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Date:   Fri, 3 May 2019 23:08:03 +0200
From:   David Disseldorp <ddiss@samba.org>
To:     ronnie sahlberg <ronniesahlberg@gmail.com>
Cc:     linux-cifs <linux-cifs@vger.kernel.org>,
        Steve French <smfrench@gmail.com>,
        Samba Technical <samba-technical@lists.samba.org>
Subject: Re: SMB2 SET_ZERO_DATA and PUNCH_HOLE
Message-ID: <20190503230803.58266ae2@samba.org>
In-Reply-To: <CAN05THSthhyGctyByj5eun6C_KK58xWgNYer+7TKBrqyObNZsw@mail.gmail.com>
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On Thu, 25 Apr 2019 17:23:56 +1000, ronnie sahlberg wrote:

> Folks.
> 
> In the cifs client  we use FSCTL_SET_ZERO_DATA when userspace wants to
> punch a hole in a file. SET_ZERO_DATA maps quite well to the
> PUNCH_HOLE semantics in that it will deallocate what it can and
> overwrite what it can not with 0.

Cool, I'd be interested to hear how things go when testing against
a Samba SMB2+ server - it similarly maps FSCTL_SET_ZERO_DATA to
PUNCH_HOLE.

> On windows 16/ntfs the deallocate blocksize is 64k.
> 
> Does anyone know if this is always 64k or if there is a way to query
> the server for this?

smbtorture4 includes a sparse_hole_dealloc test for checking this via
incremental ZERO_DATA + QAR requests:
https://git.samba.org/?p=samba.git;a=blob;f=source4/torture/smb2/ioctl.c#l3981

I don't recall seeing anything non-64k at the time, but I didn't check
that thoroughly.

Cheers, David