HV_VSS(4)
💖 NAME 💖
hv_vss — Hyper-V Volume Shadow Copy Service API. okay so this is the hv_vss(4) page from the Kernel Interfaces Manual, bestie, and we're doing part 1 of 2. It's giving kernel-side VSS broker energy, and honestly it's iconic.
✨ SYNOPSIS ✨
#include <dev/hyperv/hv_snapshot.h>
#define VSS_SUCCESS 0x00000000
#define VSS_FAIL 0x00000001
enum hv_vss_op_t {
HV_VSS_NONE = 0,
HV_VSS_CHECK,
HV_VSS_FREEZE,
HV_VSS_THAW,
HV_VSS_COUNT
};
struct hv_vss_opt_msg {
uint32_t opt; /* operation */
uint32_t status; /* 0 for success, 1 for error */
uint64_t msgid; /* an ID used to identify the transaction */
uint8_t reserved[48]; /* reserved values are all zeroes */
};💅 DESCRIPTION 💅
okay so the freeze or thaw functionality of an application is important — like, it's the whole reason you can guarantee an application consistent backup. On the Windows platform, VSS is defined to do live backup. But for a VM guest running on Hyper-V, the corresponding VSS is not defined yet. Example time: a running database server instance knows when the application's freeze/thaw should start or finish, but it is not aware of the freeze/thaw notification from the Hyper-V host. That's the gap. hv_vss is designed to notify the application about freeze/thaw requests. So it plays the role of a broker — it forwards the freeze/thaw command from the Hyper-V host to a userland application if that app registered a VSS service on the FreeBSD VM, and then it sends the result back to the Hyper-V host. icon behavior, no cap.
Generally, hv_vss_daemon(8) takes the responsibility to freeze/thaw the UFS file system, and it is automatically launched after the system boots. When the Hyper-V host wants to take a snapshot of the FreeBSD VM, it will first send a VSS capability check to the FreeBSD VM. hv_vss receives that request and forwards it to the userland application if one is registered. Only after hv_vss received the VSS_SUCCESS response from the application will hv_vss_daemon(8) be informed to check whether file system freeze/thaw is supported. If any error occurs during this period, hv_vss will inform the Hyper-V host that VSS is not supported. Also, there is a default timeout limit before sending a response to the Hyper-V host. If the total response time from the application and hv_vss_daemon(8) exceeds this value, timeout will occur and VSS unsupported is responded to the Hyper-V host. rude but fair.
After the Hyper-V host confirmed the FreeBSD VM supports VSS, it will send a freeze request to the VM, and hv_vss will first forward it to the application. After the application finished freezing, it should inform hv_vss, and then file system level freezing will be triggered by hv_vss_daemon(8). After all freezing on both the application and hv_vss_daemon(8) were finished, hv_vss will inform the Hyper-V host that freezing is done. Of course, there is a timeout limit as same as VSS capability is set to make sure freezing on the FreeBSD VM is not hang. If there is any error occurs or timeout happened, the freezing is failed on the Hyper-V side. 💀
The Hyper-V host will send a thaw request after taking the snapshot — typically, this period is very short in order not to block the running application. hv_vss firstly thaws the file system by notifying hv_vss_daemon(8), then notifies the user registered application. There is also a timeout check before sending a response to the Hyper-V host.
All the default timeout limit used in VSS capability check, freeze or thaw is the same. It is 15 seconds currently. yes bestie, one shared clock, one shared vibe. ⏳
🫶 NOTES 🫶
hv_vss only supports UFS currently. If any of the file system partitions is non UFS, the VSS capability check will fail. If the application does not register VSS, hv_vss only supports backup for file system level consistent. no app-level freeze fantasy in that case, just filesystem consistency.
🚨 The device should be closed before it was opened again. If you want to simultaneously open"/dev/hv_appvss_dev"two or more times, an error (-1) will be returned, anderrnowas set. so don't try to double-book the device like it's a group chat, okay? 🙈
If hv_vss_daemon(8) was killed after the system boots, the VSS functionality will not work. like, at all. 💀
✨ EXAMPLES ✨
okay bestie, here’s the full example — and it literally does nothing except wait 2 seconds when it gets those notifications from hv_vss 💖✨
#include <string.h>
#include <stdio.h>
#include <sys/ioctl.h>
#include <sys/param.h>
#include <sys/ucred.h>
#include <sys/mount.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <poll.h>
#include <stdint.h>
#include <syslog.h>
#include <errno.h>
#include <err.h>
#include <fcntl.h>
#include <ufs/ffs/fs.h>
#include <paths.h>
#include <sys/ioccom.h>
#include <dev/hyperv/hv_snapshot.h>
#define UNDEF_FREEZE_THAW (0)
#define FREEZE (1)
#define THAW (2)
#define CHECK (3)
#define VSS_LOG(priority, format, args...) do { \
if (is_debugging == 1) { \
if (is_daemon == 1) \
syslog(priority, format, ## args); \
else \
printf(format, ## args); \
} else { \
if (priority < LOG_DEBUG) { \
if (is_daemon == 1) \
syslog(priority, format, ## args); \
else \
printf(format, ## args); \
} \
} \
} while(0)
#define CHECK_TIMEOUT 1
#define CHECK_FAIL 2
#define FREEZE_TIMEOUT 1
#define FREEZE_FAIL 2
#define THAW_TIMEOUT 1
#define THAW_FAIL 2
static int is_daemon = 1;
static int is_debugging = 0;
static int simu_opt_waiting = 2; // seconds
#define GENERIC_OPT(TIMEOUT, FAIL) \
do { \
sleep(simu_opt_waiting); \
if (opt == CHECK_TIMEOUT) { \
sleep(simu_opt_waiting * 10); \
VSS_LOG(LOG_INFO, "%s timeout simulation\n", \
__func__); \
return (0); \
} else if (opt == CHECK_FAIL) { \
VSS_LOG(LOG_INFO, "%s failure simulation\n", \
__func__); \
return (CHECK_FAIL); \
} else { \
VSS_LOG(LOG_INFO, "%s success simulation\n", \
__func__); \
return (0); \
} \
} while (0)
static int
check(int opt)
{
GENERIC_OPT(CHECK_TIMEOUT, CHECK_FAIL);
}
static int
freeze(int opt)
{
GENERIC_OPT(FREEZE_TIMEOUT, FREEZE_FAIL);
}
static int
thaw(int opt)
{
GENERIC_OPT(THAW_TIMEOUT, THAW_FAIL);
}
static void usage(const char* cmd) {
fprintf(stderr,
"%s -f <0|1|2>: simulate app freeze."
" 0: successful, 1: freeze timeout, 2: freeze failed\n"
" -c <0|1|2>: simulate vss feature check"
" -t <0|1|2>: simulate app thaw."
" 0: successful, 1: freeze timeout, 2: freeze failed\n"
" -d : enable debug mode\n"
" -n : run this tool under non-daemon mode\n", cmd);
}
int
main(int argc, char* argv[]) {
int ch, freezesimuop = 0, thawsimuop = 0, checksimuop = 0, fd, r, error;
uint32_t op;
struct pollfd app_vss_fd[1];
struct hv_vss_opt_msg userdata;
while ((ch = getopt(argc, argv, "f:c:t:dnh")) != -1) {
switch (ch) {
case 'f':
/* Run as regular process for debugging purpose. */
freezesimuop = (int)strtol(optarg, NULL, 10);
break;
case 't':
thawsimuop = (int)strtol(optarg, NULL, 10);
break;
case 'c':
checksimuop = (int)strtol(optarg, NULL, 10);
break;
case 'd':
is_debugging = 1;
break;
case 'n':
is_daemon = 0;
break;
case 'h':
default:
usage(argv[0]);
exit(0);
}
}
openlog("APPVSS", 0, LOG_USER);
/* Become daemon first. */
if (is_daemon == 1)
daemon(1, 0);
else
VSS_LOG(LOG_DEBUG, "Run as regular process.\n");
VSS_LOG(LOG_INFO, "HV_VSS starting; pid is: %d\n", getpid());
fd = open(VSS_DEV(APP_VSS_DEV_NAME), O_RDWR);
if (fd < 0) {
VSS_LOG(LOG_ERR, "Fail to open %s, error: %d %s\n",
VSS_DEV(APP_VSS_DEV_NAME), errno, strerror(errno));
exit(EXIT_FAILURE);
}
app_vss_fd[0].fd = fd;
app_vss_fd[0].events = POLLIN | POLLRDNORM;
while (1) {
r = poll(app_vss_fd, 1, INFTIM);
VSS_LOG(LOG_DEBUG, "poll returned r = %d, revent = 0x%x\n",
r, app_vss_fd[0].revents);
if (r == 0 || (r < 0 && errno == EAGAIN) ||
(r < 0 && errno == EINTR)) {
/* Nothing to read */
continue;
}
if (r < 0) {
/*
* For poll return failure other than EAGAIN,
* we want to exit.
*/
VSS_LOG(LOG_ERR, "Poll failed.\n");
perror("poll");
exit(EIO);
}
/* Read from character device */
error = ioctl(fd, IOCHVVSSREAD, &userdata);
if (error < 0) {
VSS_LOG(LOG_ERR, "Read failed.\n");
perror("pread");
exit(EIO);
}
if (userdata.status != 0) {
VSS_LOG(LOG_ERR, "data read error\n");
continue;
}
op = userdata.opt;
switch (op) {
case HV_VSS_CHECK:
error = check(checksimuop);
break;
case HV_VSS_FREEZE:
error = freeze(freezesimuop);
break;
case HV_VSS_THAW:
error = thaw(thawsimuop);
break;
default:
VSS_LOG(LOG_ERR, "Illegal operation: %d\n", op);
error = VSS_FAIL;
}
if (error)
userdata.status = VSS_FAIL;
else
userdata.status = VSS_SUCCESS;
error = ioctl(fd, IOCHVVSSWRITE, &userdata);
if (error != 0) {
VSS_LOG(LOG_ERR, "Fail to write to device\n");
exit(EXIT_FAILURE);
} else {
VSS_LOG(LOG_INFO, "Send response %d for %s to kernel\n",
userdata.status, op == HV_VSS_FREEZE ? "Freeze" :
(op == HV_VSS_THAW ? "Thaw" : "Check"));
}
}
return 0;
}👀 SEE ALSO
bestie, go peek at these next:
🕰️ HISTORY
The daemon was introduced in October 2016 and developed by Microsoft Corp. 👑
💖 AUTHORS
FreeBSD support for hv_vss was first added by Microsoft BSD Integration Services Team <bsdic@microsoft.com>. 🫶
okay bye bestie — this page is from Debian, dated October 12, 2016, and it’s HV_VSS(4) 💅✨
📦 where this page comes from
- freebsd 15.1: base 15.1
- nixpkgs unstable: source
- Copyright (c) 2016 Microsoft Corp
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on : a language model's rewrite of the original, under its terms