Re: dump(8) using snapshot + recommended cache
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? no, in my experience it's no difference in speed with this enabled or not. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
dump(8) using snapshot + recommended cache
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. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: dump(8) using snapshot + recommended cache
I regularly use dump(8) with snapshots to back up my server. While this seems to have been working perfectly well so far, Sorry to follow-up my own post; I just realized I hadn't mentioned any version info. The docs I am reading are the ones associated with 7.1-RELEASE; I haven't checked whether this part of the dump documentation got updated with 7.1 or not. Thanks, Andrew. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: dump(8) using snapshot + recommended cache
On Sun, 1 Feb 2009 08:59:02 -0400 (AST) Andrew Hamilton-Wright andre...@ieee.org wrote: 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.*** When you dump a snapshot there are, by definition, no changes between passes. So it's saying that in that case there in no reason not to cache. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
Re: dump(8) using snapshot + recommended cache
On Sun, 1 Feb 2009, RW wrote: ***It is recommended that you always use this option when dumping a snapshot.*** When you dump a snapshot there are, by definition, no changes between passes. So it's saying that in that case there in no reason not to cache. Ah, that makes sense. That being the case, perhaps we can update the text to: If dumping from a snapshot, the filesystem is already frozen, therefore using a cache with a snapshot will ensure that consistency is maintained while also providing best performance. If that sounds good, I'll make a doc patch. Out of curiosity, under what circumstances is the improved performance the most likely? I dump from cron when the system usage is low, and haven't noticed any significant difference in time with or without cacheing -- but I haven't done any testing under heavy load, nor with limited RAM, so there are many mbufs available in any case. Thanks for the info, Andrew. ___ freebsd-questions@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-questions To unsubscribe, send any mail to freebsd-questions-unsubscr...@freebsd.org
