Synology 1813+ which raid to use? two raid5 vs one raid 6? VS no raid and have backup every hour?

1. Backup strategy
- where can I find back up strategies? ie. having multiple sets of backup in different locations? Is the a standard method that companies uses if the data is essential?

Daily incremental, weekly full backup.
At least one backup copy offsite in case of fire/flood/theft.

Here is a strategy from Microsoft:
http://technet.microsoft.com/en-us/library/cc938488.aspx

for essential data arrays I use raid 1+0, unfortunately this barely meets your storage needs since you lose half the space so for your situation I think a raid 6 suits you best. 7 drives in raid6 with 1 hot spare would yield 15tb. I owuld suggest you buy another drive or two to keep spares on hand.

As for your backups, what backup solution do you have in place equipment-wise that is.

by backup equipment I was referring to: what will you be backing up to? The synology cannot backup to itself. its got to go somewhere.

raid6 is a 2 drive fault tolerant system. Hotspares means if a drive dies a rebuild using the spare begins immediately. if two drives fail and you only have 1 hot spare then the second failed drive cant rebuild until someone gets there and physically removes the failed drive and inserts a replacement.

raid 5 is a 1 fault tolerant system. theres no point in having 2 spares as losing 2 drives at the same time means you've just lost all your data. everything.

raid 10 is a bunch of raid1's (often two drive raid1's) merged into a raid0. speed is proprtional to the number of raid1's (ie 8 drive raid10 (four raid1's) is about twice as fast as a 4 drive raid 10) fault tolerance is odd but higher than 1 drive. I'll try to explain.

in raid 5 if you loose 2 drives all data is lost. in raid 6 its the 3rd drive that critical. In raid 10 you have to loose one of the raid1's. Considering an 8 drive raid10 which is four raid1's in a raid0. if you loose 1 drive the other drive in the raid1 pair keeps you up. of the remaining 7 drives there is only 1 drive that will definitely bring down the array and thats the mirror of the failed drive. So there is a 1 in 7 (14.3%) chance of a second drive failure bringing down the array (unlike the 100% chance in a raid5). Lets say another drive fails but its not the critial one. That means two of the four raird1's are degraded (only 1 drive in each pair working) and the other remaining four drives are ok. now if two raid1 pair are degraded then if either of those 2 drives fail the array is lost but that is 2 out of 6 running drives or a 33.3% chance of loosing the array versus 100% of a raid6 failing on the third failure. Now imagine if you had 24 drives in raid10; the chance of 3 drives taking out your array drops to 2 in 22 or about 10%

unfortunately an 8 bay unit doesnt leave you much choices.
Raid6+2 (hotspares) gives you 12tb space = no room to grow.
Raid10 (all 8 drives) gives you 12tb space = no room to grow.
raid6+1 (7 drives in raid6 +1 hotsapre) gives you 15tb space.
raid 5+1 (7 drives in raid5 +1 hotspare) gives you 18tb space which is the most space and the least fault tolerant imo.

any chance you could increase to larger (more drives) unit? You never mentioned budget.

as for speed, raid 6 is by far the slowest in writes but has decent read speeds.
raid 5 is the next quickest in writes and has good/great read speeds.
raid10 is the fastest reads & writes but the full potential is not achievable with this unit. This unit is capable of about 100MB/'s thanks to the gigabit ports. There is an option to link all the ports together and get upwards of 300MB/s from the unit but no one I know has done it. Several have teamed two ports together to get about 200MB/s. (all your equipment would need to support port teaming/aggregate linking including the pc's) For comparison an 8 drive raid10 is capable of 600MB/s+ in sequential reads.

edit - if you do loose a drive and the unit is rebuilding it will be horribly slow. its also best practice to not use it while its rebuilding.

And to answer your other question, no - having hotspares does not increase the number of drives you can lose at any one time. It only effects when the rebuild process begins which directly effects when it will complete and you can get back to work.