From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from [10.34.131.19] (dhcp131-19.brq.redhat.com [10.34.131.19]) by int-mx14.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id s4MEho51027165 for ; Thu, 22 May 2014 10:43:50 -0400 Message-ID: <537E0D25.7010108@redhat.com> Date: Thu, 22 May 2014 16:43:49 +0200 From: Zdenek Kabelac MIME-Version: 1.0 References: <20140522101837.GB14236@redhat.com> In-Reply-To: <20140522101837.GB14236@redhat.com> Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] Testing the new LVM cache feature 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: LVM general discussion and development Dne 22.5.2014 12:18, Richard W.M. Jones napsal(a): > > I've set up a computer in order to test the new LVM cache feature. It > has a pair of 2 TB HDDs in RAID 1 configuration, and a 256 GB SSD. > The setup will be used to store large VM disk images in an ext4 > filesystem, to be served both locally and over NFS. > > Before I start I have some questions about this feature: > > (1) Is there a minimum recommended version of LVM or kernel to use? I > currently have lvm2-2.02.106-1.fc20.x86_64, which mentions LVM cache > in the lvm(8) man page. I have kernel 3.14.3-200.fc20.x86_64. With these new targets usually always applies - the newer the kernel and tools are - the better for you. > > (2) There is no lvmcache(7) man page in any released version of LVM2. > Was this man page ever created or is lvm(8) the definitive > documentation? It's now in upstream git as a separate man page (moved from lvm(8)) > (3) It looks as if cached LVs cannot be resized: > https://www.redhat.com/archives/lvm-devel/2014-February/msg00119.html > Will this be fixed in future? Is there any workaround -- perhaps Yes - cache is still missing a lot of feature - it needs further integration with tools like cache_check, cache_repair.... So far it's really only for a preview - I'd not consider to use it for anything serious yet. > removing the caching layer, resizing the original LV, then recreating > the cache? I really need to be able to resize LVs :-) Surely this feature will be implemented. Meanwhile - you have to drop cache, resize LV, reattach cache... (drop cache - means to remove cache) > (4) To calculate the size of the cache metadata LV, do I really just > divide by 1000, min 8 MB? It's that simple? Doesn't it depend on > dm-cache block size? Or dm-cache algorithm? How can I choose block > size and algorithm? Well this is where your experimenting may begin. However for now lvm2 doesn't allow you to play with algorithms - the lvchange interface is not yet upstream... > (5) Is there an explicit command for flushing the cache layer back to > the origin LV? To be developed... > (6) Is the on-disk format stable for future kernel/LVM upgrades? Well it's still experiemental - so if there will be found some huge problem, which requires to change/modify format it may happen. Zdenek