Flushing buffer and page cache

Flushing buffer and page cache

[Florian Weimer]

Is it possible to flush all entries in the buffer cache corresponding
to a single block device (i.e. simply drop them if they aren't dirty,
or write them to disk and drop them after this if they are dirty)?

I've got another device in my SCSI chain which writes to the disk, and
if the caches are not flushed, the computer won't see the updates.
(Synchronization is done manually, so it's not an issue---trust me, I
know what I'm doing. ;-)

Kernel version doesn't matter. ;-)

-- 
Florian Weimer 	                  Florian.Weimer@RUS.Uni-Stuttgart.DE
University of Stuttgart           http://cert.uni-stuttgart.de/
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Re: Flushing buffer and page cache

[James Bottomley]

> Is it possible to flush all entries in the buffer cache corresponding
> to a single block device (i.e. simply drop them if they aren't dirty,
> or write them to disk and drop them after this if they are dirty)?

Yes, just send the BLKFLSBUF ioctl to the device this syncs the device then 
removes all the buffers from the cache.  We use it as a tool to move a SAN 
device around a cluster, which is similar to what you want to do.

James Bottomley

Re: Flushing buffer and page cache

[Douglas Gilbert]

James Bottomley wrote:

> > Is it possible to flush all entries in the buffer cache corresponding
> > to a single block device (i.e. simply drop them if they aren't dirty,
> > or write them to disk and drop them after this if they are dirty)?
> 
> Yes, just send the BLKFLSBUF ioctl to the device this syncs the device then 
> removes all the buffers from the cache.  We use it as a tool to move a SAN 
> device around a cluster, which is similar to what you want to do.

Last time this question was raised, someone mentioned
a little utility called flushb . Here is its source:

/*
 * flushb.c --- This routine flushes the disk buffers for a disk
 */

/*
 * modified August 2000 by Juri Haberland
 * juri.haberland@innominate.de
 */

#include <stdio.h>
#include <fcntl.h>
#include <linux/fs.h>

#define NOARGS void

const char *progname;

static void usage(NOARGS)
{
        fprintf(stderr, "Usage: %s disk\n", progname);
        exit(1);
}      

int main(int argc, char **argv)
{
        int     fd;

        progname = argv[0];
        if (argc != 2)
                usage();

        fd = open(argv[1], O_RDONLY, 0);
        if (fd < 0) {
                perror("open");
                exit(1);
        }
        /*
         * Note: to reread the partition table, use the ioctl
         * BLKRRPART instead of BLKFSLBUF.
         */
        if (ioctl(fd, BLKFLSBUF, 0) < 0) {
                perror("ioctl BLKFLSBUF");
                exit(1);
        }
        return 0;
}