linux – KVM主机不会使用内存进行缓存

我有一个运行8个虚拟机的KVM主机( Linux 3.14.32).它们被过度配置(为它们分配的内存大于主机上的可用物理内存).然而,它们仅消耗大约85％的物理内存,这应该为缓存留出一些空间.但是,主机上的所有内容都是非常缓慢的,并且几乎没有内存用于缓存的免费报告：

total       used       free     shared    buffers     cached
Mem:         32224      26767       5457          0          0          3
-/+ buffers/cache:      26762       5461
Swap:         1021        414        607

如您所见,浪费了5.5 GB.关闭虚拟机后,一切恢复正常 – 内核使用260M的缓存.

的/ proc / meminfo中：

MemTotal:       32997976 kB
MemFree:         5543224 kB
MemAvailable:    5584332 kB
Buffers:            2612 kB
Cached:            83876 kB
SwapCached:        72524 kB
Active:         24721804 kB
Inactive:        2214632 kB
Active(anon):   24675412 kB
Inactive(anon):  2175048 kB
Active(file):      46392 kB
Inactive(file):    39584 kB
Unevictable:           0 kB
mlocked:               0 kB
SwapTotal:       1046520 kB
SwapFree:         622068 kB
Dirty:                84 kB
Writeback:             4 kB
AnonPages:      26806104 kB
Mapped:            28436 kB
Shmem:               488 kB
Slab:              60024 kB
SReclaimable:      25076 kB
SUnreclaim:        34948 kB
KernelStack:        3232 kB
PageTables:        62212 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:    17545508 kB
Committed_AS:   53713960 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      431560 kB
VmallocChunk:   34359138492 kB
HardwareCorrupted:     0 kB
DirectMap4k:        9728 kB
DirectMap2M:    33536000 kB

我将swappiness设置为0. VM的磁盘映像位于LVM上,缓存为= writeback.

解决方法

建议在使用RAW设备时绕过缓存(cache = none).这使用O_DIRECT模式,因此绕过主机页面缓存,I / O直接在qemu-kvm用户空间缓冲区和存储设备之间进行.

QEMU also supports a wide variety of caching modes. If you’re using
raw volumes or partitions,it is best to avoid the cache completely,
which reduces data copies and bus traffic.

参考：Tuning KVM