>I've noticed that there is a huge (more than ~3x slower) performance difference between KVM guest and host machine.
I don't know that this is relevant or not , but there is an IBM research paper (Published in 2014) "IBM Research Report - An Updated Performance Comparison of Virtual Machines and Linux Containers" http://domino.research.ibm.com/library/cyberdig.nsf/papers/0929052195DD819C85257D2300681E7B/ -> " As we would expect, Docker introduces no overhead compared to Linux, but KVM delivers only half as many IOPS because each I/O operation must go through QEMU. While the VM’s absolute performance is still quite high, it uses more CPU cycles per I/O operation, leaving less CPU available for application work. Figure 7 shows that KVM increases read latency by 2-3x, a crucial metric for some real workloads." Imre 2016-04-26 16:03 GMT+02:00 Artem Tomyuk <ad...@leboutique.com>: > Hi All. > > I've noticed that there is a huge (more than ~3x slower) performance > difference between KVM guest and host machine. > Host machine: > dell r720xd > RAID10 with 12 SAS 15 k drives and RAID0 with 2*128 GB INTEL SSD drives > in Dell CacheCade mode. > > *On the KVM guest:* > > /usr/pgsql-9.4/bin/pg_test_fsync -f test.sync > > 5 seconds per test > > O_DIRECT supported on this platform for open_datasync and open_sync. > > > Compare file sync methods using one 8kB write: > > (in wal_sync_method preference order, except fdatasync > > is Linux's default) > > open_datasync 5190.279 ops/sec 193 > usecs/op > > fdatasync 4022.553 ops/sec 249 > usecs/op > > fsync 3069.069 ops/sec 326 > usecs/op > > fsync_writethrough n/a > > open_sync 4892.348 ops/sec 204 > usecs/op > > > Compare file sync methods using two 8kB writes: > > (in wal_sync_method preference order, except fdatasync > > is Linux's default) > > open_datasync 2406.577 ops/sec 416 > usecs/op > > fdatasync 4309.413 ops/sec 232 > usecs/op > > fsync 3518.844 ops/sec 284 > usecs/op > > fsync_writethrough n/a > > open_sync 1159.604 ops/sec 862 > usecs/op > > > Compare open_sync with different write sizes: > > (This is designed to compare the cost of writing 16kB > > in different write open_sync sizes.) > > 1 * 16kB open_sync write 3700.689 ops/sec 270 > usecs/op > > 2 * 8kB open_sync writes 2581.405 ops/sec 387 > usecs/op > > 4 * 4kB open_sync writes 1318.871 ops/sec 758 > usecs/op > > 8 * 2kB open_sync writes 698.640 ops/sec 1431 > usecs/op > > 16 * 1kB open_sync writes 262.506 ops/sec 3809 > usecs/op > > > Test if fsync on non-write file descriptor is honored: > > (If the times are similar, fsync() can sync data written > > on a different descriptor.) > > write, fsync, close 3071.141 ops/sec 326 > usecs/op > > write, close, fsync 3303.946 ops/sec 303 > usecs/op > > > Non-Sync'ed 8kB writes: > > write 251321.188 ops/sec 4 > usecs/op > > > *On the host machine:* > > /usr/pgsql-9.4/bin/pg_test_fsync -f test.sync > > 5 seconds per test > > O_DIRECT supported on this platform for open_datasync and open_sync. > > > Compare file sync methods using one 8kB write: > > (in wal_sync_method preference order, except fdatasync > > is Linux's default) > > open_datasync 11364.136 ops/sec 88 > usecs/op > > fdatasync 12352.160 ops/sec 81 > usecs/op > > fsync 9833.745 ops/sec 102 > usecs/op > > fsync_writethrough n/a > > open_sync 14938.531 ops/sec 67 > usecs/op > > > Compare file sync methods using two 8kB writes: > > (in wal_sync_method preference order, except fdatasync > > is Linux's default) > > open_datasync 7703.471 ops/sec 130 > usecs/op > > fdatasync 11494.492 ops/sec 87 > usecs/op > > fsync 9029.837 ops/sec 111 > usecs/op > > fsync_writethrough n/a > > open_sync 6504.138 ops/sec 154 > usecs/op > > > Compare open_sync with different write sizes: > > (This is designed to compare the cost of writing 16kB > > in different write open_sync sizes.) > > 1 * 16kB open_sync write 14113.912 ops/sec 71 > usecs/op > > 2 * 8kB open_sync writes 7843.234 ops/sec 127 > usecs/op > > 4 * 4kB open_sync writes 3995.702 ops/sec 250 > usecs/op > > 8 * 2kB open_sync writes 1788.979 ops/sec 559 > usecs/op > > 16 * 1kB open_sync writes 937.177 ops/sec 1067 > usecs/op > > > Test if fsync on non-write file descriptor is honored: > > (If the times are similar, fsync() can sync data written > > on a different descriptor.) > > write, fsync, close 10144.280 ops/sec 99 > usecs/op > > write, close, fsync 8378.558 ops/sec 119 > usecs/op > > > Non-Sync'ed 8kB writes: > > write 159176.122 ops/sec 6 > usecs/op > > > The file system "inside" and "outside" the same - ext4 on LVM. Disk > scheduler "inside" and "outside" set to "noop". Fstab options same to, > setted to rw,noatime,nodiratime,barrier=0. OS on host and guest the same > CentOS release 6.5 (Final). > > Guest volume options: > > Disk bus: Virtio > > Cache mode: none > > IO mode: native > > > Any ideas? > > > > > > > > > >
