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authorVivek Goyal <vgoyal@redhat.com>2020-08-31 14:15:29 -0400
committerMiklos Szeredi <mszeredi@redhat.com>2020-09-02 10:58:48 +0200
commitc86243b090bc25f81abe33ddfa9fe833e02c4d64 (patch)
tree70ccd09f07ef8d70a9b97773a0e00f51b7e84db9 /Documentation/filesystems
parent235ce9ed96bc624fe929f724883cc04b7a714ac2 (diff)
downloadlinux-c86243b090bc25f81abe33ddfa9fe833e02c4d64.tar.bz2
ovl: provide a mount option "volatile"
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: Giuseppe Scrivano <gscrivan@redhat.com> Signed-off-by: Vivek Goyal <vgoyal@redhat.com> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r--Documentation/filesystems/overlayfs.rst19
1 files changed, 19 insertions, 0 deletions
diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index 8ea83a51c266..580ab9a0fe31 100644
--- a/Documentation/filesystems/overlayfs.rst
+++ b/Documentation/filesystems/overlayfs.rst
@@ -564,6 +564,25 @@ Note: the mount options index=off,nfs_export=on are conflicting for a
read-write mount and will result in an error.
+Volatile mount
+--------------
+
+This is enabled with the "volatile" mount option. Volatile mounts are not
+guaranteed to survive a crash. It is strongly recommended that volatile
+mounts are only used if data written to the overlay can be recreated
+without significant effort.
+
+The advantage of mounting with the "volatile" option is that all forms of
+sync calls to the upper filesystem are omitted.
+
+When overlay is mounted with "volatile" option, the directory
+"$workdir/work/incompat/volatile" is created. During next mount, overlay
+checks for this directory and refuses to mount if present. This is a strong
+indicator that user should throw away upper and work directories and create
+fresh one. In very limited cases where the user knows that the system has
+not crashed and contents of upperdir are intact, The "volatile" directory
+can be removed.
+
Testsuite
---------