Neil Brown wrote: >>> >>> >> Some things are not achievable with block-level raid. For example, with >> redundancy integrated into the filesystem, you can have three copies for >> metadata, two copies for small files, and parity blocks for large files, >> effectively using different raid levels for different types of data on >> the same filesystem. >> > > Absolutely. And doing that is a very good idea quite independent of > underlying RAID. Even ext2 stores multiple copies of the superblock. > > Having the filesystem duplicate data, store checksums, and be able to > find a different copy if the first one it chose was bad is very > sensible and cannot be done by just putting the filesystem on RAID. > It would need to know a lot about the RAID geometry in order not to put the the copies on the same disks. > Having the filesystem keep multiple copies of each data block so that > when one drive dies, another block is used does not excite me quite so > much. If you are going to do that, then you want to be able to > reconstruct the data that should be on a failed drive onto a new > drive. > For a RAID system, that reconstruction can go at the full speed of the > drive subsystem - but needs to copy every block, whether used or not. > For in-filesystem duplication, it is easy to imagine that being quite > slow and complex. It would depend a lot on how you arrange data, > and maybe there is some clever approach to data layout that I haven't > thought of. But I think that sort of thing is much easier to do in a > RAID layer below the filesystem. > You'd need a reverse mapping of extents to files. While maintaining that is expensive, it brings a lot of benefits: - rebuild a failed drive, without rebuilding free space - evacuate a drive in anticipation of taking it offline - efficient defragmentation Reverse mapping storage could serve as free space store too. > Combining these thoughts, it would make a lot of sense for the > filesystem to be able to say to the block device "That blocks looks > wrong - can you find me another copy to try?". That is an example of > the sort of closer integration between filesystem and RAID that would > make sense. > It's a step forward, but still quite limited compared to combining the two layers together. Sticking with the example above, you still can't have a mix of parity-protected files and mirror-protected files; the RAID decides that for you. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
