Messages in this thread | | | From | Dave Johnson <> | Date | Thu, 1 Jan 2009 11:49:40 -0500 | Subject | Re: [ext3] kjournald writing after each read despite noatime,commit=nnn |
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Bart Samwel writes: > This is the defined behaviour for laptop_mode. Whenever a *physical* > READ takes place, this is taken to indicate that the disk is spun up at > that time. The laptop_mode functionality then takes that opportunity to > sync any dirty data to disk, two seconds (or whatever value you put in > /proc/sys/vm/laptop_mode) after the physical disk activity has ceased. > The rationale behind this is that you want to sync your stuff when the > disk is spun up, and then you want to hold back writing back stuff for a > very long while. And the only way it can detect that the disk is spun up > is when there is physical disk activity. > > This is exactly what happens in your case. The READ activity reported by > block_dump is *physical* read activity: some data was needed that was > not cached in memory. block_dump does not show you what data was > retrieved from the ext3 fs *without* having to access the disk, it only > shows actual physical disk I/O.
Yep sounds good, but this happens even if there is no dirty data needing a sync back to disk.
$ grep 'Dirty\|Write' /proc/meminfo Dirty: 0 kB Writeback: 0 kB WritebackTmp: 0 kB $ cat /some/uncached/file >/dev/null
Jan 1 11:43:49 gw kernel: cat(6615): READ block 864408 on hda1 Jan 1 11:43:51 gw kernel: kjournald(760): WRITE block 2376 on hda1
Note, the reason I ask is this is a SSD so just because a physical read has taken place recently unneeded writes should be avoided.
Turning laptop_mode to 0, but leaving other settings the same resolves the uneeded write:
$ echo 0 > /proc/sys/vm/laptop_mode $ grep 'Dirty\|Write' /proc/meminfo Dirty: 0 kB Writeback: 0 kB WritebackTmp: 0 kB $ cat /some/other/uncached/file >/dev/null
Jan 1 11:47:15 gw kernel: cat(6653): READ block 864258 on hda1
-- Dave
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