> the Linux kernel Dirty page flush algorithm does not scale to large memory sizes, so anytime the Dirty page in /proc/meminfo exceeds around 1GB the writeback speed slows down progressively and eventually the /proc/sys/vm/dirty_ratio or /proc/sys/vm/dirty_bytes limit is exceeded and the kernel starts throttling all writes to keep the Dirty pages from growing any further. > > To maintain high write speed (in OPs case up to 800Mb/sec, can easily be 2 Gb/sec for a hardware RAID controller with cache) you need to counter intuitively lower the /proc/sys/vm/dirty_bytes and dirty_background_bytes to 256M and 64M respectively > > Make sure you do a sync first otherwise the system will freeze on writes for several hours until the Dirty page value in /proc/meminfo drops below the new value in /proc/sys/vm/dirty_bytes. The sync will also take several hours, but at least the system will not be frozen during this time. https://serverfault.com/a/995658