snapshot implementation

Patrick Proniewski patpro at
Tue Dec 29 11:22:21 UTC 2009

On 29 déc. 2009, at 11:51, Ronald Klop wrote:

>>> That's certainly not the case with UFS snapshots, which can take a  
>>> long
>>> time to complete (we're talking freezing your machine's disk  
>>> activity
>>> for many minutes), and are limited to 20 total.
>> UFS uses copy on write. But you say many minutes to complete? Don't  
>> you
>> speak about dump(1), that uses snapshot as a basis to dump a live  
>> file
>> system?
>> I agree, UFS snapshot creation is not lightning-fast, but many  
>> minutes
>> seems a lot to me, and I never experienced such a long creation time.
> As far as I know UFS snapshots need to create a list of currently in  
> use blocks. This is O(n) on the size of the FS and pauses the FS  
> during the snapshot. On large FS's this can take a long time.
> ZFS always maintains this list so it only needs to mark this list as  
> readonly to create a snapshot. This is O(1).

That's a very interesting precision. Thank you.


More information about the freebsd-fs mailing list