Linux 缓冲区缓存对 IO 写入的影响？

Question

我正在 Linux 服务器（内核 2.6.37、16 核、32G RAM）上的 2 个文件系统之间复制大文件（3 x 30G），但性能很差。我怀疑缓冲区缓存的使用正在扼杀 I/O 性能。

为了尝试缩小问题的范围，我直接在 SAS 磁盘上使用 fio 来监控性能。

这是 2 次 fio 运行的输出（第一次直接=1，第二次直接=0）：

配置：

[test]
rw=write
blocksize=32k
size=20G
filename=/dev/sda
# direct=1

运行 1：

test: (g=0): rw=write, bs=32K-32K/32K-32K, ioengine=sync, iodepth=1
Starting 1 process
Jobs: 1 (f=1): [W] [100.0% done] [0K/205M /s] [0/6K iops] [eta 00m:00s]
test: (groupid=0, jobs=1): err= 0: pid=4667
  write: io=20,480MB, bw=199MB/s, iops=6,381, runt=102698msec
    clat (usec): min=104, max=13,388, avg=152.06, stdev=72.43
    bw (KB/s) : min=192448, max=213824, per=100.01%, avg=204232.82, stdev=4084.67
  cpu          : usr=3.37%, sys=16.55%, ctx=655410, majf=0, minf=29
  IO depths    : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued r/w: total=0/655360, short=0/0
     lat (usec): 250=99.50%, 500=0.45%, 750=0.01%, 1000=0.01%
     lat (msec): 2=0.01%, 4=0.02%, 10=0.01%, 20=0.01%

Run status group 0 (all jobs):
  WRITE: io=20,480MB, aggrb=199MB/s, minb=204MB/s, maxb=204MB/s, mint=102698msec,    maxt=102698msec

Disk stats (read/write):
  sda: ios=0/655238, merge=0/0, ticks=0/79552, in_queue=78640, util=76.55%

运行 2：

test: (g=0): rw=write, bs=32K-32K/32K-32K, ioengine=sync, iodepth=1
Starting 1 process
Jobs: 1 (f=1): [W] [100.0% done] [0K/0K /s] [0/0 iops] [eta 00m:00s]     
test: (groupid=0, jobs=1): err= 0: pid=4733
  write: io=20,480MB, bw=91,265KB/s, iops=2,852, runt=229786msec
    clat (usec): min=16, max=127K, avg=349.53, stdev=4694.98
    bw (KB/s) : min=56013, max=1390016, per=101.47%, avg=92607.31, stdev=167453.17
  cpu          : usr=0.41%, sys=6.93%, ctx=21128, majf=0, minf=33
  IO depths    : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
     submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
     issued r/w: total=0/655360, short=0/0
     lat (usec): 20=5.53%, 50=93.89%, 100=0.02%, 250=0.01%, 500=0.01%
     lat (msec): 2=0.01%, 4=0.01%, 10=0.01%, 20=0.01%, 50=0.12%
     lat (msec): 100=0.38%, 250=0.04%

Run status group 0 (all jobs):
  WRITE: io=20,480MB, aggrb=91,265KB/s, minb=93,455KB/s, maxb=93,455KB/s, mint=229786msec, maxt=229786msec

Disk stats (read/write):
  sda: ios=8/79811, merge=7/7721388, ticks=9/32418456, in_queue=32471983, util=98.98%

我对 fio 的了解不够，无法解释结果，但我不希望使用缓冲区缓存的整体性能比使用 O_DIRECT 低 50%。

有人可以帮我解释 fio 的输出吗？
是否有任何内核调整可以解决/最小化问题？

非常感谢，

Answer 1

使用 O_DIRECT，内核绕过所有常用的缓存机制，直接写入磁盘。由于您没有使用 O_SYNC，如果启用了缓存（不使用 O_DIRECT），那么内核可能会向您撒谎说“是的，是的，我已经写了它，别担心！”，即使它没有将其写入磁盘，它仅被写入某些缓存（磁盘缓存/页面缓存/...）。