From mboxrd@z Thu Jan 1 00:00:00 1970 References: <569F78FA.4050104@redhat.com> <569F854C.5070404@gmail.com> From: Zdenek Kabelac Message-ID: <569F879B.8070401@redhat.com> Date: Wed, 20 Jan 2016 14:11:55 +0100 MIME-Version: 1.0 In-Reply-To: <569F854C.5070404@gmail.com> Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] LVM and chain of snapshots 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"; format="flowed" To: "Alexander E. Patrakov" , LVM general discussion and development , =?UTF-8?B?0JPQtdC+0YDQs9C40Lkg0JHQsNC20YPQutC+0LI=?= Dne 20.1.2016 v 14:02 Alexander E. Patrakov napsal(a): > 20.01.2016 17:09, Zdenek Kabelac wrote: >> Using lots of old-snaphosts is very bad plan - really time to look at >> thin-provisioning.... >> Old-snaps are not going to scale..... > > Well, I have already tried using lots of thin snapshots on my PC, in a context > different from what Mark wants. Namely, my snapshots were created by Snapper, > hourly. > > Thin snapshots may indeed scale well for reads and writes, but currently they > don't scale at all for the initial activation. The problem is that "vgchange > -ay" and "vgchange -an", by default, run "thin_check", and the more snapshots > I have, the more it takes for "thin_check" to finish. With 1 snapshot, it is > quick enough so that I don't notice. But with, say, 200 snapshots (some of > which include creation of an iso image), it takes more than 1.5 minutes - more > than the initramfs (or myself) is going to wait during the boot process. Well the execution of thin_check is 'rather' security feature. It's been added to capture possible errors of kernel driver. I guess now we are possibly in the age where deep checking might no longer be necessary. So if you feel the time spend on thin_checking doesn't pay-off - you can try to add option '--skip-mappings' (see lvm.conf field global/thin_check_options) Also you really should not keep 200 snapshots if you don't need them. So probably try to maintain your snapshots better (drop unneeded ones...) And final note - I do not believe activation of multi-gigabyte old-snapshot is going to give you any better results - since reading of old metadata is actually much slower then management of thin-pool & thin_check - just try it.... It's fairly naive to expect old-snaps will do any better job when we are talking about 200 snaps. Also there is no support for snaps of snaps of snaps with old-snapshot, so it's not even comparable.... Regards Zdenek