Date: Thu, 23 Sep 2010 19:38:00 +0200 From: Michael Naef <cal@linu.gs> To: "freebsd-fs" <freebsd-fs@freebsd.org> Subject: Strange behaviour with sappend flag set on ZFS Message-ID: <201009231938.09548.cal@linu.gs>
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Hi fs people
Background: When moving from pre-8.0 to FreeBSD 8.1 on ZFS I
realised that the bash history is no longer beeing written if the
file has the sappend flag set. Other programs like script (1)
however are surprisingly still able to write to such files. So it
seemed that they handle file IO differently.
(Looking at the source code of both I learned that bash uses write
(2) while script uses fwrite (3)... After some more testing and
investigation,) it seems that zfs behaves differently to ufs in one
point.
If I open (2) a file with O_APPEND the fd position pointer is set
to the start of the file. When I then write to an fd on a ufs FS in
FB6.4, it is automatically moved to the end and the bytes are
appended. No problems appear with write to the sappend-flages file.
However if I do the same on FB8.1/ZFS, the write fails as "not
permitted". When I manually move the position pointer to the end of
the file, it works as axpected:
FreeBSD 6.4/UFS:
root@fb64 [~/michi]# touch gaga
root@fb64 [~/michi]# chflags sappend gaga
root@fb64 [~/michi]# ./write
fd: 3
pos: 0
bytes written: 6
pos: 50
bytes written: 6
FreeBSD 8.1/ZFS:
root@server52 [~/michi]# chflags sappend gaga
root@fb81 [~/michi]# touch gaga
root@fb81 [~/michi]# chflags sappend gaga
root@fb81 [~/michi]# ./write
fd: 3
pos: 0
ERROR: Operation not permitted
pos: 147
bytes written: 6
The source code is:
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
int main(void){
int file= open("gaga",O_WRONLY|O_APPEND|O_CREAT,0666);
int bytes_written= 0;
printf("fd: %i\n",file);
printf("pos: %i\n",lseek(file,0,SEEK_CUR));
if ((bytes_written= write(file,"write\n",6)) == -1){
printf("ERROR: %s\n",strerror(errno));
}else{
printf("bytes written: %i\n",bytes_written);
}
lseek(file,0,SEEK_END);
printf("pos: %i\n",lseek(file,0,SEEK_CUR));
if ((bytes_written= write(file,"write\n",6)) == -1){
printf("ERROR: %s\n",strerror(errno));
}else{
printf("bytes written: %i\n",bytes_written);
}
close(file);
}
Am I missing something or is this a bug (or feature or watsoever
;-) in ZFS?
thanks a lot you (z)fs guys and cheers, Michi
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