Information Technology Reference
In-Depth Information
have already passed under the disk head, the data can be transferred im-
mediatly, rather than having to delay the request for nearly a full rotation
to reread the data.
Transfer. Once the disk head reaches a desired sector, the disk must
transfer the data from the sector to its buffer memory (for reads) or vice
versa (for writes) as the sectors rotate underneath the head. Then, for
reads, it must transfer the data from its buer memory to the host's main
memory. For writes, the order of the transfers is reversed.
To amortize seek and rotation time, disk requests are often for multiple
sequential sectors. The time to transfer one or more sequential sectors
from (or to) a surface once the disk head begins reading (or writing) the
first sector is the surface transfer time.
Definition: surface
transfer time
On a modern disk, the surface transfer time for a single sector is much
smaller than the seek time or rotational latency. For example, disk band-
widths often exceed 100 MB/s, so the surface transfer time for a 512-byte
sector is often under 5 microseconds (0.005 ms).
Because a disk's outer tracks have room for more sectors than its inner
tracks and because a given disk spins at a constant rate, the surface trans-
fer bandwidth is often higher for the outer tracks than the inner tracks.
For a disk read, once sectors have been transferred to the disk's buer
memory, they must be transferred to the host's memory over some con-
nection such as SATA (serial ATA), SAS (serial attached SCSi), Fibre
Channel, or USB (universal serial bus). For writes, the transfer goes in
the other direction. The time to transfer data between the host's memory
and the disk's buer is the host transfer time. Typical bandwidths range
Definition: host transfer
time
from 60MB/s for USB 2.0 to 2500 MB/s for Fibre Channel-20GFC.
For multi-sector reads, disks pipeline transfers between the surface and
disk buffer memory and between buffer memory and host memory; so for
large transfers, the total transfer time will be dominated by whichever of
these is the bottleneck. Similarly, for writes, disks overlap the host transfer
with the seek, rotation, and surface transfer; again, the total transfer time
will be dominated by whichever is the bottleneck.
Example: Toshiba MK3254GSY. Figure 12.3 shows some key parameters
for a recent 2.5-inch disk drive for laptop computers.
This disk stores 320 GB of data on 2 platters, so it stores 80 GB per surface.
The platters spin at 7200 revolutions per minute, which is 8.3 ms per revolution;
since each platter's diameter is about 6.3 cm, the outer edge of each platter is
moving at about 85 km/hour!
The disk's data sheet indicates an average seek time for the drive of 10.5 ms
for reads and 12.0 ms for writes. The seek time for reads and writes differs be-
