On 17/10/12 14:15, Russell Coker wrote:
Note that this zvol has compression enabled - this would be a good choice for a mail server's storage disk - mail is highly compressible. depending on available RAM in the server and the kind of mail typically received (e.g. multiple copies of the same email), de-duping the zvol may also be worthwhile.
Do be very careful with ZFS dedup -- as you increase the amount of data on the disk, the amount of memory required to hold the dedup tables in memory goes up. Initially this reduces the amount of memory available for caching, which hurts performance. After a while, the amount of memory required *exceeds* the size of the ARC, and at this point writes effectively stop working. (They do continue to work, but it is soooooooooooooooooo sloooooooooow as to be useless.) So when speccing up a machine for ZFS w/dedup, keep that in mind -- as a rough guide, you need something like 5G or ARC per TB of data, and your ARC is a 1/4 of your system memory.. so you'll need 20G of RAM for 1TB of deduped data! I've seen people suggest using an SSD as l2arc, and thus enabling the dedup tables to live there; I tried it myself and although it was better than before, it still wasn't really great. Here's an article with more info: http://constantin.glez.de/blog/2011/07/zfs-dedupe-or-not-dedupe