Hello, Every once in a while one of the hard drives in my RAID-0 array starts buzzing: seeking rapidly and regularly such that it provides a continuous tone. The tone is continuous for 0.5-2 seconds before changing frequency; the sound goes through many such steps over the course of 5-30 seconds. Meanwhile, my computer is effectively unusable: programs are starved for I/O, terminals hang, and sometimes X becomes unresponsive--I can't even move the mouse pointer. This drove me nuts for a while until I figured out the problem: reiserfs' bitmap data keeps falling out of the kernel's page cache, and re-reading the bitmap is very slow. Dropping the page cache instantly triggers the same behavior. # echo 1 > /proc/sys/vm/drop_caches # dd if=/dev/zero of=file bs=1M count=1024 It's quite common for writing a gigabyte to consist of 30 seconds of reading bitmap data followed by 7 seconds of writing. Sometimes writing a single byte takes 15 seconds of reading and 0 seconds of writing. :) I did some tests this evening that appear to confirm my analysis. I compiled two kernels: one from git immediately before this commit, and one from after. http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=5065227b46235ec0131b383cc2f537069b55c6b6 Before: - filesystem takes a long time to mount (of course) - no problems thereafter After: - filesystem mounts pretty quickly - the usual buzzing and such I don't understand why this problem is biting me so badly--I have several other reiserfs filesystems (on the same computer and on others) and I can't make any trouble happen with them. Actually, I can always force the bitmap data to be forgotten by dropping the page cache, but re-reading it only takes an moment on every other reiserfs I have. For example, when writing a 1GB file, my 185 GB single-disk filesystem reads about 600 KB of bitmap data in 1 second; my 932 GB RAID-0 is likely to read 15 MB in 30 seconds. I tried gathering information about the bitmaps on the two filesystems and how quickly they can be read. # echo 1 > /proc/sys/vm/drop_caches # time debugreiserfs -m /dev/md0 | wc -l (and the same thing for /dev/sda4) Meanwhile, I captured disk read info with dstat to see how many kilobytes of data were read. time lines kilobytes /dev/md0 55.125s 14935 29496 /dev/sda4 9.524s 2987 6680 The ratios of the above data are very close to each other and to the ratio of the filesystem sizes: fs size: 932 / 185 = 5.038 time: 55.126 / 9.524 = 5.788 lines: 14935 / 2987 = 5.000 kilobytes: 29496 / 6680 = 4.416 So, then, why does the larger filesystem have to read so much more bitmap data before writing? As I mentioned before, /dev/md0 reads up to 15 MB before writing, and /dev/sda4 reads only 600 KB. Thanks, Corey - To unsubscribe from this list: send the line "unsubscribe reiserfs-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
