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From: "Richard W.M. Jones" <rjones@redhat.com>
To: Kevin Wolf <kwolf@redhat.com>
Cc: Hu Tao <hutao@cn.fujitsu.com>,
	qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Yasunori Goto <y-goto@jp.fujitsu.com>
Subject: Re: [Qemu-devel] [PATCH v12 0/6] qcow2, raw: add preallocation=full and preallocation=falloc
Date: Fri, 22 Aug 2014 16:34:40 +0100	[thread overview]
Message-ID: <20140822153440.GK1302@redhat.com> (raw)
In-Reply-To: <20140822152233.GP32377@noname.redhat.com>

On Fri, Aug 22, 2014 at 05:22:33PM +0200, Kevin Wolf wrote:
> It's still useful because it happens to reduce the overhead in most
> implementations and it's a relatively quick operation, but the best way
> I know of to actually _fully_ preallocate is still writing zeros. Which
> of the two the user wants, is a decision that qemu can't make for them.

This is a difficult situation.  Possibly the choice is between

 - efficiently make the file fully allocated, that works in the vast
   majority of cases, but don't go crazy (ie. fallocate)

 - really really try as hard as possible to make sure that future
   allocations will never fail (ie. write random non-zero data to the
   file)

Note that neither of these is the preallocation=... option as
specified in this patch.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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software inside the virtual machine.  Supports Linux and Windows.
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  reply	other threads:[~2014-08-22 15:34 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-07-11  6:09 [Qemu-devel] [PATCH v12 0/6] qcow2, raw: add preallocation=full and preallocation=falloc Hu Tao
2014-07-11  6:09 ` [Qemu-devel] [PATCH v12 1/6] block: round up file size to nearest sector Hu Tao
2014-08-22 10:55   ` Kevin Wolf
2014-08-25  1:11     ` Hu Tao
2014-07-11  6:09 ` [Qemu-devel] [PATCH v12 2/6] raw, qcow2: don't convert file size to sector size Hu Tao
2014-07-11  6:10 ` [Qemu-devel] [PATCH v12 3/6] rename parse_enum_option to qapi_enum_parse and make it public Hu Tao
2014-07-11  6:10 ` [Qemu-devel] [PATCH v12 4/6] qapi: introduce PreallocMode and a new PreallocMode full Hu Tao
2014-08-22 10:57   ` Kevin Wolf
2014-08-25  1:12     ` Hu Tao
2014-07-11  6:10 ` [Qemu-devel] [PATCH v12 5/6] raw-posix: Add falloc and full preallocation option Hu Tao
2014-08-22 10:58   ` Kevin Wolf
2014-08-25  1:18     ` Hu Tao
2014-07-11  6:10 ` [Qemu-devel] [PATCH v12 6/6] qcow2: " Hu Tao
2014-07-11 21:07   ` Max Reitz
2014-08-22 11:00   ` Kevin Wolf
2014-08-25  1:36     ` Hu Tao
2014-07-28  8:48 ` [Qemu-devel] [PATCH v12 0/6] qcow2, raw: add preallocation=full and preallocation=falloc Hu Tao
2014-08-22 10:54   ` Kevin Wolf
2014-08-25  1:35     ` Hu Tao
2014-08-26 10:44       ` Stefan Hajnoczi
2014-08-28  5:04         ` Hu Tao
2014-08-22 12:25   ` Richard W.M. Jones
2014-08-22 12:36     ` Daniel P. Berrange
2014-08-22 13:13     ` Kevin Wolf
2014-08-22 13:26       ` Richard W.M. Jones
2014-08-22 14:20       ` Daniel P. Berrange
2014-08-22 15:22         ` Kevin Wolf
2014-08-22 15:34           ` Richard W.M. Jones [this message]
2014-08-22 15:39             ` Richard W.M. Jones
2014-08-22 15:53             ` Kevin Wolf
2014-08-22 16:00               ` Richard W.M. Jones
2014-08-25  5:18                 ` Hu Tao
2014-08-25 10:31                   ` Richard W.M. Jones
2014-08-25 13:44                   ` Richard W.M. Jones
2014-08-26  5:27                 ` Hu Tao

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