On Sat, Jan 26, 2008 at 06:44:59PM +0700, martin f krafft wrote: > also sprach Otavio Salvador <otavio@debian.org> [2008.01.26.1834 +0700]: > > Please Martin, could you comment on that? > > I have: http://lists.debian.org/debian-devel/2008/01/msg00929.html Joerg Jaspert did some tests to measure the impact of write intent bitmaps on performances. Quoting his blog entry: [1] So, having turned them on I just waited to see what happens with the machines under normal load. Not doing any stresstests, as they don’t show normal load situations anyway. And a little performance impact was expected. A little, not the thing that happened… I’m impressed, haven’t seen that much IO wait ever before. Even the old packages.debian.org code, which was really bad IO-wise (thanks to multiple greps per search), didn’t have such a bad impact. I mean - a Dual Core Athlon 3800, running a few Xen domains, usually without any noticable load, now having a load of 20 and more, while IO wait is at some 90% (as shown by top for both cores), constantly. (2 RAID1 devices). Or a Desktop System with a RAID5 (mad of 4 SATA discs), which normally unpacks a kernel tarball in some 10 seconds - now needs about 2 minutes to get it unpacked. Removing that tree now uses 5 minutes. No, bitmap internal is nothing one ever wants to use. One possibly might want to use that feature with a file for the bitmaps which is stored on a different device. That might actually make sense and wont hurt as much as internal does. I have one positive thing to say about them: Resyncing an array is indeed much much faster. But I go with longer resync times and slightly less performance during that time, the cost of having the bitmaps all the time massively impacts, a recovery of even a day doesn’t cost as much in total. I would be in favor of trusting Joerg's comments and tagging this bug as "wontfix" in order to document the issue. Any other opinions? [1] http://blog.ganneff.de/blog/2008/01/30#mdraid_bitmap_internal_bad Cheers, -- Jérémy Bobbio .''`. lunar@debian.org : :Ⓐ : # apt-get install anarchism `. `'` `-
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