Shulou(Shulou.com)06/02 Report--

In the previous section, we analyzed the principle and cause of formation of physical and logical bad channels, and also mentioned a very important point-- PLIST basic defect list and GLIST growth defect list.

Why are these two tables so important for hard drives? Because the number of bad channels will affect the speed of reading the disk. And the bad track will spread to the surrounding track, so to shield this part of the bad track, it is necessary to use the G-to-P operation.

PLIST basic defect list Primary Defect List

In fact, when each hard disk is in production, there will inevitably be areas where the magnetic media of some discs are uneven or damaged. The factory will use a PLIST to record the unstable sectors produced during the production process.

A list made by hard disk manufacturers when they make low boxes. We also call it the "permanent defect table", and these record addresses are automatically skipped while the hard drive is running, so the P list does not affect the access speed of the hard disk.

GLIST growth defect list GrownDefect List

The G list is a list that appears in the use of the hard disk, which is also called the "growth" defect table. As the user uses the hard disk, the hard disk detects possible defects (bad sectors) in some areas and automatically remaps to the space reserved at the factory.

It is used to protect the access security of user data. Since the G-list brings remapping rather than skipping, it means that the hard disk first looks for a bad area and then maps to a secure area, which slows down the access speed of the hard disk.

Looking at this, it is estimated that people will only have a sense of comparison between GLIST and PLIST. So what is the cause? Why does PLIST not affect hard disk access speed, but GlIST does?

So let's start with the sector allocation of a hard disk.

In the hard disk, in addition to the user storage work area that we usually see, there are actually two other areas-the firmware area and the reservation area.

But in practice, the firmware area and the retention area, there is no way for users to operate, and the actual number of sectors of the hard disk is larger than the capacity we see on the label. (in fact, when the factory is in production, they will not only open the 1T or 2T hard disk of the production line alone, they will mass produce the same hard disk and define the capacity for the hard disk by setting the firmware information. ) in the firmware, the factory will define the capacity of the hard disk, and those sectors that exceed the capacity will be called reserves.

As shown in figure (1), blue is the firmware area, white is the work area, green is the reserved area, and red is the bad area. When the red bad sector 6 has not been added to Plist or Glist, the number LAB of the workspace is from 0 to 29, followed by the reservation area.

Figure (1)

The situation of joining PLIST in bad ways

When the bad channel joins the PLIST, the recording address is automatically skipped when the hard disk is running, and the hard disk will no longer read or write the sector, so the lab values of all addresses in the sector have changed, and the 29th area will be extended to a sector reserved. Since all LAB has changed, a factory downgrading is required after the bad channel is added to the PLIST.

Figure (2)

Figure (3)

The situation of joining GLIST in bad ways

When the bad area is added to the GLIST, the hard disk needs to read the sector where the bad area is located, it will be relocated to a sector of the reserved area, the other sectors of the hard disk will not be affected, and the LAB will not change. Because the reserved area is relatively far away from the work area, it is in the inner channel, so when the magnetic head reads the data of the sector, it needs to move a long distance, so the reading speed of the LAB will become slower, so after joining the GLIST, it will affect the reading and writing speed of the hard disk.

In the next section, we will discuss the practical demonstration of converting Glist to Plist.

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