In order to keep my home file archive safe, I've bought a Synology NAS and configured it to use a Synology Hybrid RAID configuration. Now almost a year since I started feeling safe about the years of photos and other precious files, I've discovered that a lot of people on the Web discuss uninterrupted power supplies for Synology NAS's. Since a NAS can also work without disk fault-tolerance, I wonder: are there situations when this fool-proof configuration may fail? Doesn't RAID protect me against possible troubles? I don't care about the loss of files during writing. Only the files already on the NAS are important. I was looking for a quiet compact energy-efficient NAS and wouldn't like to have to double the footprint, noise and sleep-mode power consumption without a good reason.

I am looking for a technical explanation of why I may need a UPS if I use a RAID on a NAS. Thanks.


yes, you need a UPS as well.

RAID protects files that are completely written to disk, from being lost due to dfrive failure, and deterioration like bad blocks, etc.

UPS's prevent powerstate changes from interrupting changes currently happening to the filesystem. For instance, if a file were being written when you lost power, the file data would be lost, and the file on disk would be unusable. if you are moving a file, and lose power before it completes, the file may have been moved, but the filesystem index that tells where the file is still points to the wrong location. This issue is excerbated by disk Write-Caching, where writes to the disk are deferred until later, to avoid slowing things down now. Data in the disk write cache has not been written however, and if power is lost, they never will be.

In fact, since you are using an array, more data is written across more disks, which widens your exposure to powerstate related failures. For instance if you lost power while writing the parity stripe for a new file. now the data in the stripe is corrupt, which may make the stripe unrecoverable, or may just truncate the file. when a disk does fail, and recovery is attempted from the stripe, the corrupted data will be restored.

Last, don't assume that the only time a file or filesystem meta-data file changes is when you copy data to the NAS. There are many minuscule operations that may change the storage attributes of files at rest, of which the user is not generally aware.

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    Additionally, neither a UPS nor RAID protect you against accidentally deleting files. That's what backups are for. – ChrisInEdmonton Feb 9 '15 at 17:38
  • +1 @ChrisInEdmonton: although this remark does not relate to the topic, I agree that I shouldn't feel safe without the backup. – texnic Feb 10 '15 at 18:48
  • Frankly, I thought that RAID redundancy functioned similar to transactions in databases, i.e. the data is either written and fine or not written at all. What you've written gives me the feeling that the OS is asking the HDDs to store completely independent bits of information. In this case the situation you describe would be understandable. However if the system was a bit smarter (as I thought it was), the first disk would never report success to the OS before the writing operation was completed by the second disk. Could you maybe add any reference to confirm what you've written please? – texnic Feb 10 '15 at 18:57
  • Also, is there a difference whether I use the specialized NAS HDDs or the general purpose desktop ones? – texnic Feb 10 '15 at 18:58
  • I'd spend on either the NAS drives, or the top of the line consumer disks like the WD Caviar Blacks. somthing with a 5 year warrenty. disks on a NAS are more likely to fail without you noticing until its too late, unless you pull them regularly to check their SMART stats. – Frank Thomas Feb 10 '15 at 19:19

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