running mksnap_ffs

[Scott Oertel]

Kris Kennaway wrote:
> On Thu, Jan 11, 2007 at 11:25:34AM -0800, Scott Oertel wrote:
>   
>> Kris Kennaway wrote:
>>     
>>> On Tue, Jan 02, 2007 at 09:06:24PM +0100, Willem Jan Withagen wrote:
>>>  
>>>       
>>>> Hi,
>>>>
>>>> I got the following Filesystem:
>>>> Filesystem    Size    Used   Avail Capacity iused     ifree %iused 
>>>> /dev/da0a     1.3T    422G    823G    34%  565952 182833470    0%
>>>>
>>>> Running of a 3ware 9550, on a dual core Opteron 242 with 1Gb.
>>>> The system is used as SMB/NFS server for my other systems here.
>>>>
>>>> I would like to make weekly snapshots, but manually running mksnap_ffs 
>>>> freezes access to the disk (I sort of expected that) but the process 
>>>> never terminates. So I let is sit overnight, but looking a gstat did not 
>>>> reveil any activity what so ever...
>>>> The disk was not released, mksnap_ffs could not be terminated.
>>>> And things resulted in me rebooting the system.
>>>>
>>>> So:
>>>> - How long should I expect making a snapshot to take:
>>>> 	5, 15, 30min, 1, 2 hour or even more???
>>>>    
>>>>         
>>> Yes :) Snapshots were not designed for use in this way (they were
>>> designed to support background fsck and allow faster system recovery
>>> after power failure), so they don't scale as well as you might like on
>>> large filesystems.
>>>
>>> Kris
>>>  
>>>       
>> If snapshots were designed to support background fsck, then why did they 
>> not make it more scalable? If you can't create a snapshot without the 
>> system locking up, that means fsck won't be able to either, making 
>> background fsck worthless for systems with large storage.
>>     
>
> locking up != taking a long time to complete.  You haven't
> differentiated between those two situations yet.
>
> Kris
>   


It depends, sometimes it just takes a really long time during which the 
system is unresponsive and unstable, or it just completely locks up. 
Does it make that much of a difference? in either case, snapshotting 
large drives is not very efficient, and can't be considered for 
background fsck, or daily backup. Which are the two main purposes of 
snapshots.


--Scott