Thursday, 21 May 2015 00:00

HDD selection criteria. Part 1 – Capacity and interfaces.

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b2ap3_thumbnail_iStock_000003002918XSmall.jpgThis article is written for those who, for some reasons, decided to upgrade independently but have no idea of what is what. Here we will try to make a choice of what to be purchased and what not, as well as to develop an understanding of the main points to consider when purchasing, how to find an optimal balance of price, reliability and speed.

Hard drive is the main device of external memory and you have to be serious in selecting it if you decided to do it yourself. In this article we will review features usually claimed in the passport of device: based on those features you will be able to draw some conclusions. Additional information about device can be obtained from numerous internet-reviews of storage devices on hard magnetic disks and not only.

Hard drive’s capacity

The first selection criterion coming to my mind is the capacity. Probably the only parameter which is the more the better. Earlier, in the computer literacy books, the capacity of hard drive was viewed as factor that determines possibilities and comfort of using this or that operating system, applications and so forth. Today, it may safely be said that if you are an ordinary user, then buying a hard drive with the capacity that is not sufficient to use popular programs and packages is simply impossible.

The following capacities of hard drives prevail on today’s market: 30 to 40 GB – 18.53%, 60 to 80 GB – 17.39% and 120 GB and more – 18.03%. Installed operating system and necessary program package usually occupy not more than 5% of the total disk space. The rest of the space is usually occupied by various multimedia user data. That is why the proposed above capacity is our criterion. It has to be said that capacity and cost depend nonlinearly and until a certain moment purchasing a device with larger capacity can be economically more reasonable in terms of cost of keeping 1 GB of data. The difference in devices with capacity 40 and 60 GB is just several dollars. Take note of that. Speaking of which, I remember a HDD advertisement campaign where the price per megabyte was emphasized…

Speaking about gigabytes and megabytes we need to remember that hard drives are subject to… shrinkage! Sooner or later you will find out that the capacity of purchased hard drive displayed by programs does not correspond to the claimed on the packing. Internet content analysis shows that despite an abundant availability of information about the above mentioned fact, unfortunately, many people are still not familiar with it. No, don’t think that this is compression of platters due to high temperature impact :-))) Everything is much more prosaic than it seems: the thing is that on hard drives the capacity is indicated in millions of bytes (corresponds to prefix M) and billions of bytes (prefix G). At the same time, one kilobyte contains 1024 bytes, megabyte equals to 1 048 576 bytes and gigabyte – 1 073 741 824 bytes. The difference lies only here and there is no other factor influencing the capacity of hard drive.

Hard drives with UltraDMA interface

The condition of proper operation of hard drive in the system is correspondence of controller’s interfaces and the disk itself (for those who don’t know yet, interface – a standard, describing interaction mechanisms of two devices, and in this particular case under the mechanism we understand all its sides – electrical, mechanical…). Absolutely all motherboards are designed for mass consumption and include a controller implementing UltraDMA interface. It is the cheapest interface available and, therefore, the most popular one. If you don’t need super high indicators and you have an ordinary computer, then your choice is clear – hard drive with UltraDMA. The only thing which is worth your attention is UltraDMA mode that your motherboard supports.

The thing is that many of today’s hard drives are not able to load enough the fastest modes (UDMA 100 and UDMA133), but due to certain peculiarities, their performance substantially depends on the mode used. Not to speak of the situation when possibilities of old controller installed on the motherboard cede to possibilities of hard drive. That is why if you want to avoid problems, make it a rule, that the mode arranged by the motherboard controller should be not lower (read as not slower) that the one implemented by the hard drive controller. In contrast to the reverse situation, this option excludes the described above problems. Even if there is a necessity in using fast hard drive with slow controller, you need to make it clear, how using slow interface affects hard drive’s performance, and, of course, choose the option, where affect is minimal. Sellers shall know it, or you can easily find this info in the internet reviews.

Going through companies’ price-lists you can see instead of UDMA something like ATA and IDE. If a hard drive is new, then most probably under IDE hides one of the UDMA options, but if you really take interest in old hard drives, then under IDE you can find anything, that you have to clarify yourself. And if we are talking here about ATA, then you need to know that UDMA 33 is FastATA-3, UDMA 66 - FastATA-5, UDMA 100 - FastATA-6. It can be written both UltraATA and simply ATA indicating maximal speed of interface. It is all the same anyway. Let not the diversity of names to scare you: its origins lie in the hands of manufacturers that develop similar standards.

To connect hard drives with all named above interface versions, you need to use 40 pin connector and 40-wire cable for modes up to UDMA 33, and 80-wire for all high-speed modes. The latter has 40 additional earthing conductors which help reducing crosstalk in the ribbon cable. 80 wire ribbon cable can be used in slow modes but 40 wires cannot work in high-speed modes. It is exposed to crosstalk more and most probably a hard drive will start in a less speed mode or in a normal mode but the number of errors during transfer and subsequently the number of repeated accesses can rise to such extent that the resulting speed might be much lower than in the slow mode. You need to keep an eye on compliance of ribbon cable with transmitting mode.

Speaking about ribbon cables we shall mention so-called round ribbon cables, i.e. made in a shape of flat ribbon cables collected in a bundle or a bundle of separate wires or in any other way. They are produced by some companies and I have seen several times articles devoted to the technique of making such cables yourself. Such cables usually work flawless in modes up to UDMA-33. In higher modes there may be problems. I have already written that these modes are less error-free and require a special ribbon cable with extra screening conductors.

These conductors are randomly distributed in round ribbon cables and their presence plays no role: information cores influence each other and also catch external fields and the result is the same, many errors – many repeated requests – performance reduction. Only in a some round ribbon cables sold this factor is taken into account and screening cores distribution is well thought of and all cores can be additionally taken into one screen. But such ribbon cables are expensive. There is no doubt that round ribbon cable look better and improved air exchange, but is it worth the performance? I have warned you and you take a personal decision on whether you really need to take this adventure or not.

And finally, closing the UDMA interface question I would like to remind that if the required mode is not ensured by the controller installed on motherboard, then the problem is solved not only by replacing a motherboard, but by purchasing UDMA controller implemented as daughter board as well. This way of course has its own cons, although if we take all factors into account it can be much better than the first option.

Hard drives with Serial ATA interface

A new (relatively new) development in the field of hard drive interfaces is Serial ATA interface. A recent substantial progress in the field of transmitting and receiving devices made serial data communication on high speed possible. Using serial data communication implies reduction of number of conductors required for connection. Thus, SATA interface has a very neat multi-wire planar cable and sophisticated users don’t have to tinker with it as in case with Parallel ATA, which in its turn is the only advantage of the new interface. In the only implemented version of SATA, the interface transfer rate is 150 Mbytes/s, which is a rather little difference from 133 Mbytes/s offered by an ordinary ATA, not used by hard drives yet. Generally speaking, there are no advantages so far. Moreover, there are only disadvantages – by far, SATA controller is not integrated into every motherboard and purchasing it as daughter board has no sense; hard drives cost slightly more than similar models with traditional interface.

Using SATA hard drives won't release you from the need of using Parallel ATA for other devices. That is why purchasing hard drive with serial ATA interface can be justified only with your crave for various innovations in the field of technology. An opinion that hard drive with advanced interface is faster is false. It is modern, yes, but not faster. The speed of hard drive is determined by a number of other parameters.

Hard drives with SCSI interface

Another popular interface is SCSI (Small Computer System Interface, and it is not only hard drive interface but periphery interface in general). I won’t indulge myself much in describing it. You need to know that this interface is not for home PC for which you choose the drive. It is designed for servers, powerful workstations and so on. If you have chosen a drive to use it along, then you won’t be reading this article at the moment.

As rule, drives with SCSI have high performance and high reliability levels; they use well-proven technologies which are determined by the places they are going to be used in (see above). They are not used in mass machines since they are much more expensive. Aside from that they need an additional controller that is never integrated into mass consumed motherboards, only in boards designed to create servers, workstations (installing controller as daughter board for advanced technology bus of an ordinary PCI is not possible due to its absence: modern hard drives use Ultra160SCSI and Ultra320SCSI, throughput of which is 160 and 320 MB/s respectively, which exceeds PCI bus throughput. Clearly a hard drive does not provide such speed, but if you make an array out of such drives than there is a chance that SCSI will easily handle this task and ATA will lag far behind.).

The cost of a good controller is comparable with the cost of the drive itself. Interface cables are expensive as well. In return, the SCSI gives a number of major advantages like high data exchange speed, support of a great variety of devices, organization of instructions queue and its intellectual optimization and all that without involving CPU. Only the cost of technology clearly determines the niche of its application. I have seen the cases when with a great desire motivated by unclear reasons, a user purchased one of the old boards for PCI bus and old hard drive SCSI at the price that is not much different from modern ATA bus devices and enjoyed the understanding that his/her PC unit contains so exotic device: but only understanding because in reality it didn’t give any advantages – within home environment and ordinary tasks the technology doesn’t reveal its real potential. Therefore I suggest forgetting about SCSI and will not continue discussing standards, compatibility issues and so on. I only mentioned it here because from time to time I receive questions about hard drives with SCSI from home users who read a lot of such review in the net.

Hard drives with FC-AL and SSA interface

Another hard drive interface option is FC-AL (Fiber Channel Arbitrated Loop). Fiber interface with maximum bus length (i.e. bonding leads) of 10 km and exchange speed of 125 MB/s and lots (127) of auto configuring devices with a possibility of hot insertion and so on and so forth… Hard drives with this interface are designed to be used in the same manner as SCSI and also require an additional controller that costs a lot. The cost of the device itself is clearly different from the cost of UDMA devices. I will not go deep about such devices because they are too specific and too expensive.

In the year 1997-1998, the IBM developed its own interface SSA (Serial Storage Architecture) before releasing hard drive with this interface. It was able to ensure operation of 127 devices with throughput of 80 MB/s. Architecture point-point is used during connection. Devices use full duplex communication between each other at the speed of 20 MB/s in both directions. At that time it was really expensive. I didn’t see drives with this interface and soon didn’t even hear about them. Just an interesting fact from the world of interfaces for those who are curious...

Last modified on Thursday, 21 May 2015 19:20
Data Recovery Expert

Viktor S., Ph.D. (Electrical/Computer Engineering), was hired by DataRecoup, the international data recovery corporation, in 2012. Promoted to Engineering Senior Manager in 2010 and then to his current position, as C.I.O. of DataRecoup, in 2014. Responsible for the management of critical, high-priority RAID data recovery cases and the application of his expert, comprehensive knowledge in database data retrieval. He is also responsible for planning and implementing SEO/SEM and other internet-based marketing strategies. Currently, Viktor S., Ph.D., is focusing on the further development and expansion of DataRecoup’s major internet marketing campaign for their already successful proprietary software application “Data Recovery for Windows” (an application which he developed).

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