suspend writes for external snapshot

[Eric Anderson]

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.


Eric




-- 
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Eric Anderson        Sr. Systems Administrator        Centaur Technology
Anything that works is better than anything that doesn't.
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