From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx1.redhat.com (ext-mx08.extmail.prod.ext.phx2.redhat.com [10.5.110.12]) by int-mx10.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id oAFI8VZa016614 for ; Mon, 15 Nov 2010 13:08:31 -0500 Received: from q7.q7.com (q7.q7.com [69.168.48.248]) by mx1.redhat.com (8.13.8/8.13.8) with ESMTP id oAFI8Gcn018576 for ; Mon, 15 Nov 2010 13:08:17 -0500 Received: from charon.spiretech.com (charon.spiretech.com [69.168.63.193]) (authenticated bits=0) by q7.q7.com (8.13.8/8.13.8) with ESMTP id oAFI8CWD022515 (version=TLSv1/SSLv3 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 15 Nov 2010 10:08:16 -0800 Message-ID: <4CE1770B.8000007@q7.com> Date: Mon, 15 Nov 2010 10:08:11 -0800 From: Joe Pruett MIME-Version: 1.0 References: <4CDDBF95.9040104@q7.com> <4CDDCF7E.4070006@q7.com> <4CDDE2CE.90907@bmsi.com> In-Reply-To: Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] advice for curing terrible snapshot performance? 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: Content-Type: text/plain; charset="us-ascii" To: LVM general discussion and development >> BTW, rewriting that 1G file would be normal speed, since >> the modified chunks have already been copied to the snapshot. > i'd think that, and you'd think that, but it is not the case. most of > my tests were done by rewriting the file 4x, and while the snap %used > (monitored with the 'lvs' command) doesn't keep going up, performance > stays the same. > i did similar tests and i saw that even though i was rewriting the file, it appeared that the ext3 layer was reallocating new blocks. the snapshot usage would increase to show 3x the number of blocks i had thought it should have touched, and then it seemed to stay stable. this is one of the problems with a block level (as opposed to file level) snapshot.