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in a user process gaining visibility into the 'old' contents of a filesystem block. There were two cases: (1) when uiomove() fails (user process issues illegal write), and (2) when uiomove() overlaps a mmap() of the same file at the same offset (fault -> recursive buffer I/O reads contents of old block). Unfortunately 1.72 also had the unintended effect of forcing the filesystem to do a read-before-write in the case of a full-block-write (non append case), e.g. 'dd if=/dev/zero of=test.dat bs=1m count=256 conv=notrunc'. This destroys performance.. not only is a read forced for every write, but clustering breaks as well. The solution is to clear the buffer manually in the full-block case rather then asking BALLOC to do it (BALLOC issues the read-before-write). In the partial-block case we want BALLOC to do it because the read-before-write is necessary. This patch should greatly improve database and news-feed server performance. Found by: MKI <mki@mozone.net> MFC after: 3 days
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