[linux-lvm] [External] Re: The feasibility of implementing an alternative snapshot approach

Zhiyong Ye yezhiyong at bytedance.com
Tue Jan 10 03:48:50 UTC 2023

Hi Zdenek,

Thank you for your patience and explanations. I learned a lot from our 
discussions and thank you again for your help.



On 1/10/23 6:18 AM, Zdenek Kabelac wrote:
> Dne 09. 01. 23 v 7:21 Zhiyong Ye napsal(a):
>> Hi Zdenek,
>> Thank you for your detailed answer.
>> For the thin snapshot I will use the latest version of kernel and lvm 
>> for further testing. I want to use both snapshot methods (thin and 
>> thick) in the production environment. But if the thick snapshot is 
>> only still in the maintenance phase, then for thick lv I have to see 
>> if there is any other way to accomplish the snapshot function.
> FYI - there are still some delays with up-streaming of the latest 
> improvement patches - so stay tuned for further speedup gains & IO 
> throughput with thin provisioning)
> By the maintenance phase for old thick snapshot I mean - the development 
> of the existing thick snapshot target is basically done - the format is 
> very ancient and cannot be changed without major rewrite of the whole 
> snapshot target as such - and that's what we've made with newly 
> introduced thin-provisioning target which addressed many shortcomings of 
> the old dm-snapshot target.
>> I use lvm mainly for virtualized environments. Each lv acts as a block 
>> device of the virtual machine. So I also consider using qemu's own 
>> snapshot feature. When qemu creates a snapshot, the original image 
>> used by the virtual machine becomes read-only, and all write changes 
>> are stored in the new snapshot. But currently qemu's snapshots only 
>> support files, not block devices.
> Depending on the use-case it might matter to pick the best fitting 
> chunk-size.
> i.e. if the changes are 'localized' in the filesystem areas to match 
> thin-pool chunks (also selection of the filesystem itself might be part 
> of equation here) - even if you use snapshots a lot, you may eventually 
> get better result with bigger chunks like 128k or even 256k size instead 
> of default 64K.
> Regards
> Zdenek

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