Messages in this thread | | | Date | Tue, 14 Sep 2010 17:36:43 +1000 | From | Dave Chinner <> | Subject | Re: O_DIRECT reads appear to be cached on block device partition file? |
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On Mon, Sep 13, 2010 at 11:49:32PM -0400, Brett Russ wrote: > Running a 2.6.31 kernel on a blade chassis system with multiple > blades sharing common JBOD storage. The application intelligently > divides the drives up among the blades, but one blade in particular > is charged with monitoring. As part of this, this one monitoring > blade can perform reads of a certain 512B sector of all disks in the > system. This sector is often written by other blades, these writes > are sync'd to disk. To work around the lack of cache coherency > between the distinct blades, I'm using O_DIRECT on the monitoring > blade such that it always reads from the media to get the latest > copy of this sector. The basic steps are: > > # grab a 512B aligned buffer (use 4KB to be safe) > posix_memalign(&ptr, getpagesize(), 512B) > open(/dev/sdX3, O_RDONLY|O_DIRECT) > lseek(fd, offset, SEEK_SET) > read(fd, ptr, 512B) > > If I run the above on the monitoring blade, then sync an update to > the sector in question from another blade, then re-reun the above > code on the monitoring blade, believe it or not I appear to be > reading stale data. If I use dd with iflag=direct, reading the same > sector offset at the /dev/sdX3 partition file, I see the same stale > data as seen from the code above. If, however, I instead access > this sector offset from the /dev/sdX device file using the (offset > of partition 3 + offset of the sector) I see the intended data, > which makes me believe some caching occurred locally for /dev/sdX3.
What does blktrace tell you?
Cheers,
Dave. -- Dave Chinner david@fromorbit.com
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