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l : 9 April 2008 • 4:16AM -0400

Write-Intent Bitmaps and disk caches
by Ty! Boyack


I'm a little confused on the write-intent bitmaps and how they interact
with the disk caches - I would appreciate any clarification here.

The way I understand it, if a device fails in a raid set, the bitmap
will track regions that have changed since the failed device left the
array.  If the device is added back in, the re-sync time is much shorter
since only some blocks have to be re-sync'ed.

But in this case it's possible that blocks A, B, and C were written to
the device, and the failure was detected on block C.  Thus blocks A and
B would be probably held in the device's cache (hard drive cache, or
this problem gets worse if working with disk arrays as devices, since
they have much larger caches).  When the device was re-added, block C
would presumably be re-synced as requested by the bitmap, but would A
and B be lost forever because they fell out of the cache on the drive?

Does the bitmap, or something else, take this into account?  Or will
this eventually lead to an inconsistent read or data corruption down the
road?  Or am I just out waving my paranoid flag today?

  Ty! Boyack
  NREL Unix Network Manager
  (970) 491-1186

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