Matching Application Access Patterns to Storage Device ...

14 · Matching Application Access Patterns to Storage Device Characteristics
configurable parameters that must be manually tuned to a specific device [IBM
Corporation 2000; McKusick et al. 1984; Shriver et al. 1998].
2.3.1 Disk-specific knowledge
Much recent related work has promoted zone-based allocation and detailed diskspecific
request generation for small requests. The Tiger video server [Bolosky
et al. 1996] allocated primary copies of videos to the outer portions of each disk’s
LBN space in order to exploit the higher bandwidth of the outer zones. Secondary
copies were allocated to the lower bandwidth zones. VanMeter [1997] suggested
that there was general benefit in changing file systems to understand that different
regions of the disk provide different bandwidths.
By utilizing even more detailed disk information, several researchers have
shown substantial decreases in small request response times [Chao et al. 1992; English
and Stepanov 1992; Huang and cker Chiueh 1999; Wang et al. 1999; Yu et al.
2000]. For small writes, these systems detect the position of the head and re-map
data to the nearest free block in order to minimize the positioning costs [Huang
and cker Chiueh 1999; Wang et al. 1999]. For small reads, the SR-Array [Yu et al.
2000] determines the head position when the read request is to be serviced and
reads the closest of several replicas.
The Gamma database system [DeWitt et al. 1990] accessed data in tracksized
I/Os by simply knowing the number of sectors per track. Unfortunately,
simple mechanisms like this are no longer possible because of high-level device
interfaces and built-in firmware functions. For example, zoned geometries and
advanced defect management in current disks result in cylinders being composed
of tracks with variable number of sectors (see Table 3.1). No single value for the
number of sectors per track is correct across the device.
Using disk drive track size to set the RAID stripe unit size improves I/O performance.
Chen and Patterson [1990] developed a method for determining proper
stripe unit for a disk array and concluded that a stripe unit near track size is
optimal. Similar to the database case, however, a single value is not sufficient for
modern multi-zoned disks; it does not realize the full potential of modern disk
drives. Using values that exactly match track size, on the other hand, can do so as
illustrated in Section 5.7.6. At the file system level, aligning access to stripe unit
boundaries and writing full stripes avoids expensive read-modify-write operations
for RAID 4 and RAID 5 configurations [Chen et al. 1994; Hitz et al. 1994].