-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Corey Hickey wrote: > 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. - -- Jeff Mahoney SUSE Labs -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.4-svn0 (GNU/Linux) Comment: Using GnuPG with SUSE - http://enigmail.mozdev.org iD8DBQFHvIG7LPWxlyuTD7IRAjLgAJsGekcbqlSyMtFpT+iWI8JU1LxUyQCfUeYy cf/n+R9tlBAMQtp67e2eEnE= =we6q -----END PGP SIGNATURE----- - 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
