Hi Erik, My colleague said that when use direct I/O on USB disk that has a ext3 partition, open can succeed, but read fails. I know that filesystem must have some system calls for direct I/O, and open with O_DIRECT flag will check if these system calls exist. But why does read fail? Is it that direst I/O cannot use on USB disk? Regards, Colin ----- Original Message ----- From: "Erik Mouw" <mouw@xxxxxxxxxxxx> To: "colin" <colin@xxxxxxxxxxxxxx> Cc: <kernelnewbies@xxxxxxxxxxxx> Sent: Thursday, December 21, 2006 9:37 PM Subject: Re: How to know the file size limit, and know if O_DIRECT is supported > On Thu, Dec 21, 2006 at 04:40:03PM +0800, colin wrote: > > How do I know the file size limit on a unknown filesystem? > > Our application can use CIFS and record data to the remote site. > > We would like to know the file size limit before we start to record, but the > > "fstat" doesn't tell us about this. > > AFAIK there is no system call to figure out though the information is > in the (in-kernel) superblock. One way is to create a file and use a > binary search with ftruncate() to figure out the largest possible size. > That will only work fast on filesystems that support sparse files. > Another way is to use a safe limit (i.e.: 1GB is usually safe). > > > And another question is how do I know if Direct I/O is usable on some > > filesystem and storage? > > open() a file with O_DIRECT. If it fails, direct IO is not possible. > > > Erik > > -- > They're all fools. Don't worry. Darwin may be slow, but he'll > eventually get them. -- Matthew Lammers in alt.sysadmin.recovery -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
