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Citation very much Needed. Magnets only lose their magnetic powers very slowly. For example, samarium cobalt magnets might decrease their magnetic strength abo

Citation very much Needed. Magnets only lose their magnetic powers very slowly. For example, samarium cobalt magnets might decrease their magnetic strength about 1 percent over a decade, which contradicts your claim.

adrian_b 5y ago
Samarium cobalt are actually some of the best from this point of view, because they have a very high Curie temperature. Most magnets made from cheaper materials with lower Curie temperatures, e.g. Nd-Fe-B or ferrites, lose their magnetization at higher rates at room temperature. Losing a few percent of the magnetic strength for the whole magnet would be equivalent with massive data corruption on a HDD, because that is the percentage of magnetic domains whose magnetization has rotated, i.e. the percentage of corrupt bits. The magnetic materials used in HDDs have actually a much lower rate of spontaneous change of magnetization. Nevertheless, when a single HDD contains around 10^14 bits, even a 10^-14 per year probability of a bit flipping at ambient temperature may cause one corrupt file per year of storage per HDD, which is close to the typical rates of corruption actually seen in reality.
- gruez 5y ago
  >Nevertheless, when a single HDD contains around 10^14 bits, even a 10^-14 per year probability of a bit flipping at ambient temperature may cause one corrupt file per year of storage per HDD Hard drives add ECC info to each sector to prevent random bitflips from corrupting data[1]. Also IME actual rates of data corruption is far lower than 1 bit per year. This is on a filesystem with checksums, so any corruption would be easily detected. [1] https://en.wikipedia.org/wiki/File:Advanced_format_(4Kib)_HDD_sector.svg https://en.wikipedia.org/wiki/File:Advanced_format_(4Kib)_HD...
- dannyw 5y ago
  If you're encountering 1 bitflip per year per HDD, something is horrifically wrong. I've only observed 1 incident of corruption in 5 years of 00+ drives with ZFS, and that was due to a dying HBA controller, not the HDD.
  - adrian_b 5y ago
    Per year of cold storage. If the files are overwritten frequently, then the error rate would be much lower. Even if they are only read frequently that would lower dramatically the error rate, because the HDD controller will rewrite the blocks with corrected errors, which happen frequently. In about 200 TB of high-quality HDDs from 2 vendors and in various capacities, between 0.5 TB and 8 TB per HDD, I have seen around 100 errors after 5 years of cold storage.

adrian_b 5y ago

Samarium cobalt are actually some of the best from this point of view, because they have a very high Curie temperature. Most magnets made from cheaper materials with lower Curie temperatures, e.g. Nd-Fe-B or ferrites, lose their magnetization at higher rates at room temperature. Losing a few percent of the magnetic strength for the whole magnet would be equivalent with massive data corruption on a HDD, because that is the percentage of magnetic domains whose magnetization has rotated, i.e. the percentage of corrupt bits. The magnetic materials used in HDDs have actually a much lower rate of spontaneous change of magnetization. Nevertheless, when a single HDD contains around 10^14 bits, even a 10^-14 per year probability of a bit flipping at ambient temperature may cause one corrupt file per year of storage per HDD, which is close to the typical rates of corruption actually seen in reality.

gruez 5y ago
>Nevertheless, when a single HDD contains around 10^14 bits, even a 10^-14 per year probability of a bit flipping at ambient temperature may cause one corrupt file per year of storage per HDD Hard drives add ECC info to each sector to prevent random bitflips from corrupting data[1]. Also IME actual rates of data corruption is far lower than 1 bit per year. This is on a filesystem with checksums, so any corruption would be easily detected. [1] https://en.wikipedia.org/wiki/File:Advanced_format_(4Kib)_HDD_sector.svg https://en.wikipedia.org/wiki/File:Advanced_format_(4Kib)_HD...
dannyw 5y ago
If you're encountering 1 bitflip per year per HDD, something is horrifically wrong. I've only observed 1 incident of corruption in 5 years of 00+ drives with ZFS, and that was due to a dying HBA controller, not the HDD.
- adrian_b 5y ago
  Per year of cold storage. If the files are overwritten frequently, then the error rate would be much lower. Even if they are only read frequently that would lower dramatically the error rate, because the HDD controller will rewrite the blocks with corrected errors, which happen frequently. In about 200 TB of high-quality HDDs from 2 vendors and in various capacities, between 0.5 TB and 8 TB per HDD, I have seen around 100 errors after 5 years of cold storage.