From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jamie Lokier Subject: Re: [PATCH 1/2] fs: add SEEK_HOLE and SEEK_DATA flags Date: Sun, 24 Apr 2011 18:49:02 +0100 Message-ID: <20110424174902.GA20461@shareable.org> References: <1303414954-3315-1-git-send-email-josef@redhat.com> <20110422045054.GB17795@infradead.org> <20110422112852.GB1627@x4.trippels.de> <4DB16B72.1050702@redhat.com> <4DB1AC9D.3010706@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Eric Blake , Markus Trippelsdorf , Christoph Hellwig , Josef Bacik , linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Sunil Mushran Return-path: Content-Disposition: inline In-Reply-To: <4DB1AC9D.3010706@oracle.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org Sunil Mushran wrote: > On 04/22/2011 04:50 AM, Eric Blake wrote: > >That blog also mentioned the useful idea of adding FIND_HOLE and > >FIND_DATA, not implemented in Solaris, but which could easily be > >provided as additional lseek constants in Linux to locate the start of > >the next chunk without repositioning and which could ease application > >programmer's life a bit. After all, cp wants to know where data ends > >without repositioning (FIND_HOLE), read() that much data which > >repositions in the process, then skip to the next chunk of data > >(SEEK_DATA) - two lseek() calls per iteration if we have 4 constants, > >but 3 per iteration if we only have SEEK_HOLE and have to manually rewind. > > while(1) { > read(block); > if (block_all_zeroes) > lseek(SEEK_DATA); > } > > What's wrong with the above? If this is the case, even SEEK_HOLE > is not needed but should be added as it is already in Solaris. Apart from the obvious waste of effort (scanning *all* data for zeros is cheap but not free if the file is mostly non-hole zeros), you can't do a pread() version of the above in parallel over different parts of the same file/device. > My problem with FIND_* is that we are messing with the well understood > semantics of lseek(). fcntl() looks a better fit for FIND_HOLE/DATA anyway. -- Jamie