Date: Sun, 1 Feb 2009 08:59:02 -0400 (AST) From: Andrew Hamilton-Wright <andrewhw@ieee.org> To: freebsd-questions@freebsd.org Subject: dump(8) using snapshot + "recommended" cache Message-ID: <alpine.BSF.2.00.0902010838030.1180@qemg.org>
next in thread | raw e-mail | index | archive | help
Hi All; I regularly use dump(8) with snapshots to back up my server. While this seems to have been working perfectly well so far, upon (re)reading the man page for dump(8), I have noticed a somewhat scary pair of lines in the paragraph describing the option for -C <cachesize> (emphasis with stars mine): [Use of this option] will greatly improve performance at the cost of ***dump possibly not noticing changes in the file system*** between passes. ***It is recommended that you always use this option when dumping a snapshot.*** Does anyone know what, exactly, this means? In particular, is the first statement actually trying to say: Use of this option will greatly improve performance; however it may be that changes made to the filesystem made between _dump_ passes will be ignored. ***The resulting dumped filesystem image will be consistent and correct based on a timestamp no later than that of the point at which the dump was started***. Is this a fair statement? Is this guaranteed? Or are we trying to say that: The resulting filesystem will contain images of individual files based on a timestamp no later than that of the point at which the dump was started, however any individual files modified after the dump begins may be stored using any of the version that appeared written to disk during the period of the dump. As far as the second line goes, I am not at all clear on what this is trying to say. Why is the cache recommended? For speed? Stability? Output correctness? In particular, if a snapshot dump is made without a cache option, is it potentially corrupt? In particular, if the second attempt above is more true than the first, it seems to me that we should _not_ recommend the use of a cache with snapshots, as it seems to erode the utility of the snapshot itself. It is for this reason that I am suspecting that there is more here than meets the eye, which is why I am keen to make sure that this is clear. I am very happy to put in an update to the docs if we can make sure that we know exactly what we are trying to say here. Thanks, Andrew.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?alpine.BSF.2.00.0902010838030.1180>