• Vivek Goyal's avatar
    ovl: provide a mount option "volatile" · c86243b0
    Vivek Goyal authored
    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).
    Signed-off-by: default avatarGiuseppe Scrivano <gscrivan@redhat.com>
    Signed-off-by: default avatarVivek Goyal <vgoyal@redhat.com>
    Signed-off-by: default avatarMiklos Szeredi <mszeredi@redhat.com>
    c86243b0
readdir.c 27.3 KB