Date: Tue, 18 Jan 2000 22:11:22 +0000 (GMT) From: Terry Lambert <tlambert@primenet.com> To: bright@wintelcom.net (Alfred Perlstein) Cc: dchulhan@uwi.tt (Dale Chulhan - Away), antionline@onelist.com, chat@FreeBSD.ORG Subject: Re: Swap Files vs Swap Partitions Message-ID: <200001182211.PAA12048@usr09.primenet.com> In-Reply-To: <20000116183846.M508@fw.wintelcom.net> from "Alfred Perlstein" at Jan 16, 2000 06:38:46 PM
next in thread | previous in thread | raw e-mail | index | archive | help
> > looking for PROPER ( qualified ) answers to this question from a > > friend... > > > > > In the context of *nix...what are the distinct pros 'n cons > > > re: swap partitions vs. swapfiles???? apart from the obvious: > > > the filesystem overheads, I mean. > > exactly that. swap partitions are _much_ better than swapfiles, > note that some operating systems allow swapping to the free-space > on a filesystem, however once you get that far you are probably so > seriously hosed it's not worth the effort. Swap files go through the file system to the device driver to the disk. Swap partitions go through the device driver to the disk. Swap files are limited to the maximum file size. Swap partitions are limited to the maximum device size. If you run out of swap space on a partition, you can not recover. If you run out of swap space on a file, you can recover by permitting the growth of the file; however, if you place an administrative limit on the growth, then you in the same boat as if you wre using a partition. If your swap utilization is lowered on a swap partition, you can't recover the space for other uses. If your swap utilization is lowered on a swap file, you can recover the resources for use by other parts of the system. This is similar to other administrative resource limits, which can limit legitimate use of scare resources, in an ill-conceived attempt to "save the user" from themselves or a malicious user that you can't just kick off the machine; this works for ISPs and similar users, but for desktops and dedicated servers, it's kind of pointless to stage denial of service attacks against yourself. Administrative limits are also kind of useless when you have differential use of the machine (e.g. not the same kind of uniform load 24 hours a day, so it doesn't make sense to hog-tie your payroll runs at 3 AM to "protect" your database reports at 6 PM). In other words, swap files may seem to be a bad idea, but if you are going to disk anyway, dereferencing direct and indirect blocks are the least of your worries about latency issues, and the added flexibilty they offer when implemented correctly will generally more than make up for the problems you might encounter. As a final example, the NeXTStep OS used swap files, and so long as you dd'ed /dev/zero into the files before enabling swapping (to avoid overcommitting your available disk blocks for swapping), it was a wonderful feature that save our butts in Mathematica and FrameMaker on numerous occasions. Terry Lambert terry@lambert.org --- Any opinions in this posting are my own and not those of my present or previous employers. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-chat" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200001182211.PAA12048>