Re: [PATCH v2] Documenting the crash-recovery guarantees of Linux file systems

Amir Goldstein
2019-03-15T03:45:03Z

On Fri, Mar 15, 2019 at 5:03 AM Dave Chinner <david@fromorbit.com> wrote:
>
> On Thu, Mar 14, 2019 at 09:19:03AM +0200, Amir Goldstein wrote:
> > On Thu, Mar 14, 2019 at 3:19 AM Dave Chinner <david@fromorbit.com> wrote:
> > > On Tue, Mar 12, 2019 at 02:27:00PM -0500, Jayashree wrote:
> > > > +Strictly Ordered Metadata Consistency
> > > > +-------------------------------------
> > > > +With each file system providing varying levels of persistence
> > > > +guarantees, a consensus in this regard, will benefit application
> > > > +developers to work with certain fixed assumptions about file system
> > > > +guarantees. Dave Chinner proposed a unified model called the
> > > > +Strictly Ordered Metadata Consistency (SOMC) [5].
> > > > +
> > > > +Under this scheme, the file system guarantees to persist all previous
> > > > +dependent modifications to the object upon fsync(). If you fsync() an
> > > > +inode, it will persist all the changes required to reference the inode
> > > > +and its data. SOMC can be defined as follows [6]:
> > > > +
> > > > +If op1 precedes op2 in program order (in-memory execution order), and
> > > > +op1 and op2 share a dependency, then op2 must not be observed by a
> > > > +user after recovery without also observing op1.
> > > > +
> > > > +Unfortunately, SOMC's definition depends upon whether two operations
> > > > +share a dependency, which could be file-system specific. It might
> > > > +require a developer to understand file-system internals to know if
> > > > +SOMC would order one operation before another.
> > >
> > > That's largely an internal implementation detail, and users should
> > > not have to care about the internal implementation because the
> > > fundamental dependencies are all defined by the directory heirarchy
> > > relationships that users can see and manipulate.
> > >
> > > i.e. fs internal dependencies only increase the size of the graph
> > > that is persisted, but it will never be reduced to less than what
> > > the user can observe in the directory heirarchy.
> > >
> > > So this can be further refined:
> > >
> > > If op1 precedes op2 in program order (in-memory execution
> > > order), and op1 and op2 share a user visible reference, then
> > > op2 must not be observed by a user after recovery without
> > > also observing op1.
> > >
> > > e.g. in the case of the parent directory - the parent has a link
> > > count. Hence every create, unlink, rename, hard link, symlink, etc
> > > operation in a directory modifies a user visible link count
> > > reference. Hence fsync of one of those children will persist the
> > > directory link count, and then all of the other preceeding
> > > transactions that modified the link count also need to be persisted.
> > >
> >
> > One thing that bothers me is that the definition of SOMC (as well as
> > your refined definition) doesn't mention fsync at all, but all the examples
> > only discuss use cases with fsync.
>
> You can't discuss operational ordering without a point in time to
> use as a reference for that ordering. SOMC behaviour is preserved
> at any point the filesystem checkpoints itself, and the only thing
> that changes is the scope of that checkpoint. fsync is just a
> convenient, widely understood, minimum dependecy reference point
> that people can reason from. All the interesting ordering problems
> come from minimum dependecy reference point (i.e. fsync()), not from
> background filesystem-wide checkpoints.
>

Yes, I was referring to rename as a commonly used operation used
by application as "metadata barrier".

> > I personally find SOMC guaranty *much* more powerful in the absence
> > of fsync. I have an application that creates sparse files, sets xattrs, mtime
> > and moves them into place. The observed requirement is that after crash
> > those files either exist with correct mtime, xattr or not exist.

I wasn't clear:
1. "sparse" meaning no data at all only hole.
2. "exist" meaning found at rename destination
Naturally, its applications responsibility to cleanup temp files that were
not moved into rename destination.

>
> SOMC does not provide the guarantees you seek in the absence of a
> known data synchronisation point:
>
> a) a background metadata checkpoint can land anywhere in
> that series of operations and hence recovery will land in an
> intermediate state.

Yes, that results in temp files that would be cleaned up on recovery.

>
> b) there is data that needs writing, and SOMC provides no
> ordering guarantees for data. So after recovery file could
> exist with correct mtime and xattrs, but have no (or
> partial) data.
>

There is no data in my use case, only metadata, that is why
SOMC without fsync is an option.

> > To my understanding, SOMC provides a guaranty that the application does
> > not need to do any fsync at all,
>
> Absolutely not true. If the application has atomic creation
> requirements that need multiple syscalls to set up, it must
> implement them itself and use fsync to synchronise data and metadata
> before the "atomic create" operation that makes it visible to the
> application.
>
> SOMC only guarantees what /metadata/ you see at a fileystem
> synchronisation point; it does not provide ACID semantics to a
> random set of system calls into the filesystem.
>

So I re-state my claim above after having explained the use case.
IMO, SOMC guaranties is an important feature even in the absence
of any fsync, because of the ability to use some metadata operation
(e.g. rename, link) as metadata barriers.
I am wrong about this?

Thanks,
Amir.