Date: Mon, 06 Nov 2017 14:52:18 -0800 From: John Baldwin <jhb@freebsd.org> To: Ngie Cooper <yaneurabeya@gmail.com> Cc: Andriy Gapon <avg@freebsd.org>, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r325320 - head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs Message-ID: <188800397.5yAOiIDbtG@ralph.baldwin.cx> In-Reply-To: <5199F871-5DE2-4434-87F2-FEF88DB34037@gmail.com> References: <201711021349.vA2Dn8Yg063559@repo.freebsd.org> <5199F871-5DE2-4434-87F2-FEF88DB34037@gmail.com>
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On Thursday, November 02, 2017 10:38:49 AM Ngie Cooper wrote: >=20 > > On Nov 2, 2017, at 06:49, Andriy Gapon <avg@FreeBSD.org> wrote: > >=20 > > Author: avg > > Date: Thu Nov 2 13:49:08 2017 > > New Revision: 325320 > > URL: https://svnweb.freebsd.org/changeset/base/325320 > >=20 > > Log: > > Disable posix_fallocate(2) for ZFS > >=20 > > The generic (naive) implementation of posix_fallocate cannot provi= de the > > standard mandated guarantee that overwrites would never fail due t= o the lack > > of free space. The fundamental reason is the copy-on-write archit= ecture > > of ZFS. Other features like compression and deduplication can als= o > > increase the size difference between the (pre-)allocated dummy con= tent > > and the future content. > >=20 > > So, until ZFS can properly implement the feature it's better to re= port > > that it is unsupported rather than providing an ersatz implementat= ion. > > Please note that EINVAL is used to report that the underlying file= system > > does not support the operation (POSIX.1-2008). > >=20 > > illumos and ZoL seem to do the same. > >=20 > > MFC after: 3 weeks > > Sponsored by: Panzura >=20 > It=E2=80=99d be nice if it worked though and was reported via the fil= e system. Posix suggests it should be, as of 2013: http://austingroupb= ugs.net/view.php?id=3D687 . >=20 > Need to go poking around and see what=E2=80=99s in freebsd later on t= onight. Bug filed: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D= 223383 . The austingroup link is just about adding a new pathconf() variable. posix_fallocate() would seem to be fundamentally incompatible with COW filesystems and can never work reliably for those. Even if you reserve= d N free blocks somehow until the first write, that doesn't allow multiple writes to the same file blocks while avoiding block allocation. falloc= ate only makes sense for overwriting filesystems like UFS. --=20 John Baldwin
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