Using Guided on ZFS on a 120 GB SSD harddisk. As I understand it SSD requires some unformatted space/partition, about 10%, for writing due to its architecture or so I've read about SSD.
How to leave a partition? During installation there was no option on partitioning.
Hi,
You would probably have to use
manual partitioning to reserve disk space in a spare partition during initial installation.
However disk space reservation was generally used on SSDs prior to TRIM being available. Even then, it was only really required on SSDs that had inadequate over-provisioning, or that were used in extremely heavy duty cycle (with corresponding high wear rates).
FreeBSD ZFS has TRIM enabled by default.
It's important to understand that the TRIM protocol is just "early notice" to the SSD controller that a disk block is not being used (i.e. in the free list). Any block in the free list will eventually get TRIM'd by the SSD controller if it ever leaves the free list -- when the controller sees a write to the block, it will be TRIM'd at that point, prior to doing the write. [It actually puts the dirty block on the to-do TRIM list, gets a new clean block, fiddles the virtual block addresses, and writes to the clean block.]
A TRIM'd free list effectively augments the manufacturer's over-provisioning pool with the contents of the free list.
Conversely, in the worst case, without TRIM, the entire free list will not participate in the over-provisioning. That's not a problem if there is sufficient over-provisioning.
I have a Samsung 840 PRO (a "consumer grade" SSD) rated at 256 GB, and an Intel DC S3500 (a "data centre" grade SSD) rated at 240 GB. I reckon the missing 16 GB on the S3500 probably goes to over-provisioning.
So to return to your question, 16/256 = 6.25% might be a good starting point on a consumer grade SSD.
But wisdom has it that ZFS does funny things once it gets more than 80% full, so I would not bother reserving space on the disk. Just keep ZFS usage under 80%, leaving 20% in the TRIM'd free list.
Warren Block has a good write-upon FreeBSD SSD use
here.
Cheers,