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    SubjectRe: [ANNOUNCE] RAIF: Redundant Array of Independent Filesystems
    > >We have designed a new stackable file system that we called RAIF:
    > >Redundant Array of Independent Filesystems.
    > >
    > >Similar to Unionfs, RAIF is a fan-out file system and can be mounted over
    > >many different disk-based, memory, network, and distributed file systems.
    > >RAIF can use the stable and maintained code of the other file systems and
    > >thus stay simple itself. Similar to standard RAID, RAIF can replicate the
    > >data or store it with parity on any subset of the lower file systems. RAIF
    > >has three main advantages over traditional driver-level RAID systems:
    > >
    > >1. RAIF can be mounted over any set of file systems. This allows users to
    > > create many more useful configurations. For example, it is possible to
    > > replicate the data on the local and remote disks, and stripe the data on
    > > the local hard drives and keep the parity (or even ECC to tolerate
    > > multiple failures) on the remote server(s). In the latter case, all the
    > > read requests will be satisfied from the fast local disks and no local
    > > disk space will be spent on parity.
    > As for striping on a simplistic level, look at the Equal File
    > Distribution patch for unionfs :-)
    > Files are stored normally so that after the union is unmounted, the
    > files appear in one piece (unlike real RAID0 over two block devices).

    RAIF supports rules that describe how to store particular files or groups
    of files. A rule with RAIF level 0 (which is similar to RAID level 0) and
    a special striping unit size = '-1' will do the same (distribute the
    files on the lower file systems) for files that match any given file name
    pattern. A rule with level 4 and striping unit size = '-1' will
    distribute files on several file systems and store an extra copy of the
    files on a dedicated file system (e.g., an NFS mount with lots of space).
    Now guess what RAIF's level 6 will do with a special striping unit
    size = '-1' :-)

    Nikolai Joukov, Ph.D.
    Filesystems and Storage Laboratory
    Stony Brook University
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