Understanding usable space on each disk

What usable disk space is

A disk's usable space can be different from its physical space. Disks from different manufacturers might differ slightly in size even though they belong to the same size category. To ensure that you can replace disk from one manufacturer with a disk from another manufacturer, the appliance software automatically "right-sizes" its disks.

The appliance manages disk space so that objects never entirely fill the usable space. The following table shows the approximate usable disk space for each class of disk supported by the appliance.

Disk class	Usable disk space
1-GB disks	1,000 MB (2,048,000 blocks)
2-GB disks	2,000 MB (4,096,000 blocks)
4-GB disks	4,000 MB (8,192,000 blocks)
9-GB disks	8,600 MB (17,612,800 blocks)
18-GB disks	17,000 MB (36,864,000 blocks)
36-GB disks	34,500 MB (70,656,000 blocks)
72-GB disks	68,000 MB (139,264,000 blocks)

Understanding the overwriting-objects policy

The appliance uses a first-in, first-out (FIFO) replacement policy for writing cache objects. The FIFO policy operates by tracking inodes and using object ejection (truncation) to manage space. This process takes place asynchronously during the serving of requests. When an appliance disk reaches its upper capacity, objects are ejected until there is sufficient space to record new objects.