From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [140.186.70.92] (port=39497 helo=eggs.gnu.org) by lists.gnu.org with esmtp (Exim 4.43) id 1PiW00-00076I-EC for qemu-devel@nongnu.org; Thu, 27 Jan 2011 12:51:13 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1PiVzz-0007M4-4N for qemu-devel@nongnu.org; Thu, 27 Jan 2011 12:51:12 -0500 Received: from mail-qy0-f173.google.com ([209.85.216.173]:40370) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1PiVzy-0007Ly-Tt for qemu-devel@nongnu.org; Thu, 27 Jan 2011 12:51:11 -0500 Received: by qyl38 with SMTP id 38so7567849qyl.4 for ; Thu, 27 Jan 2011 09:51:10 -0800 (PST) Message-ID: <4D41B07E.2030508@codemonkey.ws> Date: Thu, 27 Jan 2011 11:50:54 -0600 From: Anthony Liguori MIME-Version: 1.0 Subject: Re: [Qemu-devel] [PATCH] qcow2: Add full image preallocation option References: <1296143534-13495-1-git-send-email-kwolf@redhat.com> <20110127155824.GC20364@redhat.com> In-Reply-To: <20110127155824.GC20364@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit List-Id: qemu-devel.nongnu.org List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Daniel P. Berrange" Cc: Kevin Wolf , qemu-devel@nongnu.org On 01/27/2011 09:58 AM, Daniel P. Berrange wrote: > On Thu, Jan 27, 2011 at 04:52:14PM +0100, Kevin Wolf wrote: > >> This adds a preallocation=full mode to qcow2 image creation, which does not >> only allocate metadata for the whole image, but also writes zeros to it, >> creating a non-sparse image file. >> >> Signed-off-by: Kevin Wolf >> --- >> block/qcow2.c | 45 ++++++++++++++++++++++++++++++++++++++++----- >> 1 files changed, 40 insertions(+), 5 deletions(-) >> >> diff --git a/block/qcow2.c b/block/qcow2.c >> index a1773e4..90cf2ca 100644 >> --- a/block/qcow2.c >> +++ b/block/qcow2.c >> @@ -838,7 +838,15 @@ static int qcow2_change_backing_file(BlockDriverState *bs, >> return qcow2_update_ext_header(bs, backing_file, backing_fmt); >> } >> >> -static int preallocate(BlockDriverState *bs) >> +enum prealloc_mode { >> + PREALLOC_OFF = 0, >> + PREALLOC_METADATA, >> + PREALLOC_FULL, >> +}; >> + >> +#define IO_BUF_SIZE (2 * 1024 * 1024) >> + >> +static int preallocate(BlockDriverState *bs, enum prealloc_mode mode) >> { >> uint64_t nb_sectors; >> uint64_t offset; >> @@ -846,11 +854,14 @@ static int preallocate(BlockDriverState *bs) >> int ret; >> QCowL2Meta meta; >> >> + assert(mode != PREALLOC_OFF); >> + >> nb_sectors = bdrv_getlength(bs)>> 9; >> offset = 0; >> QLIST_INIT(&meta.dependent_requests); >> meta.cluster_offset = 0; >> >> + /* First allocate metadata in _really_ big chunks */ >> while (nb_sectors) { >> num = MIN(nb_sectors, INT_MAX>> 9); >> ret = qcow2_alloc_cluster_offset(bs, offset, 0, num,&num,&meta); >> @@ -874,6 +885,28 @@ static int preallocate(BlockDriverState *bs) >> offset += num<< 9; >> } >> >> + /* Then write zeros to the cluster data, if requested */ >> + if (mode == PREALLOC_FULL) { >> + void *buf = qemu_mallocz(IO_BUF_SIZE); >> + >> + nb_sectors = bdrv_getlength(bs)>> BDRV_SECTOR_BITS; >> + offset = 0; >> + >> + while (nb_sectors) { >> + num = MIN(nb_sectors, IO_BUF_SIZE / BDRV_SECTOR_SIZE); >> + ret = bdrv_write(bs, offset>> BDRV_SECTOR_BITS, buf, num); >> > Is there a way you can calculate the total size of the qcow2 > file upfront, and just use a single posix_fallocate() call to > do the zero-filled allocation of all the data blocks. It is > many orders of magnitude faster than truely writing blocks of > zero'd data on modern filesystems. I guess if you're using > compression or encryption, we'd really have to go the slow > path, but for regular usage it'd be better to take a fast > path. > Hrm, so is the intention here to avoid sparse files or to not assume zero-fill? Regards, Anthony Liguori > Daniel > >