ZVOLs - Write Cache

When you create a ZFS volume its write cache is disabled by default meaning that all writes to the volume will be synchronous. Sometimes it might be handy though to be able to enable a write cache for a particular zvol. I wrote a small C program which allows you to check if WC is enabled or not. It also allows you to enable or disable write cache for a specified zvol.

First lets check if write cache is disabled for a zvol rpool/iscsi/vol1

milek@r600:~/progs# ./zvol_wce /dev/zvol/rdsk/rpool/iscsi/vol1
Write Cache: disabled

Now lets issue 1000 writes

milek@r600:~/progs# ptime ./sync_file_create_loop /dev/zvol/rdsk/rpool/iscsi/vol1 1000

real       12.013566363
user        0.003144874
sys         0.104826470

So it took 12s and I also confirmed that writes were actually being issued to a disk drive. Lets enable write cache now and repeat 1000 writes

milek@r600:~/progs# ./zvol_wce /dev/zvol/rdsk/rpool/iscsi/vol1 1
milek@r600:~/progs# ./zvol_wce /dev/zvol/rdsk/rpool/iscsi/vol1
Write Cache: enabled

milek@r600:~/progs# ptime ./sync_file_create_loop /dev/zvol/rdsk/rpool/iscsi/vol1 1000

real        0.239360231
user        0.000949655
sys         0.019019552

Worked fine.

The zvol_wce program is not idiot-proof and it doesn't check if operation succeeded or not. You should be able to compile it by issuing: gcc -o zvol_wce zwol_wce.c

milek@r600:~/progs# cat zvol_wce.c

/* Robert Milkowski
  http://milek.blogspot.com
*/

#include <unistd.h>
#include <stropts.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stropts.h>
#include <sys/dkio.h>


int main(int argc, char **argv)
{
 char *path;
 int wce = 0;
 int rc;
 int fd;

 path = argv[1];

 if ((fd = open(path, O_RDONLY|O_LARGEFILE)) == -1)
   exit(2);

 if (argc>2) {
   wce = atoi(argv[2]) ? 1 : 0;
   rc = ioctl(fd, DKIOCSETWCE, &wce);
 }
 else {
   rc = ioctl(fd, DKIOCGETWCE, &wce);
   printf("Write Cache: %s\n", wce ? "enabled" : "disabled");
 }

 close(fd);
 exit(0);
}