[Imported from Trac: page FAQ, version 112]
parent
2ff8107653
commit
ce0fb2dd67
10
FAQ.md
10
FAQ.md
|
@ -14,17 +14,17 @@ A: Zooko wrote [//pipermail/tahoe-dev/2011-July/006560.html a long post about th
|
|||
|
||||
**<a name="Q2_what_is_erasure_coding">Q2:</a> "Erasure-coding"? What's that?**
|
||||
|
||||
A: You know how with RAID-5 you can lose any one drive and still recover? And there is also something called RAID-6 where you can lose any two drives and still recover. Erasure coding is the generalization of this pattern: you get to configure how many drives you could lose and still recover. You can choose how many drives (actually storage servers) will be used in total, from 1 to 256, and how many storage servers are required to recover all the data, from 1 to however many storage servers there are. We call the number of total servers `N` and the number required `K`, and we write the parameters as "`K-of-N`".
|
||||
A: RAID-5 can lose one drive and RAID-6 can lose two drives and recover. Using a method of data protection in which data is broken into fragments, expanded and codified with redundancies, stored across a selected set of various places or storage servers, Erasure coding (CE). The number of records (storage / servers / nodes) used in total can be chosen from 1 to 256, and the number of storage servers that are required to recover all the data, from 1 to the total number of available storage servers. The number of overall storage servers, we call `N` and the number needed `K` and write the parameters such that it is "`K-of-N`".
|
||||
|
||||
This uses an amount of space on each server equal to the total size of your data divided by `K`.
|
||||
This uses an amount of space on each storage server equal to the total size of your data is shared over all `K`.
|
||||
|
||||
The default Tahoe-LAFS parameters are `3-of-10`, so the data is spread over 10 different drives, and you can lose any 7 of them and still recover the entire data. This gives much better reliability than comparable RAID setups, at a cost of only 3.3 times the storage space that a single copy takes. It takes about 3.3 times the storage space, because it uses space on each server equal to 1/3 of the size of the data, and there are 10 servers.
|
||||
Tahoe-LAFS having default parameters `3-of-10`, the data is spread over 10 different disks and losing any 7, continue to recover all the data. This is more reliable than comparable RAID arrangements, with a cost of only 3.3 times the storage space that a single copy carries. It takes about 3.3 times the storage space, because it uses space on each server, equal to 1/3 of the size of the data, and there are 10 servers.
|
||||
|
||||
"Forward error correction" is another term for erasure coding.
|
||||
"Forward error correction" (FEC) is another term for erasure coding.
|
||||
|
||||
Erasure coding should not be confused with "secret sharing", which has the additional security property that fewer than `K` servers cannot recover any information about the data. Tahoe-LAFS' erasure coding does not have this property, and does not need to have it because we rely on secret-key encryption (using a key in the read cap) for confidentiality.
|
||||
|
||||
"Information Dispersal Algorithm" (IDA) can refer either to an erasure code or a secret sharing algorithm depending on context, so we prefer not to use that term.
|
||||
"Information Dispersal Algorithm" (IDA) can refer either to erasure code or secret sharing algorithm according to context, so we prefer not to use that term.
|
||||
|
||||
**<a name="Q3_disable_encryption">Q3:</a> Is there a way to disable the encryption for content which isn't secret? Won't that save a lot of CPU cycles?**
|
||||
|
||||
|
|
Loading…
Reference in a new issue