Re:Announcing Ubuntu Core, with snappy transactional updates
by AaronFriel on Wednesday December 10, 2014 @10:55AM
(#8729799)
For online deduplication you are correct, but there is not much need for online deduplication on a mostly read-only system.
BTRFS currently only supports offline, and I believe the current state of ZFS is only online. A curious situation, but I imagine ZFS will eventually support offline dedupe and with that, the memory requirements will fall in terms of what needs to be cached.
And memory usage would decrease, because offline dedupe on read-only files reduces duplication in cache. Even memory-only deduplication would be sufficient. I'm not sure if zswap/zram/zcache support it, but it seems like a worthwhile feature.
This is getting into a pretty tangential discussion, but I'd be surprised if there are net memory savings from deduplication. Disk yes, but the dedup process itself has significant memory overhead (both in memory usage, and memory accesses), which would need to be offset to have a net win. At least on ZFS, it's usually recommended to turn it on only for large-RAM servers where the saving in disk space (and/or reduction in uncached disk access) is worth allocating the memory to it.