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. 2. RAIF is a file system and thus has access to the meta-data. This allows it to store different files differently. For example, RAIF can replicate important files (*.c, *.doc, etc) on all the lower file systems and stripe the multimedia files with parity at the same time. 3. It is sometimes more convenient to work with file systems than devices as the lower storage. For example, it is possible to mount RAIF over a directory on an existing file system. The data is represented as files on the lower file systems. Therefore, any lower file system is an exact replica of the RAIF file system in the case of replication. It also makes it easy to backup the data on the lower file systems using existing tools. We have performed some benchmarking on a 3GHz PC with 2GB of RAM and U320 SCSI disks. Compared to the Linux RAID driver, RAIF has overheads of about 20-25% under the Postmark v1.5 benchmark in case of striping and replication. In case of RAID4 and RAID5-like configurations, RAIF performed about two times *better* than software RAID and even better than an Adaptec 2120S RAID5 controller. This is because RAIF is located above file system caches and can cache parity as normal data when needed. We have more performance details in a technical report, if anyone is interested. We started the project in April 2004. Right now I am using it as my /home/kolya file system at home. We believe that at this stage RAIF is mature enough for others to try it out. The code is available at: <ftp://ftp.fsl.cs.sunysb.edu/pub/raif/> The code requires no kernel patches and compiles for a wide range of kernels as a module. The latest kernel we used it for is 2.6.13 and we are in the process of porting it to 2.6.19. We will be happy to hear your back. Nikolai Joukov on behalf of the RAIF team. ---------------------------------- Filesystems and Storage Laboratory Stony Brook University - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
