Why do I have a low capacity warning even though I still have unused pool capacity?
Storage Spaces provides advance notification of thinly provisioned storage spaces when the storage pool does not have enough capacity spread among a sufficient number of disks to continue to write new data. The default warning point is 70% capacity utilization. To learn when Storage Spaces will generate a warning, consider the following example.
A two column, two-way mirror space that uses thin provisioning in a four disk pool
Two of the disks have 1TB capacity and two have 2TB capacity. Because a two column, two-way mirror space needs four disks (number_of_disks = NumberOfColumns * NumberOfDataCopies), it will evenly consume all four disks as it writes new data. When capacity utilization of the two 1TB disks reaches 70%, Storage Spaces will warn of a low capacity condition. Even though the entire pool has 3.2TB free capacity, the thinly provisioned space will soon not be able to write any more data because the 1TB disks are nearly fully consumed.
You can easily keep individual storage spaces’ low capacity warning synchronized with each other and with the pool by following the guidance in the next section, “How do I increase pool capacity?” from the moment of creating the pool and through all subsequent expansions of the pool.
How do I increase pool capacity?
Storage Spaces can usually use the additional capacity from even a single additional drive. However, for optimal capacity utilization, consider the number of columns your storage spaces have and add disks in multiples of that number.
For example, consider a pool which has a four-column simple space, a one-column, two-way mirror space, and an eight-column parity space. The four-column simple space suggests you expand pool capacity in sets of four disks. The one-column, two-way mirror space suggests you expand pool capacity in sets of two disks (for mirror spaces, you have to multiple the number of columns by the number of copies). The eight-column parity space suggests you expand pool capacity in sets of eight disks. In this example, you would want to expand pool capacity in sets of eight disks as eight is a common multiple of the number of columns of each of the storage spaces in the pool.