Physical Storage

The physical storage is organized in RAID5 fashion as shown in the figure 1. The storage consists of a set of columns. We use the term columns to distinguish them from disks. A column is a block device which can be a single disk or can be a pseudo device built from multiple disks using a layered driver. The smallest column limits the size of the storage. A column is divided into contiguous regions called stripe units. A stripe is a formed by grouping one stripe unit from each column.

The size of the data portion of a stripe is N-1 times the stripe unit size (N being the number for columns), as one stripe unit from each stripe is used for parity. We use left-symmetric parity distribution scheme[#!bib:leekatz!#]. The polarity of a physical stripe is defined as the column number of its parity stripe unit. The polarity is used to ensure declustering for RAID1 stripes. These physical stripes act as backing store for logical stripes that are explained next.