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posted by janrinok on Tuesday February 06, @03:51AM   Printer-friendly
from the confidentiality-integrity-and-availability dept.

Exotic Silicon has a detailed exploration of how and why to make long term backups.

The myth...

When thinking about data backup, many people have tended to fixate on the possibility of a crashed hard disk, and in modern times, a totally dead SSD. It's been the classic disaster scenario for decades, assuming that your office doesn't burn down overnight. You sit down in front of your desktop in the morning, and it won't boot. As you reach in to fiddle with SATA cables and clean connections, you realise that the disk isn't even spinning up.

Maybe you knew enough to try a couple of short, sharp, ninety degree twists in the plane of the platters, in case it was caused by stiction. But sooner or later, reality dawns, and it becomes clear that the disk will never spin again. It, along with your data, is gone forever. So a couple of full back-ups at regular intervals should suffice, right?

Except that isn't how it usually happens - most likely you'll be calling on your backups for some other reason.

The reality...

Aside from the fact that when modern SSDs fail they often remain readable, I.E. they become read-only, your data is much more likely to be at risk from silent corruption over time or overwritten due to operator error.

Silent corruption can happen for reasons ranging from bad SATA cables and buggy SSD firmware, to malware and more. Operator error might go genuinely un-noticed, or be covered up.

Both of these scenarios can be protected against with an adequate backup strategy, but the simple approach of a regular, full backup, (which also often goes untested), in many cases just won't suffice.

Aspects like the time interval between backups, how many copies to have and how long to keep them, speed of recovery, and the confidentiality and integrity of said backups are all addressed. Also covered are silent corruption, archiving unchanging data, examples of comprehensive backup plans, and how to correctly store, label, and handle the backup storage media.

Not all storage media have long life spans.


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  • (Score: 4, Interesting) by sigterm on Tuesday February 06, @09:14AM

    by sigterm (849) on Tuesday February 06, @09:14AM (#1343308)

    "Aside from the fact that when modern SSDs fail they often remain readable"

    But do they? I'm not sure that's true at all.

    And you would be correct in your assumption.

    When a failing SSD goes read-only, it means the controller and interface are both still fully functional, but the amount of damaged cells in the flash memory chip(s) has reached the threshold that the firmware considers unacceptable. It's similar to a S.M.A.R.T. failure reported by a conventional HDD; there aren't enough free sectors/cells left to handle the growing number of defects.

    This failure mode is the best-case scenario, where the flash cells slowly succumb to wear and tear. In these scenarios you often do have ample time to back up your data, since flash cell failures are usually detected when a block is erased and cells are re-written. When the SSD controller detects a failed cell, it still has the original data cached and can redirect the write operation to an unused cell without data loss.

    However, in the case of catastrophic failure of either the SSD controller or the flash chip, immediate and total data loss can occur without any warning. I've seen this more times than I'm comfortable with, which is why I'm skeptical of using single SSDs in any configuration.

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