linux-kernel - Re: [dm-devel] LVM snapshot broke between 4.14 and 4.16

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Message-ID: <20180804052033.GA4461@thunk.org>
Date:   Sat, 4 Aug 2018 01:20:33 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     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,
        Zdenek Kabelac <zkabelac@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>, wgh@...lan.ru
Subject: Re: [dm-devel] LVM snapshot broke between 4.14 and 4.16

On Fri, Aug 03, 2018 at 03:30:37PM -0400, Mike Snitzer wrote:
> 
> I was trying to give context for the "best to update lvm2 anyway"
> disclaimer that was used.  Yeah, it was specious.

Well, it seemed to indicate a certain attitude that both Linus and I
are concerned about.  I tried to use more of a "pursuading" style to
impress why that attitude was not ideal/correct.  Linus used a much
more assertive style (e.g., "Hell, no!").

> And yeah, that isn't a good excuse to ignore it but: dm-snapshot is a
> steaming pile as compared to dm thin-provisioning...

On a side note, this is the first that I've heard the assertion that
dm-thin was better than dm-snapshot.  My impression was that
dm-snapshot was a proven code base, that only did one thing and (as
far as I could tell) did it well.  In contrast, dm-thin is much newer
code, **far** more complex, with functionality and corner cases
approaching that of a file system --- and just to be even more
exciting, it doesn't have an fsck/repair tool to deal with corrupted
metadata.

In your opinion, is it because you disagree with the assumption that
dm-thin is scary?  Or is the argument that dm-snapshot is even
scarier?

						- Ted

P.S.  It could be that my impression is wrong/out-dated, but the
kernel documentation still says that userspace tools for checking and
repairing the metadata are "under development".  As a file system
developer, the reaction this inspires is best summed up as:

     https://imgflip.com/memetemplate/50971393/Scared-Face