The rest of the paper is organized as follows. Section
2 discusses current logical volume managers and
the need for Atropos. Section 3 describes the design an
implementation of Atropos. Section 4 describes how Atropos
is used by a database storage manager. Section 5
evaluates Atropos and its value for database storage management.
Section 6 discusses related work.
2 Background
This section overviews the design of current disk array
LVMs, which do not exploit the performance benefits of
disk-specific characteristics. It highlights the features of
the Atropos LVM, which addresses shortcomings of current
LVMs, and describes how Atropos supports efficient
access in both column- and row-major orders to applications
accessing two-dimensional data structures.
2.1 Conventional LVM design
Current disk array LVMs do not sufficiently exploit or
expose the unique performance characteristics of their
individual disk drives. Since an LVM sits below the
host’s storage interface, it could internally exploit diskspecific
features without the host being aware beyond
possibly improved performance. Instead, most use data
distribution schemes designed and configured independently
of the underlying devices. Many stripe data
across their disks, assigning fixed-sized sets of blocks
to their disks in a round-robin fashion; others use more
dynamic assignment schemes for their fixed-size units.
With a well-chosen unit size, disk striping can provide
effective load balancing of small I/Os and parallel transfers
for large I/Os [13, 16, 18].
A typical choice for the stripe unit size is 32–64 KB.
For example, EMC’s Symmetrix 8000 spreads and replicates
32 KB chunks across disks [10]. HP’s AutoRAID
[25] spreads 64 KB “relocation blocks” across
disks. These values conform to the conclusions of early
studies [4] of stripe unit size trade-offs, which showed
that a unit size roughly matching a single disk track (32–
64 KB at the times of these systems’ first implementations)
was a good rule-of-thumb. Interestingly, many
such systems seem not to track the growing track size
over time (200–350KB for 2002 disks), perhaps because
the values are hard-coded into the design. As a consequence,
medium- to large-sized requests to the array result
in suboptimal performance due to small inefficient
disk accesses.
2.2 Exploiting disk characteristics
Track-sized stripe units: Atropos matches the stripe
unit size to the exact track size of the disks in the volume.
In addition to conforming to the rule-of-thumb as
disk technology progresses, this choice allows applications
(and the array itself [11]) to utilize track-based accesses:
accesses aligned and sized for one track. Recent
research [22] has shown that doing so increases disk efficiency
by up to 50% for streaming applications that
share the disk system with other activity and for components
(e.g., log-structured file systems [17]) that utilize
medium-sized segments. In fact, track-based access provides
almost the same disk efficiency for such applications
as would sequential streaming.
The improvement results from two disk-level details.
First, firmware support for zero-latency access eliminates
rotational latency for full-track accesses; the data
of one track can be read in one revolution regardless of
the initial rotational offset after the seek. Second, no
head switch is involved in reading a single track. Combined,
these positioning delays represent over a third
of the total service time for non-aligned, approximately
track-sized accesses. Using small stripe unit sizes, as
do the array controllers mentioned above, increases the
proportion of time spent on these overheads.
Atropos uses automated extraction methods described
in previous work [21, 22] to match stripe units to disk
track boundaries. As detailed in Section 3.3, Atropos
also deals with multi-zoned disk geometries, whereby
tracks at different radial distances have different numbers
of sectors. that are not multiples of any useful block
size.
Efficient access to non-contiguous blocks: In addition
to exploiting disk-specific information to determine
its stripe unit size, Atropos exploits disk-specific information
to support efficient access to data across several
stripe units mapped to the same disk. This access
pattern, called semi-sequential, reads some data from
each of several tracks such that, after the initial seek,
no positioning delays other than track switches are incurred.
Such access is appropriate for two-dimensional
data structures, allowing efficient access in both rowand
column-major order.
In order to arrange data for efficient semi-sequential
access, Atropos must know the track switch time as well
as the track sizes. Carefully deciding how much data to
access on each track, before moving to the next, allows
Atropos to access data from several tracks in one full revolution
by taking advantage of the Shortest-Positioning-
Time-First (SPTF) [12, 23] request scheduler built into
disk firmware. Given the set of accesses to the different
tracks, the scheduler can arrange them to ensure
efficient execution by minimizing the total positioning
time. If the sum of the data transfer times and the track
switch times equals the time for one rotation, the scheduler
will service them in an order that largely eliminates
rotational latency (similar to the zero-latency feature for
single track access). The result is that semi-sequential
accesses are much more efficient than a like number of
random or unorchestrated accesses.
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