Bad sectors, part II - reallocation

Since it is known in advance that it is impossible to create a perfect magnetic surface, a number of spare sectors are reserved on the drive.

When a surface defect appears, the sector with the defect is replaced with a good one from the pool of a reserved sectors. Obviously, there is no surface repair involved. Instead the special record is made in the address table, like "if the write/read request arrives for the sector 123, use the sector 456 instead". This results in a certain loss of performance because it is now required to move the head to the reserved sectors zone and back again instead of just reading a contiguous chunk of data. On top of that, the data which was stored in the bad sector is lost. Nevertheless, theoretically you can use the drive further as if there are no bad sectors at all.

This process is called “reallocation”. The S.M.A.R.T. attribute named “Reallocated Sectors Count” shows a number of the reallocated (replaced) sectors.

If the drive idles long enough, it can start a self-test, reading random sectors to make sure that they are not corrupted. The sectors with defects are queued and then subjected to the reallocation if needed. Another S.M.A.R.T. attribute – “Current Pending Sector Count” - is designated for monitoring of the queue status.

The first surface check is done during production of the drive, and the new drive (just from the factory) may already have several reallocated sectors. However, these "factory-certified" defects are not shown in the S.M.A.R.T. counters.

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