[Qemu-discuss] How can I make serial port redirected to file use line buffering?

[Priedhorsky, Reid]

Hello,

I am running QEMU with three serial ports redirected to files: the guest Linux 
console and two other logs. It appears that these serial ports are writing 
unbuffered to the file. Specifically, when I tail -F several files from 
different QEMU processes at once, output is intermingled character-by-character 
rather than line-by-line.

I believe this is causing boot performance problems, as boot is much faster 
with these files on some filesystems rather than others. Therefore, I’d like to 
use line buffering instead, to reduce the number of writes to disk. How can I 
do this?

Here is my QEMU command line. Some paths have been trimmed.

$ qemu-system-x86_64 -boot order=dc -cpu host -device virtio-blk,drive=vd0 
-device virtio-blk,drive=vd1,scsi=off,x-data-plane=on -drive 
file=./charlie-test/run/0.overlay.qcow2,id=vd0,if=none,aio=native,format=qcow2, 
-drive file=/chscratch1/0.tmp,id=vd1,if=none,aio=native,format=raw,cache=none 
-k en-us -m 26786008K -machine q35,accel=kvm -net 
nic,model=virtio,macaddr=0C:00:AC:16:0C:01 -net 
tap,ifname=tap0,script=no,downscript=no -serial 
file:./charlie-test/out/0_console.out -serial file:./charlie-test/out/0_job.out 
-serial file:./charlie-test/out/0_job.err -smp 2 -vga cirrus -virtfs 
local,security_model=none,path=[...]/charliecloud/opt,mount_tag=opt -virtfs 
local,security_model=none,path=./charlie-test/meta,mount_tag=meta -virtfs 
local,security_model=none,path=testout/data4,mount_tag=data4 -virtfs 
local,security_model=none,path=testout,mount_tag=data1 -virtfs 
local,security_model=none,path=testout/data3,mount_tag=data3 -virtfs 
local,security_model=none,path=testout/data2,mount_tag=data2 -display none

I can provide more details if needed. Just let me know what questions you have.

Thanks,
Reid