low for at least five years. HDDs are designed for assumed application and usage conditions for these failure rates.

Some HDDs are made for high usage and long power-on applications that may be common in enterprise storage systems. Examples of these types of high usage and long power-on HDDs are some of the products in the Cinemastar product line from HGST (a division of Western Digital) and the Enterprise Performance HDDs from Seagate. Enterprise storage systems combine a number of HDDs together in an array with almost continuous power and frequent writing and reading of data.

Other HDDs may be designed for personal computer application where the yearly power-on hours are less than for enterprise applications because these HDDs tend to be used for fewer hours per day (e.g., about eight hours per day). In addition, HDDs in personal computers are subject to less frequent writing and reading of information because these devices are serving individual users rather than the many simultaneous users common for an enterprise storage array. There are also special HDDs that may be designed for particular applications, such as those in a home digital video recorder or for video surveillance recording.

Digital Cinema Storage Within and Outside the Integrated Media Block

The temperature of the Xenon lamp used for illumination in a digital projector is very high, since these devices have electrical power levels between 900 W and 15 kW. Although either liquid or air-cooling is used inside a digital projector to control the heat load, the temperature inside of the projector can easily exceed 100 degreesC.

This elevated temperature can place additional stress on components inside the projector. The integrated media block is placed within the projector. As a consequence, the temperature of the IMB can be considerable. This may have important implications for an IMB that contains HDDs.

HDD long-term reliability is often discussed as the annual failure rate (AFR). This is the expected rate at which a large number of HDDs would fail over a year’s use. The annual failure rate is designed so that there will be a relatively small number of HDDs that would fail during the HDD warranty period and over the designed life of the HDD if the HDD is used within the designed operation conditions.

The annual failure rate of an HDD will increase from this design point if the drive is subject to additional stresses. These stresses can include additional power-on- hours per year (related to the use of the drive during each day), the internal temperature of the HDD (related to the external thermal load outside the HDD, as well as the effectiveness of the dissipation of heat generated by the hard disk drive itself). Other factors, such as physical shock and excessive vibration, can also cause problems with an HDD. In order to get the greatest life from a storage system using HDDs, these additional stresses need to be understood and controlled.

Since the temperature inside a modern digital theatre projector can run hot and can even exceed 100 degreesC, we need to look at the impact of temperature (as well as power-on hours) on the annual failure rate of an HDD. Data from Seagate Technology shows the annual failure rate as a function of the external HDD case temperature for a Seagate Video Surveillance HDD. These HDDs are used for applications involving intensive writing of video images with occasional reading.

Although video surveillance differs from the more frequent reading of data that is common for projection in a digital cinema, the trends for AFR increase with temperature would also occur. If we stick with the lower power-on-hour curve (2,400 POH), which corresponds to roughly five days per week, eight hours per day of use, increasing the HDD case temperature from 30 degreesC (close to room temperature) to 70 degreesC increases the AFR by about 3.4 times (from 0.21% to 0.71%). Earlier HDD AFR temperature acceleration studies showed a similar increase in AFR over a similar temperature range. Also, the AFR is higher as the annual power-on hours increase (8,760 hours corresponds to 24-hour-per-day use) and seems to result in a threefold increase in AFR at 30 degreesC compared to the lower power-on-hour case.

Since the temperature in the projector, and thus the IMB, can exceed 100 degreesC (and will likely be over 50 degreesC), this heat load, combined with the internal heat generated by the HDDs embedded inside an IMB, will increase the annual failure rate of these HDDs versus an HDD located in a less high ambient temperature environment. For instance, with an external storage system with well-designed thermal control, the internal temperature can probably be kept in the range of 30 degreesC to 50 degreesC. As a consequence of this lower temperature operation, we expect lower AFRs and the expected drive life for an HDD in an external IMB storage system should be higher than those used inside an IMB.

Longer life for the HDDs in an external storage system means that the HDDs need to be replaced less often than those inside an IMB and this increases the operational time for the projector it is attached to—and thus the projector could require less maintenance. In addition, replacing an HDD inside a well-designed external storage system will be easier than replacing an HDD enclosed in a digital projector. This easier access also makes it easier to upgrade HDD storage capacity as HDD technology evolves than would be the case for HDDs embedded in a digital projector.

The Future of Hard Disk Drives

IBM introduced the first hard disk drive in 1956. Since that time, these useful devices have increase over a billion times in storage capacity while shrinking to the size of today’s 2.5-inch and smaller HDDs. Advances in HDD technology have involved the development of head, medium (disk), motors, servo systems and drive electronics, as well as enclosure and contamination protection technologies. Together, these advances have created a reliable low-cost digital storage technology that is used in many applications and that is the repository for much of the world’s information, including video content.

Higher HDD areal densities (the number of bytes of information that can be stored per surface area of the disk) provide more digital storage capacity per disk and thus per disk drive. Note that other factors, such as the number of disks in an HDD, are also important factors in the total HDD storage capacity. However, in terms of staying ahead of other storage technology in $/TB, such as flash memory, increases in HDD areal density are the chief tool.

Current HDD areal density growth trends (Giga-bits per square inch, or Gbpsi) are in the range of 15% annually. However, the actual advances in HDD AD do not occur in any sort of regular predictable order, where maximum HDD areal density per quarter is plotted. Instead, the areal density grows in fits and starts and may stay constant for many quarters. We can create some sort of average growth that represents the overall trend, but it is important to realize that with such irregular growth, smooth mathematical growth-trend predictions will most likely be wrong at any given point in time.

3.5-inch HDDs have been announced with up to 10 TB of storage capacity. These should be available in 2015. The development of 8 and now 10 TB 3.5-inch HDDs involves the introduction of a few technologies such as shingled magnetic recording (SMR) and hermetically sealed He-filled hard disk drives. SMR involves the partial overwriting of a previously written track by a new track, reducing the effective track width of the prior track. As a consequence of SMR, the effective track density of an HDD can be increased at the cost of not being able to do direct overwrite of previous data. Creation of a hermetically sealed HDD full of helium rather than air allows packing more disks into the HDD and thus a higher capacity in the drive.

Recent announcements of a 3 TB 2.5-inch HDD show that these HDD form factors are increasing in storage capacity as well. Both 2.5-inch and 3.5-inch HDDs benefit from the increasing areal density technology.

The SMR technology probably cannot add too much more to areal density growth, and putting helium in a hard drive can only allow adding so many more disks to a drive. Higher storage capacities require the introduction of new digital storage technology. The Advanced Storage Technology Consortium (ASTC), a roadmap group for the HDD industry, released the 2014 roadmap for HDD areal density (the amount of digital storage that can be stored on a given surface area of an HDD).

According to the ASTC road map, Heat Assisted Magnetic Recording (HAMR) should be introduced into HDDs by 2017, increasing the average annual areal density growth rate to 30% (it is currently about 15%).

Interestingly, the road map shows that a technology called bit patterned media (with the magnetic media broken into small regions on the disk surface) will be introduced about 2021, combined with SMR or an extension to SMR called Two Dimensional Magnetic Recording (TDMR). This will be combined with HAMR, which will result in up to 10 Terra-bit-per-square-inch (Tbpsi) areal densities by 2025. Note that today’s shipping HDDs have an areal density as high as 0.86 Tbpsi. This implies that a 3.5-inch HDD built with that technology in 2025 could have about ten times the capacity possible today, or 100 TB.

The Future of Digital Cinema Storage

The size of digital storage devices used in digital cinema is important because the digital storage requirements are increasing. Current digital cinemas show 2K and 4K content, but in the future even higher resolution formats may be used, sometimes in combination with 3D. Resolution (and thus the number of pixels), frame rate and the color depth of the pixels are all factors in the total size of a digital-cinema file. There is work in the industry to develop 8K X 4K video that may be introduced into theatres after 2020.

An external storage system feeding content to an IMB can be larger than one built into the IMB, since space outside a projector is less constrained than inside the projector. This allows more disk drives to be used and also allows higher capacity 3.5-inch rather than lower capacity 2.5-inch HDDs to be used. All the HDDs built into IMBs are 2.5-inch HDDs due to the limited space available in digital projectors.

As we discussed earlier, using 3.5-inch rather than 2.5-inch HDDs allows more storage capacity and a lower $/TB price. As a consequence, an external HDD storage system serving encrypted data to an IMB with 3.5-inch HDDs can provide higher reliability, easier upgrades and greater storage capacity for a lower price than is the case for HDD storage within an IMB.

As the size of digital-cinema content increases with higher resolution and higher frame rates, having more storage capacity available for a lower price can be an important consideration.

In addition to larger-size content with higher pixel count and higher frame rate with higher resolution content, the data-rate requirements are greater as well. 3.5-inch HDDs, such as those that can be used in an external storage system supplying content to a digital-cinema IMB, generally have a higher data rate than the 2.5-inch HDDs integrated within some IMBs (over 1 Gbps for 3.5-inch versus about 500 Mbps for 2.5-inch HDDs). This is due to the higher rotational speed resulting from higher radii for 3.5-inch disks and the higher RPM on these drives (7,200 RPM) versus most 2.5-inch disk drives (5,400 RPM).

This is about a 2:1 difference in average data rate, favoring the 3.5-inch drives. Digital-cinema files are generally between 100 and 500 GB in size depending upon resolution and whether it is 3D or 2D. Although data rates in digital cinema are currently as high as 500 Mbps, for video alone DCI files are reconstructed from multiple blocks of data and additional bandwidth is needed to combine them together.

In addition, if multiple video streams come out of one storage system, the required bandwidth is multiplied. Thus, 3.5-inch HDDs can be a better choice for high-resolution rich media content than 2.5-inch drives, particularly as the content resolution increases or if multiple video streams are necessary.

Conclusions

Higher capacity on each disk in an HDD is important for digital cinema, as fewer disks can be used to achieve a given storage capacity. Also, the overall size of digital-cinema files is likely to increase with the introduction of higher color depth in cinema display files, along with higher frame rates and higher resolutions. Larger cost-effective storage devices, such as 3.5-inch HDDs, will be a key to contain and display such high-resolution, large-size content because of their higher storage capacity and data rate.

Using external HDD storage connected to a digital-cinema projector, rather than HDDs built into the Integrated Media Block (IMB), allows these HDDs to operate at lower temperatures, thus improving their reliability and longevity. Using external storage also allows using 3.5-inch rather than 2.5-inch HDDs, which have higher capacities and data rates more suited to higher-resolution video content-streaming. As HDD technologies develop, they will be able to support future video content with higher resolution, greater color depth and higher frame rates.

Tom Coughlin, President of Coughlin Associates, is a widely respected storage analyst and consultant. He has over 30 years in the data-storage industry, with multiple engineering and management positions at high-profile companies.