On Mon, May 11, 2015 at 10:03:32PM +0200, Hendrik Friedel wrote: > Hello, > > I need your advice for the following scenario: > I do have three -identical- 3TB HDD. On these I store: > -Backups (Images of my Families Laptop; Backup of the Machines (the > one with the HDD) running System). > -Videos (recordings, not essential) > -HomeVideos (Familiy, essential) > -Photos > -Documents > > Now I intend to: > * increase the available disc space > * stop worrying how much space is available on which drive > * create redundancy > * have low operating cost (power-consumption) > > > So, I need to buy one additional HDD. > And I want to combine the drives to one big volume. > > I am aware that raid is no backup. > Thus, I backup the HomeVideos and the Photos on an external drive > not connected to the power supply and USB. > > I think, that I do not need raid5, depending on the definition of > 'not essential'. Nevertheless, raid5 would efficiently (at low cost) > decrease the likelyhood of loss of the 'non-essential' data. > Furthermore, raid5 would fill the gap that I have between backups > (the fact, that I do these backup manually means that I do not do > them every day) and at least safe me from hardware failures. > > So, yes, I think I do want raid5 and I want to combine. > But what size of HDD do I want to buy? The best value (cost per TB) > I get for 3TB drives. So I could by two 3TB drives, one for > additional space, the other for the parity information. > Or I could by one bigger drive, with potentially lower power > consumption. But how would the raid5 look in this case? I fear, only > 3TB of the 6 would be redundant and the other 3 would be not > redundant, right?
No, 3 TB of the new drive would be usable, giving you a grand total of (4-1) * 3TB = 9 TB of usable space on the array, with 3 TB of raw space unusable (the remainder of the 6 TB device). http://carfax.org.uk/btrfs-usage may be helpful to you here. > Now the last point: Power consumption: Under which conditions can > the drives spin down in case of raid5? I assume that all drives have > to run as in case the data is written on any one of the drives, > right? > Is that also true during reading of data, i.e. is the parity also > checked for read operations? Parity is spread evenly across all devices, so a read of more than a trivial quantity of data would almost certainly spin up all devices anyway. Hugo. -- Hugo Mills | A gentleman doesn't do damage unless he's paid for hugo@... carfax.org.uk | it. http://carfax.org.uk/ | PGP: E2AB1DE4 | Juri Papay
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