Articles: Storage

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The free space on our computers’ hard disks is getting consumed at a tremendous rate in our never-ending search for higher quality. HD video, 15-megapixel photographs, lossless audio, high-resolution textures of latest games are the reasons why we don’t measure hard disk capacities in just gigabytes anymore. It doesn’t matter if a hard drive has a few gigabytes more or less now that we use the term terabyte to describe it.

After a long period of exponential development hard disk drives have slowed their pace down in the last year and a half. Besides the obvious manufacturing difficulties, the problem is that moving beyond the 2-terabyte barrier is not as easy as it seems.

One aspect of this problem is in the Master Boot Record, a table located at the beginning of a partitioned disk and designed for choosing a partition an OS is going to be booted from. The MBR wouldn’t be necessary if we all had but one partition on each of our hard disks. The computer’s BIOS would initialize the disk and pass execution to code contained at a certain address. But as we do have multiple partitions, we cannot do without the MBR. The BIOS loads the MBR which in its turn loads the OS located in one of the partitions described in the MBR. The problem is that the MBR was invented when HDDs used to have much smaller storage capacities than today (by the way, the alignment requirement for today’s HDDs with 4-kilobyte sectors goes back to that time, too). The MBR could originally address only 224 sectors. One sector being 512 bytes large, this equaled 8.4 gigabytes. And even this method required some tricks with CHS (cylinder-head-sector) addressing which involved presenting an HDD as a monster with as many as 255 heads. Some users may still remember that problem. Later on, the LBA addressing mechanism was introduced to replace CHS and expand the addressable space to 232 sectors. This is about 2.2 terabytes. Thus, the MBR cannot access sectors beyond 2.2 TB. No new partitions can begin there, which means that a single-partition disk can have a maximum storage capacity of 2 terabytes. As HDD makers hold that 2 terabytes means 2 trillion (1012) bytes rather than 241 (which is a bigger number) as operating systems think, today’s 2TB HDDs are free from that problem, but something had to be done about 3TB models.

Like in RAID arrays, a high-capacity disk could be sliced into logical partitions (LUNs), each smaller than 2 terabytes, and presented as individual physical HDDs, each with its own MBR, to the BIOS. The manufacturers prefer another solution, though. They use the GUID Partition Table, a new and advanced disk partitioning method. It is a table of partitions with unique partition IDs and no code to execute. It is the Extensible Firmware Interface, a new standard devised to replace the conventional BIOS, that is responsible for orchestrating the boot-up process. The GPT can address up to 9.4 zettabytes (9.4x1021) which should suffice for a long time. It stores a backup copy of the main table and checksums for partition IDs, supports up to 128 partitions and even has an MBR at the beginning to solve some compatibility issues. There are such issues, actually. The GPT is supported by every 64-bit version of Windows, by Windows 2003 Server and Vista SP1, but old OSes, including such popular ones as 32-bit Windows XP and Windows 2000, won’t work with GPT-enabled HDDs. The replacement of the MBR with the GPT involves one more important nuance: the standard BIOS cannot work with GPT partitions. That is, the OS will see such partitions, but must be installed on an MBR one for the computer to boot it up. It is only computers with the mentioned EFI (instead of BIOS) that can boot up from GPT partitions but EFI mainboards are very few as yet.

Besides, the HDD controller’s drivers must support disk partitions larger than 2 terabytes. That’s quite an important requirement because otherwise the OS would only see about 800 megabytes instead of 3 terabytes. Such drivers for Intel chipsets were only released in December 2010 and generic Windows 7 disk drivers had had to be used until that.

For all these pitfalls, the transition to the new storage capacity could not be delayed much longer and the HDD manufacturers got to work on it. Their websites now offer special sections detailing the subtleties of the move to 3 terabytes, and we’ve got three HDDs of the record-breaking capacity, two of which can already be found in retail shops.

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