Capacity scaling and segmentation

The 3592 tape drive use capacity scaling and segmentation to place data in a designated section of tape to speed access and manage efficient capacity.

The 3592 tape drive supports capacity scaling for tape cartridges of media types JA, JB, JC, JD, and JE over a broad range of capacities. The effect of capacity scaling is to contain data in a specified fraction of the tape, which yields faster locate and read times. Alternatively, you can purchase economy tapes (the JJ, JK, JL, or JM media type) to achieve this faster performance.

The 3592 J1A tape drive divides tape into longitudinal segments. Using this capability, it is possible, for example, to segment 300 GB (279.39 GiB) data tapes into two segments: one segment with 64.42 GB (60 GiB) fast access, and another 200 GB (186.26 GiB) segment for more capacity. You can purchase 300 GB (279.39 GiB) data tapes that are pre-formatted with these segments, or you can segment and capacity scale them later. Segmentation is only available within a specified range of settings for capacity scaling. Capacity scaling is not supported for economy or write-once-read-many (WORM) tapes. For information about implementing segmentation and capacity scaling, refer to the README files that pertain to your device driver at Fix Central.

For more technical information about WORM, capacity scaling, and segmentation, see also the IBM 3592 Tape Drive SCSI Reference.

All TS1120 and later tape drives also support multiple format options, such as scaling and segmentation modes that trade capacity for improved access times. While 256 settings of the Capacity Scaling byte (and resulting fractional capacities) are supported on these drives, the following three primary settings are recommended for use:
  • Full capacity default mode
  • 20% scaled fast access mode (20% capacity that is scaled, front of tape used). The Capacity Scaling byte is x'35'.
  • Performance scaling for 87% capacity and a segmented format with recursive accumulating backhitchless flush (RABF) capability (a non-volatile caching technique) for the full cartridge. For WORM firmware for the 3592 tape drive, the Capacity Scaling byte is x'E0'.
These settings are fully certified and are available as labeled and initialized part-numbered cartridges. For the exact Mode Select commands and settings that are necessary to invoke scaling, see the IBM 3592 Tape Drive SCSI Reference.

Scaling support in drive

Capacity scaling in the TS1120 and later tape drives is controlled by the host program that performs a Scaling operation. The Scaling operation uses the Capacity Scaling byte and the Capacity Scaling Valid control bit in Mode page X'23'. These tape drives do not change their current cartridge scaling, except for a special condition. The drives change scaling when a SCSI Mode Select command that specifies Mode Page X'23' (with appropriate non-default parameter settings) is received while the cartridge is positioned at the beginning of the tape. The drive can sense and report the scaling state of the current medium by using a Mode Sense command that specifies Mode Page X'23'. The default unscaled capacity is 300 GB (279.39 GiB) for a JA cartridge in J1A density, 500 GB (465.66 GiB) for a JA cartridge in E05 density, 700 GB (651.93 GiB) for a JB cartridge in E05 density, and 1 000 GB (931.32 GiB) for a JB cartridge in E06 density. The default unscaled capacity for a JY cartridge in E07 density is 4 TB (3.64 TiB).The default unscaled capacity for a JZ cartridge in E08 density is 10 TB (9.1 TiB).
  • The cartridge can be rescaled from any current Capacity Scaling byte value to any supported new value. The tape is logically erased by this (End of Data mark that is written at beginning of tape), but not physically erased as with the long erase command. Scaling or rescaling one cartridge does not cause rescaling of the next cartridge; an explicit command must be issued for each cartridge to be rescaled.
  • The drive provides the option of setting the scaling values of N/256ths of full capacity, where N ranges from X'16' (22 -- equals about 8% capacity) to X'EC' (236 -- equals about 92%).
  • For scaling factors N, between X'4B' and X'EB', the drive scales to the specified amount and creates a fast-access 20% capacity segment in the beginning of the scaled region. (Not applicable for JE cartridge types or 60F format cartridges.)
  • At all scaling factors, the drive supports early warning at the end of the scaled region (with the appropriate unit attention to inform the software that it flushes buffers and close volume) and reports a physical end-of-tape check condition at the end of the scaled region, just as it would if unscaled tape reached the real physical end of the tape.
Capacity scaling is not offered on any of the short length (economy) cartridge types (JJ, JK, JL, or JM), or on the WORM cartridges (JR, JV, JW, JX, JY, and JZ). Capacity scaling is only offered on the JA, JB, JC, and JD cartridge types (not JE).
Three important attributes are controlled by the setting of the Capacity Scaling byte value:
  • The total Medium Capacity
  • The ability to perform the RABF function on an entire cartridge, including last wraps
  • Information about whether the format is segmented (not supported on JE cartridges or 60F format). If the format is segmented, a fast-access segment is created on the front part of the tape followed by a larger remainder segment that occupies the remainder of the tape. The fast access segment is always filled (written) first, followed by the filling of the remainder segment. For some applications that want improved access attributes for partially filled cartridges but still want to use full capacity (if required) without rescaling, this option is available.
It is important to note that the scaled state and attributes (segmentation, RABF) of the cartridge format is retained when a cartridge is reformatted between the J1A and E05 logical formats, although the exact resulting used capacity as a percentage of full capacity is not identical for all mapped settings.