[Sheepdog] object sizes

[Liu Yuan]

On 11/12/2011 01:43 AM, Christoph Hellwig wrote:

> On Sat, Nov 12, 2011 at 12:04:47AM +0800, Liu Yuan wrote:
>> I am not sure what is reflink. is it synonym to snapshot? IIC, yes, the
>> new storage infrastructure named 'farm' will support clsuter-wide snapshot.
> 
> It basically is a file level snapshot, see here for an introduction:
> 
> 	http://oss.oracle.com/osswiki/OCFS2/DesignDocs/ReflinkOperation
> 


Thanks for the URL.

>> Simply put, it somewhat resembles git a lot (both code and idea level).
>> there are three object type, named 'data, trunk, snapshot' that is
>> similar to git's 'blob, tree, commit'.
>>
>> 'data' is just sheepdog's data object, only named by its sha1ed content.
>> So the data objects with the same content will be mapped to only single
>> sha1 name, thus achieve node-wide data sharing.
>>
>> 'snapshot' object will serve to support snapshot which contains the
>> snapshoted trunk, that is 'directory' of the that-time data objects on
>> each node. The trunk object will provide a means to find data objects.
>> This will support cluster-wide snapshot.
>>
>> The 'farm' doesn't has any constraint to object data size. Hope this helps.
> 
> How well do snapshots / COW images work with large objects?
> 
> Either way, I'm looking forward to see your work.
> 


I can not give a simple answer yet before I finish it. For your
description about large data size, I have no doubt that it will benefit
rotational disk utilizing the underlying extent-like feature and
neutralize the seek overhead, but how about network performance for big
data transfer? It seems to me that we trade off the fragmentation
against wasteful bandwidth. A single 128MB object will fill the 1000Mb
nic to the full, so parallel data transfer seems very restricted. Being
that said, though I don't have much thought on this issue and my
networking knowledge is not qualified to make the pertinent remark.

Thanks,
Yuan