using O_DIRECT

using O_DIRECT

[Bob McElrath]

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I have written a small program to use O_DIRECT (attached), after
applying your patch o_direct-14 to kernel 2.4.9.  Opening the file with
O_DIRECT is successful, but attempts to write to the fd return EINVAL.

Am I doing something wrong?  Should I have to recompile glibc too?

(0)<root@draal:/usr/src/mjpeg_play> gcc -o testopen testopen.c
(0)<root@draal:/usr/src/mjpeg_play> strace ./testopen > /dev/null
execve("./testopen", ["./testopen"], [/* 54 vars */]) = 0
uname({sys="Linux", node="draal.physics.wisc.edu", ...}) = 0
brk(0)                                  = 0x120010d90
open("/etc/ld.so.preload", O_RDONLY)    = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY)      = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=62527, ...}) = 0
mmap(NULL, 62527, PROT_READ, MAP_PRIVATE, 3, 0) = 0x20000030000
close(3)                                = 0
open("/lib/libc.so.6.1", O_RDONLY)      = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0&\220\1\0\0\0`\201\3"..., 1024) = 1024
fstat(3, {st_mode=S_IFREG|0755, st_size=9653762, ...}) = 0
mmap(NULL, 1558648, PROT_READ|PROT_EXEC, MAP_PRIVATE, 3, 0) = 0x20000040000
mprotect(0x2000019a000, 141432, PROT_NONE) = 0
mmap(0x200001a0000, 98304, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_FIXED, 3, 0x150000) = 0x200001a0000
mmap(0x200001b8000, 18552, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x200001b8000
close(3)                                = 0
munmap(0x20000030000, 62527)            = 0
getxpid()                               = 2387
open("testopen.txt", O_RDWR|O_CREAT|O_SYNC|0x100000, 0644) = 3
fstat(1, {st_mode=S_IFCHR|0666, st_rdev=makedev(1, 3), ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x2000001e000
ioctl(1, TCGETS, 0x11fffea50)           = -1 ENOTTY (Inappropriate ioctl for device)
write(3, "Welcome to the winter of my disc"..., 40) = -1 EINVAL (Invalid argument)
close(3)                                = 0
write(1, "Got fd 3.\nUnable to write to fil"..., 77) = 77
munmap(0x2000001e000, 8192)             = 0
exit(0)                                 = ?

Cheers,
-- Bob

Bob McElrath (rsmcelrath@students.wisc.edu) 
Univ. of Wisconsin at Madison, Department of Physics

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/* Test opening of a file with O_DIRECT */

#include <asm/types.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <linux/types.h>
#include <asm/fcntl.h>  /* Note: <fcntl.h> DOES NOT HAVE O_DIRECT! */
                        /* <fcntl.h> is a glibc header... */
#include <errno.h>
#include <stdio.h>

int main(void) {
    char message[] = "Welcome to the winter of my discontent.\n";
    int fd = open("testopen.txt", O_RDWR|O_CREAT|O_SYNC|O_DIRECT, S_IRUSR|S_IRGRP|S_IROTH|S_IWUSR);
    int len;
    if(fd < 0) {
        printf("Unable to open file, errno is %d.\n", errno);
    } else {
        printf("Got fd %d.\n", fd);
        if((len = write(fd, message, strlen(message))) < 0) {
            printf("Unable to write to file, len is %d, errno is %d.\n", len, errno);
        } else {
            printf("%d bytes written to file.\n", len);
        }
    }
    close(fd);
    printf("End of program...\n");
    return 0;
}

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Re: using O_DIRECT

[Andrea Arcangeli]

On Mon, Sep 03, 2001 at 03:13:42PM -0500, Bob McElrath wrote:
> I have written a small program to use O_DIRECT (attached), after
> applying your patch o_direct-14 to kernel 2.4.9.  Opening the file with
> O_DIRECT is successful, but attempts to write to the fd return EINVAL.
> 
> Am I doing something wrong?  Should I have to recompile glibc too?

eh, the alignment and size of the buffer have basically the same
restrictions of running read/write on a raw device, with the only
difference that for rawio the granularity is the hardblocksize, while
for O_DIRECT the granularity is the softblocksize of the filesystem.

We'll have to relax the granularity of the I/O down to the hardblocksize
for O_DIRECT too eventually.

Andrea

Re: using O_DIRECT

[Andrea Arcangeli]

On Tue, Sep 04, 2001 at 03:32:43AM -0500, Bob McElrath wrote:
> I don't really care about the granularity.  The bigger the better,
> actually, since I'm streaming data to disk quickly.  ;)

Yes ;).

Andrea

Re: using O_DIRECT

[Bob McElrath]

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Andrea Arcangeli [andrea@suse.de] wrote:
> On Mon, Sep 03, 2001 at 03:13:42PM -0500, Bob McElrath wrote:
> > I have written a small program to use O_DIRECT (attached), after
> > applying your patch o_direct-14 to kernel 2.4.9.  Opening the file with
> > O_DIRECT is successful, but attempts to write to the fd return EINVAL.
> > 
> > Am I doing something wrong?  Should I have to recompile glibc too?
> 
> eh, the alignment and size of the buffer have basically the same
> restrictions of running read/write on a raw device, with the only
> difference that for rawio the granularity is the hardblocksize, while
> for O_DIRECT the granularity is the softblocksize of the filesystem.

Ah, thanks, it's working now.  Makes sense that it should use blocksize
granularity...

> We'll have to relax the granularity of the I/O down to the hardblocksize
> for O_DIRECT too eventually.

I don't really care about the granularity.  The bigger the better,
actually, since I'm streaming data to disk quickly.  ;)

Thanks,
-- Bob

Bob McElrath (rsmcelrath@students.wisc.edu) 
Univ. of Wisconsin at Madison, Department of Physics

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