linux-ext4 - Re: inline_data status (e2fsprogs)

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Date:   Mon, 16 Sep 2019 11:20:20 +0200
From:   Pas <pas@...g.hu>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: inline_data status (e2fsprogs)

Thanks for the blazing fast and detailed reply!

Am I correct assuming that the jbd2 problem is only (mostly?) relevant
in case of data=journal mode?

> It also generally doesn't buy you much for most file system
workloads, so it hasn't been high on my priority list to fix.

Completely understandable. I wasn't really hunting for those juice
performance bits, however it seemed like a nice optimization that's in
the kernel for quite long, plus enabling it is just a bit flip, then
why not? But then I haven't found anything conclusive on it.

Thanks again for clearing things up!

Pas

On Sun, 15 Sep 2019 at 01:28, Theodore Y. Ts'o <tytso@....edu> wrote:
>
> On Sat, Sep 14, 2019 at 08:06:04PM +0200, Pas wrote:
> >
> > I hope this is the correct forum to ask these questions. (If not, then
> > sorry for the noise! Though then could you recommend where to ask
> > them?)
>
> There are some known issues with with the inline_data feature; in
> particular, it violates some of the jbd2 rules about how to jorunal
> data blocks.  As such, bad things(tm) can happen on crash recovery.
> For the most part it works OK for the original intended use case, was
> for bigalloc file systems where there was a desire to handle small
> directories more efficiently, which was how the developer who created
> the feature used said feature.  I didn't realize this particular
> rather serious bug until after the feature went into the kernel, and
> it's been on my todo list to fix; I just haven't had the time.
>
> It doesn't happen all that often; you need to start files that are
> small enough such that they can fit in an inline_data file, and then
> grow them via an appending write so that they need to be shifted to an
> block allocated file, and then force an unclean shutdown at an
> inopportune time.
>
> But yeah, there is a good reason why it's not a default-enabled
> feature.  It also generally doesn't buy you much for most file system
> workloads, so it hasn't been high on my priority list to fix.
>
> Regards,
>
>                                         - Ted