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Zoned constant angular velocity (ZCAV) is a disk-recording arrangement that groups tracks into zones, with a fixed number of sectors per track inside each zone. In hard drives, outer zones can fit more sectors per track than inner ones, using their longer circumference to increase capacity. The trade-off is that transfer rate can vary depending on where data sits on the disk.
How ZCAV works on a hard drive
A disk platter has concentric tracks. Because an outer track has a longer circumference than an inner track, it can hold more sectors at a comparable recording density. ZCAV takes advantage of that difference by dividing tracks into zones: tracks within a zone share the same number of sectors per track, while outer zones can use a higher count.
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This is a zoned layout, not one sectors-per-track count applied uniformly across the entire recording surface. By recording more sectors on the longer outer tracks, the arrangement increases the disk’s usable capacity. Rodney Van Meter’s 1997 USENIX paper describes ZCAV as “a technique adopted by hard disk manufacturers to increase the capacity of disk drives.” Read the paper, “Observing the Effects of Multi-Zone Disks.”
How ZCAV differs from CAV and CLV
The names describe different ways of handling disk rotation and recording across the surface. In the hard-drive comparison in Van Meter’s paper, CAV keeps the same sectors-per-track count, while ZCAV groups tracks into zones with different counts. CLV changes rotational speed to maintain linear velocity. These distinctions matter because a disk’s layout and motion affect how much data can be read as the head moves across the surface.
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| Method | How it handles the disk | Implication for data rate across position |
|---|---|---|
| CAV | Constant angular velocity; the hard-drive comparison uses the same sectors per track. | Not established as uniform by the cited comparison. |
| CLV | Changes rotational speed to maintain linear velocity. | Not stated in the cited comparison. |
| ZCAV | Groups tracks into zones with a set sectors-per-track count per zone; outer zones can have more sectors per track. | Can vary by sector address or head position. |
Why transfer rate can vary
Because zones have different sectors-per-track counts and data may be located at different positions, ZCAV does not guarantee one transfer rate across the whole disk. Van Meter’s 1997 analysis reported a roughly 25% peak transfer-rate drop depending on head position in the paper’s specific analysis and measurements using a BSD Fast File System. That figure is not a universal benchmark for current drives.
ZCAV on optical media
ZCAV is not exclusively a hard-drive term. ECMA-280, the first-edition optical disk cartridge standard published in December 1998, lists ZCAV as a control-track velocity-mode value alongside CAV, CLV, ZCLV, and Logical ZCAV. The standard treats ZCAV and ZCLV as distinct modes; the optical-media usage should not be assumed to describe a hard drive’s implementation. See ECMA-280.
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What the term means for a drive buyer
ZCAV explains a recording layout, not a user-facing setting. The cited technical sources establish the concept but do not establish that current drive models advertise ZCAV or let buyers select it. For a practical choice, use the specifications and performance information for the particular drive rather than treating ZCAV as a guaranteed speed feature.
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