From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx3.redhat.com (mx3.redhat.com [172.16.48.32]) by int-mx1.corp.redhat.com (8.13.1/8.13.1) with ESMTP id m54E7qML000726 for ; Wed, 4 Jun 2008 10:07:52 -0400 Received: from server.klug.on.ca (server.klug.on.ca [205.189.48.131]) by mx3.redhat.com (8.13.8/8.13.8) with ESMTP id m54E7eLi026072 for ; Wed, 4 Jun 2008 10:07:40 -0400 Received: from linux.interlinx.bc.ca (unknown [67.193.220.102]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by server.klug.on.ca (Postfix) with ESMTP id 970572803 for ; Wed, 4 Jun 2008 10:07:39 -0400 (EDT) Received: from [10.75.22.1] (pc.ilinx [10.75.22.1]) by linux.interlinx.bc.ca (Postfix) with ESMTP id B5F8C808A for ; Wed, 4 Jun 2008 10:07:37 -0400 (EDT) Subject: Re: [linux-lvm] ANNOUNCE: an experimental implementation of snapshot merging From: "Brian J. Murrell" In-Reply-To: References: <1212525167.29076.191.camel@pc.ilinx> Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="=-1PBN8MUlKJn3tHfTgY9r" Date: Wed, 04 Jun 2008 10:07:37 -0400 Message-Id: <1212588457.29076.252.camel@pc.ilinx> Mime-Version: 1.0 Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: To: LVM general discussion and development --=-1PBN8MUlKJn3tHfTgY9r Content-Type: text/plain Content-Transfer-Encoding: quoted-printable On Wed, 2008-06-04 at 07:07 -0400, Mikulas Patocka wrote: >=20 > > A pedantic example would be to create an origin, O1 and then a snapshot > > of O1 called S1. Make a bunch of changes to S1 and then create another > > snapshot of O1 called S2 and then block copy from S1 to S2 (presumably, > > although I don't know for sure, S2 only contains the same changed block= s > > as S1). >=20 > No, if you block copy over /dev/vg/snapshot device, it will contain all=20 > the blocks (regardless if they differ from the origin or not). So my premise of copying from S1 to S2 and the result that S2 would only contain real blocks for what differs to the origin and pointers for blocks that don't differ is false? > There could theoretically be compare function, testing if the write equal= s=20 > to actual data and dropping the write eventually --- but it would be too=20 > much coding overhead for too little practical advantage. For certain use cases I think this would be a great advantage. Think about this: 1. Create an LV and install a linux distro into it. 2. Make a snapshot and customize to the settings for a given configuration (i.e. a group of hosts) 3. Now I want to make an (initially) identical (to the snapshot customized in step 2) snapshot for each node in that configuration Being able to do: # lvcreate -L5G -n pristine node_group [ install linux distro into /dev/node_group/pristine ] # lvcreate -s -L$size -n config1_master pristine # for node in $nodelist; do > lvcreate -s -L$size -n $node pristine > dd if=3D/dev/node_group/config1_master of=3D/dev/node_group/$node > done And have each of those snapshots be as efficient as config1_master. > Snapshots are never shrunk (except when being merged) --- the metadata=20 > format doesn't allow to remove arbitrary exception from the exception=20 > table. Pity. b. --=-1PBN8MUlKJn3tHfTgY9r Content-Type: application/pgp-signature; name=signature.asc Content-Description: This is a digitally signed message part -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQBIRqGpl3EQlGLyuXARAhZwAJ0bce1ElP6US/V5gOjPSWbGo2vtPQCg0pcL wnUapbxwMhZ6HxiZIJgLbAU= =olBf -----END PGP SIGNATURE----- --=-1PBN8MUlKJn3tHfTgY9r--