最近发现环境中 KVM 虚拟机磁盘利用率查不准，使用 virsh 命令查看磁盘使用情况得到如下结果：
# virsh domblkinfo 20 vda --human
Capacity: 2.000 GiB
Allocation: 2.000 GiB
Physical: 2.000 GiB显然是有问题的，正常的数值三个应该不通，进入系统查看磁盘使用率也仅有 2% 左右，因此试图通过检查源码的方式查看是否正确。
- libvirt: 5.6.0
- os: Centos
跟踪记录
首先找到 libvirtd 的 PID：
ps -aux | grep libvirtd
root 1907 0.0 0.0 1385796 25116 ? Ssl Aug26 5:22 /usr/sbin/libvirtd --timeout 120使用 GDB 开始跟踪他：
gdb libvirtd 1907首先在源码中全局搜索 domblkinfo 关键字，找到该命令的执行函数： tools/virsh-domain-monitor.c→cmdDomblkinfo 。
分析源码找到获取信息的函数 src/libvirt-domain.c -> virDomainGetBlockInfo ：
if (virDomainGetBlockInfo(dom, device, &info, 0) < 0)
goto cleanup;
if (!cmdDomblkinfoGet(ctl, &info, &cap, &alloc, &phy, human))
goto cleanup;
vshPrint(ctl, "%-15s %s\n", _("Capacity:"), cap);
vshPrint(ctl, "%-15s %s\n", _("Allocation:"), alloc);
vshPrint(ctl, "%-15s %s\n", _("Physical:"), phy);这其中的 info 包含了所需信息，看一下填充该字段的 virDomainGetBlockInfo 函数实现，用 GDB 跟一下它吧.
跟踪 src/libvirt-domain.c -> virDomainGetBlockInfo
先打个断点：
(gdb) break virDomainGetBlockInfo
Breakpoint 1 at 0x7f4d4394a760: file libvirt-domain.c, line 6094.再打开一个终端，执行一下命令：
[root@compute-01 ~]# virsh list
Id Name State
-----------------------------------
2 instance-000001b6 running
3 instance-000001b8 running
4 instance-000001b9 running
[root@compute-01 ~]# virsh domblkinfo 4 vda此时会发现终端卡住了，看一下 GDB 已经将程序中断，单步调试看一下：
[Switching to Thread 0x7f4d32ef0700 (LWP 1918)]
Breakpoint 1, virDomainGetBlockInfo (domain=domain@entry=0x7f4cfc00aeb0, disk=0x7f4cfc00cc60 "vda",
info=info@entry=0x7f4d32eefac0, flags=0) at libvirt-domain.c:6094
6094 {
(gdb) n
6097 VIR_DOMAIN_DEBUG(domain, "info=%p, flags=0x%x", info, flags);
(gdb) n
6094 {
(gdb) n
6097 VIR_DOMAIN_DEBUG(domain, "info=%p, flags=0x%x", info, flags);
(gdb) n
6099 virResetLastError();
(gdb) n
6101 if (info)
(gdb) n
6102 memset(info, 0, sizeof(*info));
(gdb) n
6104 virCheckDomainReturn(domain, -1);
(gdb) n
6105 virCheckNonEmptyStringArgGoto(disk, error);
(gdb) n
6106 virCheckNonNullArgGoto(info, error);
(gdb) n
6110 if (conn->driver->domainGetBlockInfo) {
(gdb) n
6112 ret = conn->driver->domainGetBlockInfo(domain, disk, info, flags);
(gdb) s
qemuDomainGetBlockInfo (dom=0x7f4cfc00aeb0, path=0x7f4cfc00cc60 "vda", info=0x7f4d32eefac0, flags=0)
at qemu/qemu_driver.c:12413
12413 {发现在 6112 行跳到了另一个函数，继续跟踪它.
跟踪 src/qemu/qemu_driver.c -> qemuDomainGetBlockInfo
(gdb) n
12421 virCheckFlags(0, -1);
(gdb) n
12413 {
(gdb) n
12414 virQEMUDriverPtr driver = dom->conn->privateData;
(gdb) n
12421 virCheckFlags(0, -1);
(gdb) n
12419 qemuBlockStatsPtr entry = NULL;
(gdb) n
12414 virQEMUDriverPtr driver = dom->conn->privateData;
(gdb) n
12421 virCheckFlags(0, -1);
(gdb) n
12423 if (!(vm = qemuDomObjFromDomain(dom)))
(gdb) n
12426 cfg = virQEMUDriverGetConfig(driver);
(gdb) n
12428 if (virDomainGetBlockInfoEnsureACL(dom->conn, vm->def) < 0)
(gdb) n
12431 if (qemuDomainObjBeginJob(driver, vm, QEMU_JOB_QUERY) < 0)
(gdb) n
12434 if (!(disk = virDomainDiskByName(vm->def, path, false))) {
(gdb) n
12440 if (virStorageSourceIsEmpty(disk->src)) {
(gdb) n
12448 if (!virDomainObjIsActive(vm)) {
(gdb) n
12460 if (qemuDomainBlocksStatsGather(driver, vm, path, true, &entry) < 0)
(gdb) n
12463 if (!entry->wr_highest_offset_valid) {
(gdb) n
12466 if (virStorageSourceGetActualType(disk->src) == VIR_STORAGE_TYPE_FILE &&
(gdb) n
12468 info->allocation = entry->physical;
(gdb) n
12466 if (virStorageSourceGetActualType(disk->src) == VIR_STORAGE_TYPE_FILE &&
(gdb) p info->allocation
$2 = 0
(gdb) n
12470 info->allocation = entry->wr_highest_offset;
(gdb) n
12484 if (entry->physical == 0 || info->allocation == 0 ||
(gdb) p info->allocation
$3 = 32870912
(gdb) p entry->wr_highest_offset
$4 = 32870912至此，我们知道了 info -> allocation 的值来自 entry->wr_highest_offset ，接下来查看源码， entry->wr_highest_offset 的值应该是在这里被赋予的：
if (qemuDomainBlocksStatsGather(driver, vm, path, true, &entry) < 0)
goto endjob;下面将断点打在 qemuDomainBlocksStatsGather 看一下其中的 entry->wr_highest_offset 是在哪里被赋值.
跟踪 src/qemu/qemu_driver.c -> qemuDomainBlocksStatsGather
将之前的断点删除，打上新的断点
(gdb) info breakpoints
Num Type Disp Enb Address What
1 breakpoint keep y 0x00007f4d4394a760 in virDomainGetBlockInfo at libvirt-domain.c:6094
breakpoint already hit 1 time
(gdb) delete 1
(gdb) break qemuDomainBlocksStatsGather
Breakpoint 2 at 0x7f4d208e3700: file qemu/qemu_driver.c, line 11427.之后在 GDB continue ，之后一直按回车，直到程序正常运行了，再执行一下获取磁盘信息的命令，继续跟踪。
Breakpoint 1, qemuDomainBlocksStatsGather (driver=driver@entry=0x7f4d180f99b0, vm=0x7f4d100b8890,
path=path@entry=0x7f4d0c00ae50 "vda", capacity=capacity@entry=true, retstats=retstats@entry=0x7f4d336f0980)
at qemu/qemu_driver.c:11427
11427 {
(gdb) n
11428 qemuDomainObjPrivatePtr priv = vm->privateData;
(gdb)
11429 bool blockdev = virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_BLOCKDEV);
(gdb)
11439 if (*path) {
(gdb)
11440 if (!(disk = virDomainDiskByName(vm->def, path, false))) {
(gdb)
11445 if (blockdev) {
(gdb)
11448 if (!disk->info.alias) {
(gdb)
11458 qemuDomainObjEnterMonitor(driver, vm);
(gdb)
11459 nstats = qemuMonitorGetAllBlockStatsInfo(priv->mon, &blockstats, false);
(gdb)
11461 if (capacity && nstats >= 0) {
(gdb)
11465 rc = qemuMonitorBlockStatsUpdateCapacity(priv->mon, blockstats, false);
(gdb)
11468 if (qemuDomainObjExitMonitor(driver, vm) < 0 || nstats < 0 || rc < 0)
(gdb)
11471 if (VIR_ALLOC(*retstats) < 0)
(gdb)
11474 if (entryname) {
(gdb)
11475 if (!(stats = virHashLookup(blockstats, entryname))) {
(gdb)
11481 **retstats = *stats;
(gdb) p stats
$12 = (qemuBlockStats *) 0x7f4d0c001000
(gdb) p *stats
$13 = {rd_req = 712, rd_bytes = 17435136, wr_req = 130, wr_bytes = 418816, rd_total_times = 527027278,
wr_total_times = 86798718, flush_req = 20, flush_total_times = 94396427, capacity = 2147483648,
physical = 2147483648, wr_highest_offset = 32870912, wr_highest_offset_valid = true, write_threshold = 0}
(gdb) c
Continuing.分析这一调用过程，发现我们跟踪的 restats 变量来自 stats，而该值在这一行被填充：
11475 if (!(stats = virHashLookup(blockstats, entryname))) {值来自哈希表查询结果，从 blockstats 中查询 entryname ，而该哈希表在这两行被赋值：
11459 nstats = qemuMonitorGetAllBlockStatsInfo(priv->mon, &blockstats, false);
11465 rc = qemuMonitorBlockStatsUpdateCapacity(priv->mon, blockstats, false);之后就可以跟踪源码了，经过一番探索，发现他们最终都调用了同一个函数来从 QEMU 获取设备信息，即 src/qemu/qemu_monitor_json.c -> qemuMonitorJSONQueryBlock ，看一下它的函数实现：
static virJSONValuePtr
qemuMonitorJSONQueryBlock(qemuMonitorPtr mon)
{
virJSONValuePtr cmd;
virJSONValuePtr reply = NULL;
virJSONValuePtr devices = NULL;
if (!(cmd = qemuMonitorJSONMakeCommand("query-block", NULL)))
return NULL;
if (qemuMonitorJSONCommand(mon, cmd, &reply) < 0 ||
qemuMonitorJSONCheckReply(cmd, reply, VIR_JSON_TYPE_ARRAY) < 0)
goto cleanup;
devices = virJSONValueObjectStealArray(reply, "return");
cleanup:
virJSONValueFree(cmd);
virJSONValueFree(reply);
return devices;
}继续探索会发现 libvirt 在这里调用了 QEMU 提供的 QMP 协议，其中的查询关键词为 query-block ，返回的结果中含有 wr_highest_offset 字段。
最终得到一张 libvirt 查询磁盘使用情况的调用栈示意图：
如果继续探索，可能就需要去跟踪 QEMU 源码了，下篇文章见。
参考文献
- 使用gdb debug libvirt 心得
- GDB常用命令
- GDB禁用和删除断点
- qmp—QEMU Machine Protocol介绍_北方南方-程序员宅基地
- QMP ( qemu monitor protocol ) and Different ways of accessing it
- Unix Socket – Server Examples
- About QEMU
