Partitioning in LTO5 and LTFS

The LTO5 specification includes support for partitions on LTO5 data tapes. This partition support can be classified into two groups: Partition Aware and Partition Enabled.

Here is the relevant definition from the LTO 5 specification:

“All Generation 5 LTO Ultrium tape drives will support at least one partition when writing to or reading from a Generation 5 cartridge. Drives that support more than one partition will implement all requirements for partitions as described in this document. Drives that support only one partition will reject a cartridge that has more than one partition.”

Partition Aware drives can identify partitioned media but will refuse to operate with partitioned media. Partition Aware are supported in the LTO5 specification as an approach to allow drive vendors time to explore and implement support for partitioning. The idea was that a vendor could choose to undertake minimal work to have their drive firmware identify partitioned media and refuse to operate over the partitioned media and still be in compliance with the specification.

Partition Enabled drives are supported in the LTO5 specification as drives that will identify, create, and work with partitioned LTO5 media. At the time of writing (June 2011) all LTO5 drive vendors offer Partition Enabled hardware.

Partitioning in LTO5 allows a maximum of 2 partitions on the tape media. Media utilizing one partition mirrors the format of previous LTO generations. That is, the whole empty tape is a single area of writable sequential media.

LTO5 media utilizing two partitions provides two tape areas of arbitrary size separated by a guard wrap. Each partition can be written to as sequential media without impacting the data stored on the other partition. Each partition can be of an arbitrary size but the media is allocated in terms of wraps with a guard wrap between the partitions. Since partitions are allocated as complete wraps the smallest practical partition size is one wrap, or approximately 37.5GB of uncompressed data storage. The guard wrap is necessary to provide sufficient physical separation on the media to ensure that writes to each partition will not interfere with data written to the other partition.

The LTFS software utilizes two partitions to construct the on media format for LTFS. The “Index Partition” is created to use one whole wrap on the media. The LTO5 partitioning scheme uses an additional wrap as a guard wrap between partition 0 and partition 1. This guard wrap is a buffer to ensure that writes to each partition cannot interfere with data on the other partition.

The LTFS software creates a second “Data Partition” utilizing the remaining wraps available on the media to consume the balance of the tape area. LTO5 media has a rated capacity of 1.5TB. Therefore, LTO5 media formatted for use by LTFS has a storage capacity of 1.43TB (1.5TB – (37.5GB + 37.5GB)).