On Fri, Nov 6, 2020 at 7:59 PM Sargun Dhillon <sargun@xxxxxxxxx> wrote: > > On Mon, Aug 31, 2020 at 11:15 AM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > > > > Container folks are complaining that dnf/yum issues too many sync while > > installing packages and this slows down the image build. Build > > requirement is such that they don't care if a node goes down while > > build was still going on. In that case, they will simply throw away > > unfinished layer and start new build. So they don't care about syncing > > intermediate state to the disk and hence don't want to pay the price > > associated with sync. > > > > So they are asking for mount options where they can disable sync on overlay > > mount point. > > > > They primarily seem to have two use cases. > > > > - For building images, they will mount overlay with nosync and then sync > > upper layer after unmounting overlay and reuse upper as lower for next > > layer. > > > > - For running containers, they don't seem to care about syncing upper > > layer because if node goes down, they will simply throw away upper > > layer and create a fresh one. > > > > So this patch provides a mount option "volatile" which disables all forms > > of sync. Now it is caller's responsibility to throw away upper if > > system crashes or shuts down and start fresh. > > > > With "volatile", I am seeing roughly 20% speed up in my VM where I am just > > installing emacs in an image. Installation time drops from 31 seconds to > > 25 seconds when nosync option is used. This is for the case of building on top > > of an image where all packages are already cached. That way I take > > out the network operations latency out of the measurement. > > > > Giuseppe is also looking to cut down on number of iops done on the > > disk. He is complaining that often in cloud their VMs are throttled > > if they cross the limit. This option can help them where they reduce > > number of iops (by cutting down on frequent sync and writebacks). > > [...] > There is some slightly confusing behaviour here [I realize this > behaviour is as intended]: > > (root) ~ # mount -t overlay -o > volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir > none /mnt/foo > (root) ~ # umount /mnt/foo > (root) ~ # mount -t overlay -o > volatile,index=off,lowerdir=/root/lowerdir,upperdir=/root/upperdir,workdir=/root/workdir > none /mnt/foo > mount: /mnt/foo: wrong fs type, bad option, bad superblock on none, > missing codepage or helper program, or other error. > > From my understanding, the dirty flag should only be a problem if the > existing overlayfs is unmounted uncleanly. Docker does > this (mount, and re-mounts) during startup time because it writes some > files to the overlayfs. I think that we should harden > the volatile check slightly, and make it so that within the same boot, > it's not a problem, and having to have the user clear > the workdir every time is a pain. In addition, the semantics of the > volatile patch itself do not appear to be such that they > would break mounts during the same boot / mount of upperdir -- as > overlayfs does not defer any writes in itself, and it's > only that it's short-circuiting writes to the upperdir. > > Amir, > What do you think? How do you propose to check that upperdir was used during the same boot? Maybe a simpler check is that upperdir inode is still in cache as an easy way around this. Add an overlayfs specific inode flag, similar to I_OVL_INUSE I_OVL_WAS_INUSE. Thanks, Amir.
