I think the other part of information that's missing is that we COW at
the block level, NOT at the file level.

So, the extra blocks are in use only during the update, and it's only
blocks, not whole files...

Hope this helps..

Nathan.


On Thu, 2006-07-13 at 14:10, Chad Lewis wrote:
> It uses extra space in the middle of the write, in order to hold the  
> new data, but once
> the write is complete, the space occupied by the old version is now  
> free for use.
> 
> ckl
> 
> On Jul 12, 2006, at 8:05 PM, Robert Chen wrote:
> 
> > I still could not understand why Copy on Write does not waste file  
> > system capacity.
> >
> > Robert
> >
> >
> > Raymond Xiong ??:
> >> Robert Chen wrote:
> >>> question is ZFS uses COW(copy on write), does this mean it will  
> >>> double usage of capacity or waste the capacity? What COW really  
> >>> do? No mirror also has COW? Please help me, thanks. Robert  
> >>> _______________________________________________ zfs-discuss  
> >>> mailing list zfs-discuss@opensolaris.org http:// 
> >>> mail.opensolaris.org/mailman/listinfo/zfs-discuss
> >> It doesn't. Page 11 of the following slides illustrates how COW  
> >> works in ZFS: http://www.opensolaris.org/os/community/zfs/docs/
> >> zfs_last.pdf "Blocks containing active data are never overwritten  
> >> in place; instead, a new block is allocated, modified data is  
> >> written to it, and then any metadata blocks referencing it are  
> >> similarly read, reallocated, and written. To reduce the overhead  
> >> of this process, multiple updates are grouped into transaction  
> >> groups, and an intent log is used when synchronous write semantics  
> >> are required."(from http://en.wikipedia.org/wiki/ZFS) IN snapshot  
> >> scenario, COW consumes much less disk space and is much faster.  
> >> Raymond _______________________________________________ zfs- 
> >> discuss mailing list zfs-discuss@opensolaris.org http:// 
> >> mail.opensolaris.org/mailman/listinfo/zfs-discuss
> >
> > _______________________________________________
> > zfs-discuss mailing list
> > zfs-discuss@opensolaris.org
> > http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
> 
> _______________________________________________
> zfs-discuss mailing list
> zfs-discuss@opensolaris.org
> http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
-- 

_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss

Reply via email to