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Reader Question: DiskTester and OS X Cache

Reader Frank W writes:

Do DiskTester files land in a system (or other) cache?

MPG: DiskTester bypasses the OS X unified buffer cache so that all its tests are actually reading and writing to the drive. This is mandatory for any meaningful results.

The drive (HDD, SSD) or a controller card might have its own cache; this is all “the drive” and is necessarily part of the test.

The DiskTester fill-volume test is useful for characterizing drive performance, whatever the type of drive.

DiskTester fill-volume produces output that can be pasted into a (supplied) Excel graph template so that volume performance can be interpreted at a glance (“volume”, not “drive” — a single drive or multi-drive RAID or an SSD or whatever is tested as a file system volume).

  • For hard drives, the graph shows how performance declines across the volume (see The Fastest 2TB Volume is on a 4TB Drive and related articles). It also shows if there are any oddball irregularities, which could indicate a marginal drive.
  • For solid state drives (SSDs), the graph makes any performance breakdowns or irregularities visible.
  • For RAID or any other type of device, the same ideas apply.

For a Fusion drive, the graph below makes clear the abrupt loss of performance when the SSD fills up.

Write and read speed of 256GB 2nd batch (512GB total) on 480GB SSD + 2TB Fusion volume
Write and read speed of 256GB 2nd batch (512GB total) on 480GB SSD + 2TB Fusion volume
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