>>> On Mon, 22 Oct 2007 10:18:59 -0700 (PDT), Peter >>> <thenephilim13@xxxxxxxxx> said: [ ... ] thenephilim13> I can understand that if a RMW happens it will thenephilim13> effectively lower the write throughput thenephilim13> substantially but I'm not sure entirely sure why thenephilim13> this would happen while writing new content, It does not really depend on new vs. old content, but on whether the filesystem knows the real address of the blocks it is writing to and assembles writes in sequences beginning at the appropriate boundary and of the approriate length. XFS (at least some recetn versions) makes a valiant attempt to deduce the right values from the underlying storage system, but double checking usually helps. >> [ ... ] My impression is that something that takes less than >> 5% on a developers's system does not get looked at, even if >> it takes 50% on your system. The Linux kernel was very >> efficient when most developers were using old cheap PCs >> themselves. "scratch your itch" rules. thenephilim13> This is a rather unfortunate situation, it seems thenephilim13> that some of the roots are forgotten, If somebody's roots are to be a poor postdoc/postgrad scrounging used/broken PC parts and then they get hired with a huge salary and given a 4-CPU 4GB PC to play with, usually they are very happy to forget those roots and don't think it unfortunate. Just another instance also of the "who pays the piper calls the tunes" principle. thenephilim13> especially in a case like this where one would thenephilim13> think running a file server on a modest CPU thenephilim13> should be enough. Let's say that I have been myself been astonished by how CPU intensive is the Linux page cache. But then I think that quite a few aspects of the Linux page/IO management subsystems (never mind sick, mad horrors like 'vm/page-cluster') could be substantially improved by someone who had read the relevant literature (I am not volunteering unfortunately) instead of trying to wing it and flounder. [ ... ] >> Misaligned writes and page cache CPU time most likely. thenephilim13> What influence would adding more harddrives to thenephilim13> this RAID have? If you can guarantee that writes (and reads, less importantly) be aligned and of the right size adding more drives improves performance as long as this does not hit the bus bandwidth limits and the CPU ones (and your system likely is near those); unfortunately adding more drives makes it less likely that writes will be properly aligned and of the right size, never mind all the other downsides. A 3-drive RAID5 is one of only two common (or uncommon) cases in which RAID5 is a tolerable idea. thenephilim13> I know in terms of a Netapp filer they always thenephilim13> talk about spindle count for performance. NetApp use something very different from RAID5 (and which is not quite RAID4)and they also have a custom filesystem described in an interesting paper: http://www.netapp.com/library/tr/3002.pdf http://blogs.sun.com/val/entry/zfs_faqs_freenix_cfp_new - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
