Hello Sergey, > I think some people in here are familiar with the idea of creating > temporary filesystems in RAM, that gives a huge performance boost at > some workloads (e.g. MySQL). However, those setups never got widely > used because of one problem: the whole thing crashes if you run out of > space in the ramdisk. One guy (Yves Trudeau) said "Let's use a union
Since aufs is file-based vertual filesystem, it depends upon the number of files and their sizes. If you have MySQL file whose size is 10MB and aufs puts it on tmpfs (or ramdisk), then the file will stay on tmpfs and keep consuming 10MB of tmpfs. It will never be moved to another place automatically neither "entirely" nor "partially". If you want to move it manually, it is possible. But there are some issues you shoule care. - if you move a file on tmpfs during other process accessing the file, you may succeed but the file still remains invisible. this is very similar to "open + unlink" behaviour. And the changes made to the invisible file will never be reflected to the file at the new place. - you should specify udba=notify aufs mount option. > to find the proper mount options. I need a creation policy that fills > the ramdisk first, and after it's filled, writes to the HDD. Looks > like I need either "top-down-freespace" that doesn't care about > parents and writes to the highest branch that has enough free space, > or a "least-free-space" policy with out-of-disk errors suppression. The aufs mount option "create=most-free-space" may help you. But again, note that aufs is file-based. I suppose what you want is "ramdisk + dm-snapshot" which handles blocks instead of files. By the way, I have a plan to implement a new feature "move-down" to aufs, which is a converse of "copy-up". It will allow users to move-down files from the upper branch to lower without stopping accesses to them. But several restrictions will surely exist. J. R. Okajima ------------------------------------------------------------------------------ RSA(R) Conference 2012 Save $700 by Nov 18 Register now http://p.sf.net/sfu/rsa-sfdev2dev1
