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Originally Posted by eagleamon
So the question: regarding reliability and disk longevity, is it recommended to have them spin down while not used (most of the day, as these are storage unit for movies, pictures, documents..) or let them spin whatever.. ?
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There are longevity issues both ways: spinning up/down puts more stress on the drive motor than running it continuously, so you don't want to do it too often; but running the drive motor continuously will wear out the bearings.
So the question is not whether to have a spin down, but what length of time the spin down parameter should be. It probably should be in the minutes range (see
man hdparm for details on what the value means, for example 120 = 600s).
As long as the spindown is something reasonable, then other factors will also play a part in longevity, such as protecting the drive from shocks and temperature extremes.
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what is the best configuration here ? My 2 raid 1 stripes, or a raid 5 ? I know that two disks kill the raid 5, and they have to be of the same stripe to kill the lvm on the raid1, but what are the odds ?
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The advantages of different RAID configurations are complex, but you don't get something for nothing; it involves trade-offs.
I think you are comparing RAID 1+0 (or RAID 0+1) to a 4 disk RAID 5 here.
The RAID 0+1 will be more reliable, because as you say, the second disk failure is less likely to cause total failure. But it will also give you less storage than a 4 disk RAID 5, and will be slower.
A more equal comparison would be RAID 0+1 with a 4 disk RAID 6. Here the space efficiency is the same, and the speed comparable. But the RAID 6 will take 3 failures to cause total failure, so is more reliable.
Another consideration is that parity style RAID (such as RAID 5 and RAID 6) can be harder to rebuild than a straight mirror. This is not such an issue in a commercial environment, where you might have access to duplicate hardware, but in a home environment it is nice to be able to mount the drives after a failure occurred without rebuilding (eg you can mount the good striped pair as RAID 0).
You should also take care that you are not using the RAID mirror as a substitute for backups. It would be better in some circumstances to use the second pair of disks as offline backups, rather than an online mirror. The reason for this is that there are other failure modes: spikes through power supplies, accidental deletion of data, RAID controller failures, local incidents such as fire or flood, and so on, that can affect all online disks at the same time. The purpose of redundancy in RAID configurations is to provide continuity of access (server uptimes), rather than data backup.