Re: Ext4 and xfs problems in dm-thin on allocation and discard

[Eric Sandeen <sandeen@redhat.com>]

On 6/19/12 9:19 AM, Ted Ts'o wrote:
> On Tue, Jun 19, 2012 at 04:09:48PM +0200, Lukáš Czerner wrote:
>>
>> With thin provisioning you'll get totally different file system
>> layout than on fully provisioned disk as you push more and more
>> writes to your drive. This unfortunately has great impact on
>> performance since file systems usually have a lot of optimization on
>> where to put data/metadata on the drive and how to read them.
>> However in case of thinly provisioned storage those optimization
>> would not help. And yes, you just have to expect lower performance
>> with dm-thin from the file system on top of it. It is not and it
>> will never be ideal solution for workloads where you expect the best
>> performance.
> 
> One of the things which would be nice to be able to easily set up is a
> configuration where we get the benefits of thin provisioning with
> respect to snapshost, but where the underlying block device used by
> the file system is contiguous.  That is, it would be really useful to
> *not* use thin provisioning for the underlying file system, but to use
> thin provisioned snapshots.  That way we only pay the thinp
> performance penalty for the snapshots, and not for normal file system
> operations.  This is something that would be very useful both for ext4
> and xfs.

I agree, and have asked for exactly the same thing... though I have no
idea how hard it is to disentangle allocation-aware snapshots from thing
provisioned storage.

-Eric
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