On Thu, May 12, 2016 at 5:33 PM, Steven Burgess <[email protected]> wrote:
> Snapshots take up some space, but 2M per sound much higher than expected. > I ran an experiment on an otherwise idle machine: > > zpool get -Hp -o value free testpool > > 7362543104 > > zfs get -rHp -o value -t snapshot name testpool/testfs | wc -l > > 5000 > > Zfs destroy testpool/testfs@first%last > > zpool get -Hp -o value free testpool > > 7351909376 > > So by destroying those 5000 snapshots we freed up 10 MiB, that's 2126 > bytes (0.002 MiB) per snapshot. > Which is closer to what I expected. > How did you confirm that the snapshots took no space? If files are shared > between snapshots, it is possible to free space by destroying a large set > of snapshots where each snapshots used is 0. You can use zfs destroy -nv to > see this kind of space usage. > This is all after the fact. But beforehand zfs list said: NAME USED AVAIL REFER MOUNTPOINT tank/a 300M 169G 300M /a so USED and REFER were identical - there's no evidence of any significant space used by snapshots there. Which matches what I would expect based on usage. I have 2 similar machines, in both cases I removed 35k snapshots; in both cases I came back to find that I had another 70G free in the pool. And yes, I can match up the steps on the free space graph with when the snapshots were destroyed. This isn't really a problem, I was just curious because the result was unexpected. > On Thu, May 12, 2016 at 11:14 AM, Dan McDonald <[email protected]> wrote: > >> >> > On May 12, 2016, at 7:01 AM, Peter Tribble <[email protected]> >> wrote: >> > >> > Does anyone know what the overhead of a zfs snapshot is? >> > >> > I'm not talking about the data, I'm talking about whatever space >> > zfs needs to allocate internally for housekeeping. >> > >> > A quick estimate indicates something around 2M of overhead. >> > >> > (I've just deleted ~35k snapshots on a system, each taking no space. >> > I was a little surprised - although pleased - to get ~70G of space back >> > in the pool.) >> >> I'd highly recommend asking this question and sharing your observations >> with the ZFS developer's list. In fact, why don't I just do that for you... >> >> Dan >> > > -- -Peter Tribble http://www.petertribble.co.uk/ - http://ptribble.blogspot.com/ ------------------------------------------- openzfs-developer Archives: https://www.listbox.com/member/archive/274414/=now RSS Feed: https://www.listbox.com/member/archive/rss/274414/28015062-cce53afa Modify Your Subscription: https://www.listbox.com/member/?member_id=28015062&id_secret=28015062-f966d51c Powered by Listbox: http://www.listbox.com
