On Thursday 2015-06-18 09:35, Erik Cumps wrote: | On Tue, Jun 16, 2015 at 3:56 PM, Erik Cumps <erik.cu...@esaturnus.com> wrote: | > The context is a 16 GB 32-bit intel debian workstation, using an ext4 | > filesystem with journalling, on a lvm SATA3 SSD disk, with relatively | > recent stock kernels from 3.2 onwards to 4.0, running some KVM virtual | > machines. The host system (so not the virual machines) shows sporadic | > extremely slow write performance (around 4 megabytes per second). | > However, if we use the debian 3.2.0 kernel this problem does not | > manifest itself. [...] | Actually, it is the *synchronous*, direct IO that matches the expected | raw write performance of the device. | | The "regular IO" test is doing roughly this: | | echo 3 > /proc/sys/vm/drop_caches | dd if=ramdisk_file of=test_file bs=1M count=100 | dd if=ramdisk_file of=test_file bs=1M count=100 | dd if=ramdisk_file of=test_file bs=1M count=100 | | The direct IO test is doing roughly this: | | echo 3 > /proc/sys/vm/drop_caches | dd if=ramdisk_file of=test_file oflag=sync,direct bs=1M count=100 | dd if=ramdisk_file of=test_file oflag=sync,direct bs=1M count=100 | dd if=ramdisk_file of=test_file oflag=sync,direct bs=1M count=100
I'm seeing this as well here on a number of new Dell Optiplex 7020 machines and one older Optiplex 780, all with 8GB RAM and running Ubuntu 14.04 in 32-bit mode. A simple dd command shows the problem: $ dd bs=1M count=10 if=/dev/zero of=/tmp/ddtest 10+0 records in 10+0 records out 10485760 bytes (10 MB) copied, 7.02392 s, 1.5 MB/s $ dd bs=1M count=10 if=/dev/zero of=/tmp/ddtest oflag=sync,direct 10+0 records in 10+0 records out 10485760 bytes (10 MB) copied, 0.535397 s, 19.6 MB/s In my case, running: echo 3 > /proc/sys/vm/drop_caches will restore the normal speed for a limited time: $ dd bs=1M count=10 if=/dev/zero of=/tmp/ddtest 10+0 records in 10+0 records out 10485760 bytes (10 MB) copied, 0.0123759 s, 847 MB/s There is an old Ubuntu bug report describing the same issue: https://bugs.launchpad.net/ubuntu/+source/linux-meta-lts-trusty/+bug/1333294 -- Dick -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/