Dell Studio 1558 (Early 2010) - Manual (Page 86)

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Drives and Media
RAID Level 0+1 Configuration
A RAID 0+1 array combines the high data access rate of a RAID level 0 array
and the data protection (redundancy) of a RAID level 1 mirror by striping
data across two drives and mirroring that striped data on a second set of two
drives.
If a drive failure occurs, subsequent read and write operations are directed to
the other surviving drives. A replacement drive can then be rebuilt using the
data from the surviving drives. Also, because data is duplicated on the primary
and additional drives, four 120-GB RAID level 1 drives collectively have a
maximum of 240-GB on which to store data.