Linux buffer cache effect on IO writes?
- by Patrick LeBoutillier
I'm copying large files (3 x 30G) between 2 filesystems on a Linux server (kernel 2.6.37, 16 cores, 32G RAM) and I'm getting poor performance. I suspect that the usage of the buffer cache is killing the I/O performance.
To try and narrow down the problem I used fio directly on the SAS disk to monitor the performance.
Here is the output of 2 fio runs (the first with direct=1, the second one direct=0):
Config:
[test]
rw=write
blocksize=32k
size=20G
filename=/dev/sda
# direct=1
Run 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%
Run 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%
I'm not knowledgeable enough with fio to interpret the results, but I don't expect the overall performance using the buffer cache to be 50% less than with O_DIRECT.
Can someone help me interpret the fio output?
Are there any kernel tunings that could fix/minimize the problem?
Thanks a lot,
