Date: Wed, 19 Jun 2002 02:39:41 -0700 (PDT) From: Matt Dillon <dillon@FreeBSD.org> To: cvs-committers@FreeBSD.org, cvs-all@FreeBSD.org Subject: cvs commit: src/sys/ufs/ufs ufs_readwrite.c Message-ID: <200206190939.g5J9dfc51512@freefall.freebsd.org>
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dillon 2002/06/19 02:39:41 PDT
Modified files:
sys/ufs/ufs ufs_readwrite.c
Log:
In rev 1.72 a situation related to write/mmap was fixed which could result
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
Revision Changes Path
1.90 +11 -7 src/sys/ufs/ufs/ufs_readwrite.c
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