Configuration for mbuffer buffer size

[ShaRose]

Can you add an option to configure the mbuffer size? At the moment it's hard-coded to 512M, which is fine in most cases, but I was using zfs on a VPS with a gig of ram (for the snapshotting and easy backup) and 512M might not always be available. On the other side of the spectrum, you might want to increase the buffer size.

Similarly, it might be a good idea to add a configuration option for the block size since both are effectively used in exactly the same spot.

[yboetz]

I will reduce the mbuffer size in the future anyway, as 512MB is almost never used, I think I will use something like 128MB. That is something any system using ZFS should be able to handle, as ZFS itself needs quite a bit of RAM already :). Adding more config options just adds code and makes it more complicated, so I hesitate about implementing something which almost noone will notice/use.