Articles: Storage

Bookmark and Share

Pages: [ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 ]

Advanced Format

A little bit of history first. The idea of transitioning to 4KB sectors rose up back in 1998 but it is only in 2007 that the International Disk Drive Equipment and Materials Association released a document that summarized the 7-year work on the Long Data Sector technology (it is sometimes referred to as Long Data Block, too). A variation of this technology is implemented by Western Digital under the name of Advanced Format.

The idea of long sectors is simple: the disk is divided into 4KB rather than 512-byte sectors. Thus, a 4-kilobyte chunk of data will require one sector to be stored instead of eight sectors as in today’s HDDs. The eightfold reduction in the number of sectors per track ensures a significant saving in terms of auxiliary data such as the Sync/DAM and ECC blocks accompanying each sector and in terms of intervals between sectors. The ECC block becomes larger with the larger sector, but the total space occupied on the disk by these auxiliary data is indeed reduced.

What is the point of all this? First, the larger ECC block increases the probability of recovering data read errors which is highly important for today’s HDDs with high recording density (and, consequently, with a very low signal-to-noise ratio). If ECC cannot help recover the data, the disk has to read the sector once more, which requires a full rotation of the platter. Thus, the higher ECC recover rate means that there will be fewer idle rotations and a lower probability of an irrecoverable error. Western Digital claims a 50% improvement in ECC-based data recovery.

Another good thing about the 4KB sectors is that the auxiliary information takes less space and this saved space can be used for user data. From an end-user’s point of view, it means that the developer can produce a larger-capacity HDD, the other technologies being the same. Although this advantage (about 7-11%) is not as high as we see each time when the developers move on to higher-density platters, it is good all the same.

And finally, this technology increases the disk’s linear recording density. It helps increase the amount of data fitting into each track and we can expect an appropriate increase in the drive’s linear speed.

What are the downsides of this technology? It puts a higher load on the drive’s electronics responsible for ECC but that’s not a big problem today. More importantly, there is a lot of software, from BIOS code to applications, that has been developed with 512-byte sectors in mind and it is impossible to switch to the new sector size in one move. Something has been done already. Microsoft’s new operating systems starting from Windows Vista (and including Windows 7) support 4KB hard disks. LDS technology is also supported by many RAID controllers. Mainboards are acquiring this support in their BIOS code, too. However, there is no full compatibility as yet.

It is because of this possible incompatibility that Western Digital chose the following strategy for its new HDDs. Physically, the HDD’s platters are divided into 4KB sectors. However, it pretends that it has 512-byte sectors. Each physical sector contains eight logical sectors. All the required address translations are done inside the HDD, ensuring maximum compatibility but at the expense of performance.

There is one biggest downside to this emulation that made Western Digital develop a special solution. The still popular Windows XP (and all of its predecessors) uses a curious way of formatting the hard disk. When creating a partition, the first 63 sectors (from sector 0 to sector 62) are reserved while the partition itself begins at sector 63. With 4KB physical sectors, this sector 63 proves to be the eighth logical sector in the eighth physical sector. Then, the NTFS file system accesses the hard disk in 4KB data blocks, producing a very bad effect. Due to the 512-byte difference (sector 63), each cluster of the file system resides in two physical 4KB sectors, although a cluster is the same size as a sector. When reading data from the disk, more information than necessary has to be read and the required data has to be extracted from it. When writing a cluster, the disk has to read two physical sectors, change the eight logical sectors corresponding to the cluster’s address, and write them back. Writing a 512-byte sector (system data are changed by means of such requests), the hard disk has to read a physical 4KB sector, change the required logical 512-byte sector, and write the modified physical 4KB sector back.

It resembles RAID5, doesn’t it? :)

Of course, this has a negative effect on the drive’s performance as you will see shortly. We don’t know why Microsoft chose the offset size of 63 sectors but it used to show up with RAID arrays (where disk stripes would be shifted relative to the OS clusters) and SSDs (due to block-based access) and has become a real problem by now. The good news is that Windows Vista and its successors (including Windows 7) are free from this problem, taking a different approach to formatting hard disks.

If you still want to use new disks under old OSes, Western Digital offers two solutions. First, you can set the HDD’s jumper to pins 7 and 8 and the HDD will move its logical structure by one logical sector so that sector 63 from the OS’s point of view is actually sector 64, i.e. the beginning of a physical sector. This shift can be easily implemented on the logical level: each incoming sector address is just augmented by one when translated into the disk. The disk’s capacity does not suffer because the last sector becomes sector 0 due to the cyclic shift.

This solution is recommended to create one partition on the disk. If you want to create multiple partitions, Western Digital recommends another method which is also used to transfer disk images.

You take your “unaligned” disk and run the WD Align tool (downloadable from the Western Digital website). This tool shifts the existing disk partitions to align them. This method is universal but takes more time because data are actually transferred from one sector to another. Empty sections will be transferred quickly but full ones will take some time. You must make sure that there won’t be power outages during the process. Again, the disk capacity does not change after this procedure but we wonder what WD Align would do with an unaligned and absolutely full partition the size of the whole disk?

WD Align won’t allow you to align an ordinary (one that doesn’t support 4KB sectors) or an already aligned disk. This protection can be easily bypassed if you insert the jumper after having aligned the disk. We hope you won’t do that, though. And we also hope you won’t insert the jumper into a disk formatted in Windows Vista 7 because the sectors will be additionally shifted by one 512-byte sector and the previously created partition won’t be accessible. So, don’t do that!

And now, let’s move on to our tests.

Pages: [ 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 ]


Comments currently: 10
Discussion started: 03/08/10 04:10:41 PM
Latest comment: 05/18/11 07:31:55 AM

View comments

Add your Comment