From Amahi Wiki
Storage Pooling is a technology to pool disk drives and make them look as if they were all part of a single pool of disk space.
The implementation technology in Storage Pooling is Greyhole.
Greyhole is a system module that uses Samba to create a storage pool of all your available hard drives (whatever their size, however they're connected).
From this pool, space is allocated for all Shares participating in the pool.
In addition, it also allows you to create redundant copies of the files you store in shares. You can use this to prevent data loss when part of your hardware fails.
In short, Greyhole offers:
- JBOD storage pool (Just A Bunch of Disks)
- Per share redundancy: file replication on a per-share
- Easily recoverable files: you can read them with other systems
Pooling and File Replication
Greyhole allows file replication, to provide protection against some hardware failures.
Greyhole offers four options for each share:
- Disabled: Share is not allocated from the pool
- One: Initially, files in a share in the pool are stored once in the pool, without extra copies stored (a '-' in the option)
- Many: You can specify how many extra copies you want for files in that share. The total limit in the number of copies is the number of partitions joined in the pool.
- Max: Alternatively, you can chose this option "Always Max" so that the number of copies matches your partitions, even if you reduce or increase the number of partitions participating in the pool. You want this option for valuable files.
Check out some diagrams explaining things in detail in the Greyhole home page.
NOTE: Greyhole is not a backup solution. If a file is removed, all copies are removed.