Tape capacity and backup performance explained

Native tape capacity: size the backup before compression

Native capacity is the amount a cartridge holds without assuming compression. Use it as the starting point when deciding whether a save will fit on one tape. A TS2240 LTO 4 cartridge holds 800 GB natively; a TS2290 LTO 9 cartridge holds 18 TB.

The drive generation and the cartridge generation must match a supported combination. Compare LTO cartridge generations before changing the drive that reads an existing archive.

Compressed capacity: the ratio is an assumption

Compressed capacity estimates what fits when the drive can reduce the incoming data. The published figure is not extra physical tape. A backup that is already compressed or encrypted may gain very little.

These are generation-specific assumptions. Size the order from actual backup data rather than the larger number alone.

Library capacity: count data cartridges, then apply the media size

A library total combines the number of cartridges available for data with native capacity per cartridge. Physical slots may also hold cleaning media or be assigned to import/export. Licensed capacity can be lower than the installed slot count.

The TS3200 example is 48 physical positions multiplied by 6 TB per LTO 7 cartridge, or 288 TB native before positions are reserved for other uses. Open the TS3200 capacity table for its generation-specific totals. Understand physical and licensed slots.

Native transfer rate: drive speed and backup speed differ

Native transfer rate measures how quickly the drive writes or reads data without a compression multiplier. It is normally stated in MB/s. A compressed rate or an interface signalling rate measures something different.

Full-height and half-height drives can have different native rates within the same LTO generation. TS4300 LTO 9 configurations rate the half-height drive at 300 MB/s and the full-height drive at 400 MB/s. Check the selected TS4300 drive configuration.

A drive must receive enough data to keep streaming. Source storage, host adapters and backup software can all limit the job. Record the real save size and elapsed time when sizing a replacement.

Drive buffer: temporary memory inside the tape drive

The drive buffer holds data temporarily between the host and tape mechanism. It helps handle short differences in data arrival and tape movement. Its size is memory capacity, not cartridge capacity or a sustained backup rate.

Keep the buffer figure with the exact drive model instead of assuming the same memory across generations.

Cache in a virtual tape appliance

VTL cache is memory used by the disk-based backup appliance. It is separate from the disk space that holds retained virtual tapes. More cache does not add the same amount of retained backup capacity.

For example, the CIT3324 has a published 32 GB base cache configuration while the CIT8524 starts at 1 TB. Those are appliance-specific configurations. Plan usable VTL disk capacity separately from cache and virtual-drive licences.

Data cartridges, cleaning cartridges and WORM media

A cleaning cartridge services the tape drive and does not store a backup. A data cartridge holds the saved data. WORM means Write Once, Read Many: compatible WORM media prevents normal rewriting of recorded data.

Match each cartridge type to the drive generation and library. WORM media does not remove the need for encryption keys, readable hardware or a tested restore. Plan tape encryption and recovery.

Get help with your tape configuration

Send us the installed model and the backup or restore you need to support. We can match the equipment and media before quoting the configuration. Request tape configuration help.

Tape product references

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