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bug#51433: cp 9.0 sometimes fails with SEEK_DATA/SEEK_HOLE

From: Paul Eggert
Subject: bug#51433: cp 9.0 sometimes fails with SEEK_DATA/SEEK_HOLE
Date: Sun, 31 Oct 2021 22:52:04 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.1.2

On 10/31/21 09:13, Pádraig Brady wrote:

The attached uses statfs()->f_type which is usually available,
to avoid using SEEK_DATA on ZFS.  This should be fairly lightweight I think,
and only used for files that might be sparse.

Couldn't we be even lazier about invoking fstatfs? It needs to be invoked only if the file appears to be sparse (as you mentioned), and also only if lseek+SEEK_DATA fails (with errno==ENXIO) at a position less than the file's length, and also at most once per input file descriptor.

When copying multiple files (e.g., cp -r) we could also cache the previous file's st_dev and ZFS status, so in the typical case we could skip fstatfs entirely except for the first sparsish file that satisfies the abovementioned criteria.

Also, as I understand it the problem occurs with OpenZFS but not on Solaris or its descendants, so we don't need to do any of this if __sun is defined.

Also, if we're going to use fstatfs it'd probably be better to hoist all the fstatfs portability mess out of stat.c and into a new file (say, gl/lib/pstatfs.c) that gives us a portable way of getting statfs-like data out of filesystems, and using that new file in both stat.c and copy.c.

One more idea: I've seen reports that the problem goes away if you use 'read' to read just the first byte of the false hole; that might be a simpler workaround than getting into the fstatfs portability mess.

We could also disable SEEK_DATA for remote file systems,
but that would be a pity since SEEK_DATA seems especially useful there.


Don't know if you're following <https://github.com/openzfs/zfs/issues/11900> but rincebrain reports a working workaround on the OpenZFS side tho more testing needs to be done. This OpenZFS bugs affects GNU tar and star (and surely other programs) as well as coreutils, and we may be better off overall if we merely ask people to fix their ZFS implementation rather than our trying to work around the bug in coreutils.

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