linux-kernel - Re: LVM snapshot broke between 4.14 and 4.16

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Date:   Sat, 4 Aug 2018 14:18:47 -0400
From:   Mike Snitzer <snitzer@...hat.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     Zdenek Kabelac <zkabelac@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Sagi Grimberg <sagi@...mberg.me>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>, dm-devel@...hat.com,
        Ilya Dryomov <idryomov@...il.com>, wgh@...lan.ru
Subject: Re: LVM snapshot broke between 4.14 and 4.16

On Sat, Aug 04 2018 at 12:22pm -0400,
Theodore Y. Ts'o <tytso@....edu> wrote:

> On Sat, Aug 04, 2018 at 10:36:50AM +0200, Zdenek Kabelac wrote:
> > dm-snapshost has really outdated design - it's been useful in the old age
> > where megabyte was hell lot of space.
> > 
> > Nowadays, when users do need to handle snapshots in multi gigabyte sizes and
> > moreover have number of snapshots from the same volume taken over the time,
> > want to take snapshot of snapshot of snapshot, the old snapshot simple kills
> > all the performance, uses tons of resources and becomes serious bottleneck
> > of your system and has lots of usability limitation.
> 
> Fair enough.  I don't think I would consider that makes dm-snapshot a
> "steaming pile".  For me, protection against data loss is Job One.

What's your point Ted?  Do you have _any_ intention of actually using
anything DM or is this just a way for you to continue to snipe at it?
 
> > That's where  thin provisioning will shine....
> 
> The dm-thin development might want to take a look at what's currently
> in Documentation/device-mapper/thin-privisioning.txt:
> 
>    Status
>    ======
> 
>    These targets are very much still in the EXPERIMENTAL state.  Please
>    do not yet rely on them in production.  But do experiment and offer us
>    feedback.  Different use cases will have different performance
>    characteristics, for example due to fragmentation of the data volume.
> 
>    If you find this software is not performing as expected please mail
>    dm-devel@...hat.com with details and we'll try our best to improve
>    things for you.
> 
>    Userspace tools for checking and repairing the metadata are under
>    development.
> 
> Saying that dm-snapshot is a steaming pile and dm-thin is what
> everyone should use doesn't seem to be consistent with the above.

Maybe read your email from earlier today before repeating yourself:
https://lkml.org/lkml/2018/8/4/366