Re: dummy file read periodically for external USB Hard Disk

[NeilBrown <neilb@suse.de>]

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On Wed, 9 Jul 2014 22:13:28 +0800 loody <miloody@gmail.com> wrote:

> hi NeilBrown:
> we use below c source but there is no read command firing from usb
> host to device.
> except O_DIRECT, is there any flag we need to use?
> appreciate all your kind help,
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <fcntl.h>
> 
> char message[] = "/mnt/usb/4854344154343452/
> test.txt";
> int main()
> {
>    int fd;
>    char buffer[5];
>    int count = 0;
>    char *buf="1234567890";
>    if((fd=open(message,O_CREAT|O_TRUNC|O_RDWR|O_DIRECT, 0777))<0)

I said "Open the device with O_DIRECT".  You are opening a file in the
filesystem which is mounted from the device.  That is a different thing.

>    {
>        perror("open");
>        return -1;
>    }
>    printf("fd=%d\n", fd);
>    write(fd, buf, strlen(buf));
>    while(1){
>        lseek(fd,0,SEEK_SET);
>        sleep(3);
>        count = read(fd, buffer, 3);

Did you do any research to understand how O_DIRECT works?
I recommend the man page for "open(2)".

You need to read thoroughly, but towards the end it says:

       Under Linux 2.4, transfer sizes, and the alignment of the user buffer and  the
       file  offset  must all be multiples of the logical block size of the file sys-
       tem.  Under Linux 2.6, alignment to 512-byte boundaries suffices.

Neither your buffer nor your IO size is 512-byte aligned.

NeilBrown


>        printf("count=%d,%x,%x,%x\n", count,buffer[0],buffer[1],buffer[2]);
>    }
> 
> }
> 
> 2014-07-08 11:01 GMT+08:00 NeilBrown <neilb@suse.de>:
> > On Mon, 7 Jul 2014 22:42:29 +0800 loody <miloody@gmail.com> wrote:
> >
> >> hi all:
> >>     we met a USB Hard Disk that will go to suspend if host stop
> >> sending scsi read command over 5mins.
> >>     To save the IO, kernel will keep the file in page cache as much as
> >> he can and under this circumstances, the read command may disappear
> >> for a while longer enough to cause the device suspend.
> >>
> >>     is there any kernel config or module parameter can do the dummy
> >> read periodically, even the sector 0 (MBR) is fine.
> >>     or is there any kernel api I can use to read sector 0(MBR) maybe
> >> every 4mins?
> >>
> >> appreciate your help in advance,
> >> --
> >> To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
> >> the body of a message to majordomo@vger.kernel.org
> >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >
> > Open the device with O_DIRECT and read a block every 4 minutes.
> > That should keep it awake.
> >
> > NeilBrown
> 
> 
> 


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