Date: Mon, 19 Jan 2015 18:04:42 +0100 From: Fabian Keil <freebsd-listen@fabiankeil.de> To: freebsd-questions@freebsd.org Subject: Re: ZFS and sparse file backed md devices Message-ID: <263b82a2.0cfdaef0@fabiankeil.de> In-Reply-To: <20150119160056.7c7ece19f5d0fccded7e913f@sohara.org> References: <20150119160056.7c7ece19f5d0fccded7e913f@sohara.org>
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--Sig_/w3jskIZlxm.Vo09E_hrb9b9 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable Steve O'Hara-Smith <steve@sohara.org> wrote: > I tried to follow the suggestions for converting a ZFS mirror (mine > was a three way mirror) to a RAIDZ (or in my case a RAIDZ2) when tight on > discs by creating a pool using sparse file backed md devices to stand in > for the missing discs. Fortunately I experimented with a dry run using > nothing but sparse file backed md devices first. >=20 > I'm using FreeBSD 10.1-RELEASE-p3. >=20 > The first surprise was when I created four 2TB sparse file backed > md devices using truncate and mdconfig and then tried to make a zfs pool > out of them. The sparse files became not sparse - or at least tried to but > of course there wasn't 8TB of space to use in /tmp so it filled up and it > took a reboot to kill the zpool create run. Next experiment was more > modest, four 128MB sparse files, sure enough once the zpool create finish= ed > they were four 128MB files and not sparse. Creating a pool on real discs > certainly doesn't write on all the blocks - so why did my sparse files get > filled in ? My first suspect would be vdev trimming. On recent FreeBSD releases it's enabled by default, even if none of the disks actually support trimming. I set vfs.zfs.trim.enabled=3D0 on all my systems were it doesn't work (all of them, as I use geli below ZFS). > A little more experimenting revealed that I could offline the 128MB > md devices one by one, destroy the device, truncate the file up to 2TB, > recreate the device, wipe the ZFS meta data and replace the offlined > device without filling in the sparse file. All was well until I did this = to > the fourth device and the pool tried to autoexpand - after a few seconds > the box locked up and became completely unresponsive to everything except > pings. Anybody have any idea why ? See above. =20 > At this point I decided that the sparse file method was a > non-starter and rebuilt my pool using four 1TB partitions out of the two > available drives, copied the data, and then replaced the partitions one by > one with whole drives[1], eventually winding up where I wanted to be with= my > three drive mirror converted to a four drive RAIDZ2. Still I am puzzled as > to why the sparse file md device route no longer works. I'm frequently using sparse files for testing and at least for me it works as expected: fk@r500 ~ $sudo mdconfig -f /tank/scratch/testfile md0 fk@r500 ~ $zogftw import 2015-01-19 17:52:18 zogftw: No pool name specified. Trying all unattached l= abels: test 2015-01-19 17:52:18 zogftw: No geli keyfile found at /home/fk/.config/zogft= w/geli/keyfiles/test.key. Not using any. 2015-01-19 17:52:21 zogftw: 'test' attached 2015-01-19 17:52:23 zogftw: 'test' imported fk@r500 ~ $zpool list NAME SIZE ALLOC FREE EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT tank 228G 196G 32.0G - 51% 85% 1.00x ONLINE - test 1016P 218K 1016P - 0% 0% 1.00x ONLINE - Fabian --Sig_/w3jskIZlxm.Vo09E_hrb9b9 Content-Type: application/pgp-signature Content-Description: OpenPGP digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iEYEARECAAYFAlS9ORkACgkQBYqIVf93VJ3JfwCguAvHIHBga+64R17Y/exzvGEa Rj8AnA7vr28sFUZSkXqvWiO7Xp+vPqtD =WRUC -----END PGP SIGNATURE----- --Sig_/w3jskIZlxm.Vo09E_hrb9b9--
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