[ZT] 7 Reasons Why BSD Is Better than Linux
7 Reasons Why BSD Is Better than Linux
http://www.intranetjournal.com/articles/200809/pij_09_22_08a.html
September 24, 2008
September 17, 2008
[ZT] Why isn’t PostgreSQL using my index?
...PostgreSQL's query optimizer is smart, really smart and unless you have evidence otherwise you should trust what it is doing. In this particular case, the optimizer realizes that if a table has only a few rows that using the index is actually slower than just spinning through the entire table. Just because PostgreSQL isn't using your index today with a small number of rows, does not mean it won't choose to use it later when you have more data or the query changes...September 16, 2008
September 15, 2008
[ZT] Best practices for using gjournal with gmirror
FROM: http://unix.derkeiler.com/Mailing-Lists/FreeBSD/hackers/2007-01/msg00220.html
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[Q:] When using gjournal and for a gmirror volume, does the journal need to be mirrored as well to maintain redundancy? If so, when storing the journal on the same physical disks as the mirror, is it better to mirror at the slice level (journal and fs on different partitions in the same mirror) or at the partition level (journal and fs each have their own mirror) or does it matter?
[A:] The problem with mirroring each partition/slice separately is that when you have a crash, on boot, gmirror will start to rebuild all partitions at once, which may be problematic. On the other hand, when you mirror each partition/slice separately, and some partitions weren’t modified in last few seconds before the crash, gmirror will not resync them on boot, so not entire disk will be synchronized.
注：Gmirror如果对整个硬盘作镜像，系统崩溃重启后，gmirror将有较大可能需要自动重建所有分区(如果是1T HD，会是个大问题)。
如果对每个分区单独做镜像，如果某些个镜像分区在崩溃前几秒钟没有发生修改/写操作的话，镜像状态不会降级损坏，gmirror 就不需要在重启后重建这个分区，（如果将需要频繁写入的分区划为小于20G的独立分区做镜像，就可以一定程度上绕开以上的问题）。
When you run gjournal on top of gmirror/graid3 there is no need for resync after a crash, so bascially all cons against mirroring the whole disks and against mirroring partitions are no longer true. Both configurations will work the same. In that case I’d suggest mirroring the whole disks, because when one of your disks dies, you may just replace it and be down with it. If you mirror partitions separately, you first have to create partitions and insert each of them into their mirrors, which is more complex than simple ‘gmirror insert foo newdisk’.
注： 如果在Gmirror上启用gjournal, 就可以很大程度上避免在崩溃重启后重新同步(?) 这种情况下，推荐对整个硬盘做mirror。
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[Q:] does a gjournal volume need to be fsck’ed after a crash? If not, will it just work (e.g. fsck -p sees that the filesystem is clean) or does it need to be disabled somehow?
[A:] Gjournaled file system has to be fscked, but only to handle orphaned files. Such fsck on multiterabyte provider takes seconds, not hours.
注：gjournal 系统同样需要fsck, 但是非常迅速。
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