I believe the ZFS deduplication consumes a ton of ram (somewhere around 1GB of ram per TB of disk) to keep the block checksums in ram. Said list is checked for

I believe the ZFS deduplication consumes a ton of ram (somewhere around 1GB of ram per TB of disk) to keep the block checksums in ram. Said list is checked for every block write. I don't believe it's necessary to check for reads, since the file metadata already tells you which blocks to look for. As a result ZFS with deduplication on uses a ton of ram and has slow writes. This is so big an impact that most folks just turn it off, despite the tempting lure of "free" disk space.