UFS2 snapshots on large filesystems

[user]

Eric,

Thanks a bunch for your comments.  See below:

On Sun, 6 Nov 2005, Eric Anderson wrote:

> I have several 2TB filesystems, which I do snapshots on.  I can report 
> that it indeed takes a long time to run, but works nevertheless.  One 
> thing to keep in mind though - fsck'ing a 2TB filesystem can take 2GB of 
> memory (depends on how many files you have, and a few other factors).  I 
> have 4GB of memory in this box, and what I've seen so far is between 
> 1.5GB and 2.5GB of memory required for fsck to run smoothly on a single 
> partition.  You definitely also want to turn off background fsck, unless 
> you have extreme amounts of time on your hands.  An fsck of a 70% full 
> 2TB filesystem with a ton of files on it takes many hours, so I often 
> mount the filesystems unclean (FreeBSD lets me do this) with rw, and 
> continue my work until I can unmount, fsck, and remount the fs.


Can you elaborate ?  Namely, how long on the 2GB filesystems ?

As far as the fsck is concerned, this only happens on an ungraceful
reboot, right ?  Assuming a snapshot on a 2GB FS, and assuming no crashes,
no long-wait processes like fsck will ever occur, right ?

Any other comments ?  Do you experience instability/crashes often on
systems of this nature ?

Again, thanks.