Date: Thu, 11 Oct 2018 07:47:15 +0000 From: Lorenzo Salvadore <phascolarctos@protonmail.ch> To: FreeBSD Questions <freebsd-questions@freebsd.org> Subject: Re: swap to a sparse file Message-ID: <TmVhvYYFMip6ATUlnJkKj2jq4FtJzspncxgcet4keU1-6m58EJpZaJkzKzxuQ4jry_by7SdUjdzZ4YJzjoMT-K0trnJ7_8OdgeoM3fbKirs=@protonmail.ch> In-Reply-To: <20181011055236.138279b2.freebsd@edvax.de> References: <20181011014705.GA17798@admin.sibptus.ru> <20181011055236.138279b2.freebsd@edvax.de>
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> On Thu, 11 Oct 2018 08:47:05 +0700, Victor Sudakov wrote: > > > I've inherited a swapless FreeBSD 11.2 host with no unused disk space. > > I would like to add some swap to be on the safe side: in case there is > > a memory leak etc. > > On the other hand, I don't like to waste several gigabytes on a > > precious SSD because perhaps the system will never need this swap > > space anyway. > > Exactly my thoughts. :-) > > > The FreeBSD Handbook prescribes creating a swap file with "dd if=3D/dev= /zero ..." > > which would waste the space. > > A regular file as swap space also places additional > load into the virtual file system, whereas one of the > key advantages of a swap partition is that no file > access (through a filesystem) has to take place. > > > Is there any good reason I can't just "truncate -s2G /swap0" and > > make the swap a sparse file? > > I'm using a "pseudo-file" approach which hasn't shown any > problems so far. I'm using dd, but no actual disk space > will be allocated for the target file. That file then is > used like a regular swap partition via mdconfig's virtual > node facility, and also deleted. > > Sure, it probably isn't the ideal solution, but it works. > > Maybe you can use this for further inspiration? > > In /etc/rc.local: > > SWAP=3D"/swap0" > /bin/rm -f $SWAP > /bin/dd if=3D/dev/zero of=3D$SWAP bs=3D16m seek=3D500 count=3D0 > /sbin/mdconfig -a -t vnode -u 0 -f $SWAP || /bin/sh > /bin/chflags nodump $SWAP > /bin/rm $SWAP > /sbin/swapctl -a /dev/md0 > echo -n " swap" > > In /etc/rc.shutdown.local: > > /sbin/swapctl -d /dev/md0 > /dev/null 2>&1 > > /sbin/mdconfig -d -u 0 > echo -n " swap" Another solution if you have a zfs filesystem is to create a new filesystem for swap that would contain only a swapfile. That way the dedicated filesystem would use space only when needed and shrink when unneeded, but you could also set a minimal amount of space and a maximal amount. I do not know what is your exact situation with this host you inherited, but maybe it would not be a bad idea to just reinstall a brand new FreeBSD on it with the partitioning scheme you want (of course, you might need to backup something). Lorenzo Salvadore.
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