Date: Tue, 07 Jul 2009 09:54:50 +0200 From: "Frank Batschulat (Home)" <Frank.Batschulat@Sun.COM> To: =?utf-8?B?xaBpbXVuIE1pa2VjaW4=?= <numisemis@yahoo.com>, Dan Naumov <dan.naumov@gmail.com> Cc: "freebsd-fs@freebsd.org" <freebsd-fs@freebsd.org> Subject: Re: ZFS: swap on a ZVOL Message-ID: <op.uwotloct046apg@opteron> In-Reply-To: <354003.91539.qm@web37307.mail.mud.yahoo.com> References: <354003.91539.qm@web37307.mail.mud.yahoo.com>
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On Tue, 07 Jul 2009 09:21:54 +0200, Šimun Mikecin <numisemis@yahoo.com> wrote: > > Dan Naumov wrote: > As far as I know, using swap on top of a "non-trivial" filesystem like > ZFS is considered "unsupported", but it does in fact work. You can > create a ZVOL of arbitrary size (say, 4G) and then do the following: > zfs set org.freebsd:swap=on pool/swapvolname > to have /etc/rc.d/zfs enable swap on said ZVOL on every boot. You can > also do this in an ugly way: put swapon > /dev/zvol/<poolname>/<swapvolname> into your /etc/rc.local (without > having to pass the "org.freebsd:swap=on" option to the ZVOL). > > Now the question remains, what kind of issues are expected to arise > when using swap on a ZVOL and is there any work going to in order to > resolve them? One of the issues mentioned is that ZVOL swap cannot > handle kernel dumps and another, more serious potential issue is a > race condition where "more swap is needed to swap". Assuming I have a > machine with 2gb ram, if I use a 4gb ZVOL swap, am I likely to run > into any serious issues assuming that under normal operation, the > system uses from none to very little swap? > > AFAIK, it was said that race condition you mentioned also exists in OpenSolaris (that was back in the ZFS v6 days). > But, AFAIK new versions of OpenSolaris do use swap on ZFS volume as by default (correct me if I'm wrong). > Somebody more knowledgeable should answer this, but this made me thinking that maybe, just maybe that race condition was solved in some ZFS version >v6. Right, we do use a zvol as swap device, and we also do use a dedicated zvol as the dump device for the kernel crash dump. http://www.opensolaris.org/os/community/zfs/boot/zfsbootFAQ/#zfsswap http://www.solarisinternals.com/wiki/index.php/ZFS_Troubleshooting_Guide#ZFS_Swap_and_Dump_Devices http://docs.sun.com/app/docs/doc/819-5461/zfsboot-1?a=view http://www.opensolaris.org/os/community/zfs/boot/zfsboottalk.0910.pdf http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=5008936 was part of the ZFS boot project: http://www.opensolaris.org/os/community/zfs/boot/ afai remember neither the dump nor swap part itself required changes to the ondisk format, zpool version 6 just covered storing some new pool properties like 'bootfs' -> http://www.opensolaris.org/os/community/zfs/version/6/ hth frankB
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