Re: [LSF/MM ATTEND] Stackable Union Filesystem Implementation

[Jan Kara <jack@suse.cz>]

On Wed 08-01-14 23:56:57, Saket Sinha wrote:
> >> One of the big problems was that too many copyups were made on the
> >> read-write file system. So we decided to implement an union file
> >> system designed for diskless systems, with the following
> >> functionalities:
> >>
> >> 1. union between only one read-only and one read-write file systems
> >>
> >> 2. if only the file metadata are modified, then do not
> >> copy the whole file on the read-write files system but
> >> only the metadata (stored with a file named as the file
> >> itself prefixed by '.me.')
> >   So do you do anything special at CERN so that metadata is often modified
> > without data being changed? Because there are only two operations where I
> > can imagine this to be useful:
> > 1) atime update - but you better turn atime off for unioned filesystem
> >    anyway.
> > 2) xattr update
> >
> As already mentioned that the issue that we were facing was that "too
> many copyups were made on the  read-write file system".
  But my question is: In which cases specifically do you want to avoid
copyups as compared to e.g. Overlayfs?

> Writes to a file system in a  unioning file system will produce many
> duplicated blocks in memory since it uses a stackable filesystem
> approach so response time for a particular operation is also a
> concern.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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