suspend writes for external snapshot

[Scott Long]

Eric Anderson wrote:
> Robert wrote:
> 
>> Eric Anderson wrote:
>>
>>> Kris Kennaway wrote:
>>>
>>>> On Wed, Apr 26, 2006 at 04:07:32PM -0500, Robert wrote:
>>>>
>>>>> Greetings,
>>>>> I am trying to determine if there is a way to suspend write 
>>>>> activity on a FreeBSD filesystem (currently 6.0R) in order to take 
>>>>> a snapshot with external SAN software (Falconstor IPStor)?
>>>>
>>>>
>>>>
>>>> Use a FreeBSD snapshot and then image that?
>>>
>>>
>>>
>>> He can't do it that way because the Falconstor is a raw block device 
>>> and has no idea the difference between a snapshot and other 
>>> filesystem bits.
>>>
>>>
>>> Robert - we have some very alpha patches in house that allow 
>>> suspension of UFS filesystems for things like this (also working on 
>>> online UFS growing).  I'm not sure how close we really are to letting 
>>> it loose in the wild, but we will definitely be making all the 
>>> patches available to whomever wants it.
>>>
>>>
>>> Eric
>>>
>>>
>>>
>>
>> Sounds great thank you, I will keep an eye out for the patches.
>>
>> I am still unclear as to the necessity of suspending writes based on 
>> the following information from http://www.mckusick.com/softdep/
>>
>> "By ensuring that the only inconsistencies are unclaimed blocks or 
>> inodes, soft updates can eliminate the need to run a filesystem check 
>> program after every system crash. Instead, the system is brought up 
>> immediately.
>>
>> When it is convenient, a snapshot is taken and a background task can 
>> be run on on that snapshot to reclaim any lost blocks and inodes. The 
>> use of a snapshot allows normal filesystem activity to continue 
>> concurrently. "
>>
>> Does this mean that I can take a snapshot of an in use filesystem and 
>> the only ill effects will be the lost blocks and inodes which can be 
>> reclaimed with background fsck once the snapshot is booted?
> 
> 
> I believe so, yes.  Someone correct me here if I am wrong.
> 
> 

Ideally, yes.  But for the purposes of snapshots, you want the 
filesystem to be flushed.  Otherwise you might catch an in-progress
operation like a file add/delete/truncate that would be better off
finished before taking the snapshot

Scott