overlayfs : Another unionfs ? Kernel 3.18

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overlayfs : Another unionfs ? Kernel 3.18

Karl Godt
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Reading kernelnewbies.org/Linux_3.18

1.1. Overlayfs

An overlay filesystem combines two filesystems, an 'upper' filesystem and a 'lower' filesystem,
into a single file system namespace,
modifications will be done to the upper filesystem.

It has many uses, but it is most often used for live CDs,
where a read-only OS image is used as lower filesystem and a writeable RAM-backed filesystem is used as the upper one.

Any modifications will be done in the upper filesystem,
thus allowing users to run the OS image provided normally.
Overlayfs differs from other "union filesystem" implementations in that after a file is opened all operations go directly to the underlying,
lower or upper, filesystems.
This simplifies the implementation and allows native performance in these cases.

It is possible for both directory trees to be in the same filesystem
and there is no requirement that the root of a filesystem be given for either upper or lower.
The lower filesystem can be any filesystem supported by Linux and does not need to be writable.
The lower filesystem can even be another overlayfs.
The upper filesystem will normally be writable and if it is it must support the creation of trusted.* extended attributes,
and must provide valid d_type in readdir() responses, so NFS is not suitable.



May need some stubs in /init (and rc.sysinit) to care for that ..
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Re: overlayfs : Another unionfs ? Kernel 3.18

q5sys
I've used Overlay FS with Fedora Atomic for about a year or so now.  While its technically possible to overlay one on top of another, it's really a mess, and imho doesnt offer any advantages over AUFS.

On Mon, Dec 28, 2015 at 3:34 PM, Karl Godt [via woof-CE] <[hidden email]> wrote:
Reading kernelnewbies.org/Linux_3.18

1.1. Overlayfs

An overlay filesystem combines two filesystems, an 'upper' filesystem and a 'lower' filesystem,
into a single file system namespace,
modifications will be done to the upper filesystem.

It has many uses, but it is most often used for live CDs,
where a read-only OS image is used as lower filesystem and a writeable RAM-backed filesystem is used as the upper one.

Any modifications will be done in the upper filesystem,
thus allowing users to run the OS image provided normally.
Overlayfs differs from other "union filesystem" implementations in that after a file is opened all operations go directly to the underlying,
lower or upper, filesystems.
This simplifies the implementation and allows native performance in these cases.

It is possible for both directory trees to be in the same filesystem
and there is no requirement that the root of a filesystem be given for either upper or lower.
The lower filesystem can be any filesystem supported by Linux and does not need to be writable.
The lower filesystem can even be another overlayfs.
The upper filesystem will normally be writable and if it is it must support the creation of trusted.* extended attributes,
and must provide valid d_type in readdir() responses, so NFS is not suitable.



May need some stubs in /init (and rc.sysinit) to care for that ..



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