Board index » database » Linux and broken O_DIRECT

2004-10-29 03:56:49 PM
dist/configure.ac in BDB 4.2 contains the following comment:
# Linux has a broken O_DIRECT flag, but we allow people to override it from
# the command line.
In what way is Linux broken? Are there problems with metadata
updates, like on other systems? Is the brokenness specific to certain
kernel versions?
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Florian Weimer <fw@deneb.enyo.de>wrote in message news:<871xfiot32.fsf@deneb.enyo.de>...
+ Systems with O_DIRECT in their include files, but on which
the open system call will fail if O_DIRECT is specified,
+ Systems where the open calls will succeed when the O_DIRECT
flag is specified, but any subsequent read or write using
the file descriptor returned by the open call will fail,
+ Systems where O_DIRECT worked with some filesystems but not
with others,
+ Systems where buffers require specific alignment if they
are to be written to a file descriptor for which O_DIRECT
was specified; if a buffer isn't properly aligned the
read/write call will fail (rather than the system falling
back to a slower read/write).
problems to that level.
In the Berkeley DB 4.3 release, we default to not using the O_DIRECT
flag. You can always override the default and configure O_DIRECT
explicitly, however, using:
env db_cv_open_o_direct=yes ../dist/configure [args]
Regards,
--keith
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Keith Bostic bostic@sleepycat.com
Sleepycat Software Inc. keithbosticim (ymsgid)
118 Tower Rd. +1-781-259-3139
Lincoln, MA 01773 www.sleepycat.com
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* Keith Bostic:
We've seen a variety of behaviors, including:
[...]
+ Systems where buffers require specific alignment if they
are to be written to a file descriptor for which O_DIRECT
was specified; if a buffer isn't properly aligned the
read/write call will fail (rather than the system falling
back to a slower read/write).
Linux 2.6 requires that block boundaries in the user-space buffer do