Disk Caching & Performance


Great stuff from – https://lonesysadmin.net/2013/12/22/better-linux-disk-caching-performance-vm-dirty_ratio/

 # sysctl -a | grep dirty
 vm.dirty_background_ratio = 10
 vm.dirty_background_bytes = 0
 vm.dirty_ratio = 20
 vm.dirty_bytes = 0
 vm.dirty_writeback_centisecs = 500
 vm.dirty_expire_centisecs = 3000

vm.dirty_background_ratio is the percentage of system memory that can be filled with “dirty” pages — memory pages that still need to be written to disk — before the pdflush/flush/kdmflush background processes kick in to write it to disk. My example is 10%, so if my virtual server has 32 GB of memory that’s 3.2 GB of data that can be sitting in RAM before something is done.

vm.dirty_ratio is the absolute maximum amount of system memory that can be filled with dirty pages before everything must get committed to disk. When the system gets to this point all new I/O blocks until dirty pages have been written to disk. This is often the source of long I/O pauses, but is a safeguard against too much data being cached unsafely in memory.

vm.dirty_background_bytes and vm.dirty_bytes are another way to specify these parameters. If you set the _bytes version the _ratio version will become 0, and vice-versa.

vm.dirty_expire_centisecs is how long something can be in cache before it needs to be written. In this case it’s 30 seconds. When the pdflush/flush/kdmflush processes kick in they will check to see how old a dirty page is, and if it’s older than this value it’ll be written asynchronously to disk. Since holding a dirty page in memory is unsafe this is also a safeguard against data loss.

vm.dirty_writeback_centisecs is how often the pdflush/flush/kdmflush processes wake up and check to see if work needs to be done.

# cat /proc/vmstat | egrep "dirty|writeback"
nr_dirty 9556
nr_writeback 0
nr_writeback_temp 0

9556 dirty files waiting to be written to disk


The kernel buffers always stay in main memory, because they have to. Applications and cache don’t need to stay in RAM, though. The cache can be dropped, and the applications can be paged out to the swap file. Dropping cache means a potential performance hit. Likewise with paging applications out. The vm.swappiness parameter helps the kernel decide what to do. By setting it to the maximum of 100 the kernel will swap very aggressively. By setting it to 0 the kernel will only swap to protect against an out-of-memory condition. The default is 60 which means that some swapping will occur.

# sysctl -a |grep swappiness
vm.swappiness = 60

Edit in:



sysctl –p