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Subject: Re: posix_fallocate on ZFS
From: Ian Lepore <ian@freebsd.org>
To: Alan Somers <asomers@freebsd.org>, Willem Jan Withagen <wjw@digiware.nl>
Cc: freebsd current <freebsd-current@freebsd.org>
Date: Sat, 10 Feb 2018 12:43:19 -0700
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On Sat, 2018-02-10 at 11:24 -0700, Alan Somers wrote:
> On Sat, Feb 10, 2018 at 10:28 AM, Willem Jan Withagen 
> wrote:
> 
> > 
> > Hi,
> > 
> > This has been disabled on ZFS since last November.
> > And I do understand the rationale on this.
> > 
> > BUT
> > 
> > I've now upgraded some of my HEAD Ceph test systems and they now fail,
> > since Ceph uses posix_fallocate() to allocate space for the
> > FileStore-journal.
> > 
> > Is there any expectation that this is going to fixed in any near future?
> > 
> > --WjW
> > 
> No.  It's fundamentally impossible to support posix_fallocate on a COW
> filesystem like ZFS.  Ceph should be taught to ignore an EINVAL result,
> since the system call is merely advisory.
> 
> -Alan

Unfortunately, posix documents that the function returns EINVAL only
due to bad input parameters, so ignoring that seems like a bad idea.

Wouldn't it be better if we returned EOPNOTSUP if that's the actual
situation?  That could be safely ignored.

-- Ian