Do I need to reserve disk space for UFS snapshots?

[Kris Kennaway]

On Wed, Apr 26, 2006 at 03:23:39PM -0500, Eric Anderson wrote:
> Kent Ho wrote:
> >Hi,
> >
> >I'm planing to use FreeBSD snapshots.  It seems to be the snapshop image 
> >is placed on the same file system.  The question is how do I manage or 
> >reserve space for the snapshots?  Can someone give me some guidelines or 
> >pointers.
> >
> >Here's my netapp it has a separate area allocated for the snapshots, I'm 
> >keeping 2 weekly snapshots in there(weekly.0 & weekly.1) using around 30G.
> >
> >Filesystem          kbytes       used      avail capacity  Mounted on
> >/vol/vol0/          305052552  272495320   32557232    89% /vol/vol0/
> >/vol/vol0/.snapshot  76263136   30658028   45605108    40% 
> >/vol/vol0/.snapshot
> >
> >Filesystem               iused      ifree  %iused  Mounted on
> >/vol/vol0/            10802387   89822586    11%   /vol/vol0/
> >
> >If copy all content in vol0 to a FreeBSD and make 2 snapshot will it be 
> >around the same? 272G + 30G = 302G?  So if I df on the FreeBSD box 
> >around 302G will be used?
> >
> >Any help and information is highly appreciated.
> 
> 
> FreeBSD snapshots do not work the same way as NetApp snapshots.  First, 
> they must reside on the same filesystem as the snapshotted filesystem. 
> They will initially eat up some space upon creation equal to the space 
> used by the cylinder group summaries and superblocks, but it isn't a 
> whole lot.  You'll need to have however much space is changed weekly for 
> each snapshot, so if 10Gb per week is changed, then you'll need that 
> much space per snapshot file done.

Also be aware that bad things happen (panics, etc) if your filesystem
fills up while snapshots are active!  This will hopefully be addressed
in a future release.

Kris
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