hillsoli.blogg.se - Maczfs vs zevo

In this case, you don't just read the data, but you validate all files to make sure they are still what they are supposed to be. While this should help, you have no way to know if problems are fixed, or even if there were problems in the first place: the hard disk controller will do its magic silently. You just force the disk to periodically scrub its entire surface, hoping that no sectors could have gone bad since the last time you did that, and trusting the controller to move data to new sectors whenever necessary. This is the minimum protection that you can have. forcing the disk to read all the data stored on it periodically). There are several things that can be done against this phenomenon, and all involve some amount of scrubbing (i.e. I do not know if manufacturers publish these figures for consumer-grade disks, but you can assume that they are sensibly worse.

This is small, but not so small to make it totally irrelevant, and it is only for enterprise-grade hardware. According to Wikipedia, in 2013, manufacturers claimed a non-recoverable error every $latex 10^$ bits read (which is 1.25 Peta-bytes, or 1,250 TB). The reality is more complex than this, and hard disks store redundant information, which allows them to silently correct most errors. Given that a sector contains several bits, it is likely that many of them will be corrupted at the same time. This process has a weakness: what if you don't access your data for a long time? In this scenario, it is possible that a sector is accessed only when it's too late for the controller to clearly understand whether some of the bits stored there are actually zeros or ones.

If, after this operation, there are still doubts, the sector is flagged as corrupted, and replaced with a new one from the spare pool (all data on the corrupted sector is moved to the new one while it's still readable).

If the magnetic field gets weaker (basically making a zero look more like a one or vice versa), the hard disk controller tries to magnetize it again.

When you access the data for reading, the hard disk head reads the data if the magnetic field is still strong, all is good.

The physical portion of a disk which correspond to a specific bit of your data is magnetized in a precise way, indicating whether that specific bit is a binary zero, or a one.

Modern hard disks are smart enough to detect this problem, and silently replace corrupted sectors with new ones before they are unreadable (a new hard disk contain some amount of spare sectors, on top of the specified capacity, which are used for this).

Space on hard disks is subdivided in sectors, and the density is so high that minimal degradations of the disk inevitably lead to corruption. Imagine losing your old family photos, movies of your kids, or the private key of your Bitcoin wallet. This leads to all sort of issues, especially with data which is not accessed often and not easily replaceable, if at all. To make it simple, it means that there is a slight chance that your data might change over time, getting corrupted. Bit rot, in case you do not know, is a scary word which makes you lose all confidence in your storage.