Hi Dave, Dave Johnson wrote: > 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 This looks like it's a generic property of syncing an ext3 file system. Try turning off laptop_mode and then running "sync". You will probably see the same behaviour. > 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: For your SSD I guess you need to get rid of the sync-after-disk-activity, but keep the other VM behaviours of laptop_mode (such as avoiding swapping out pages / writing back dirty pages in order to free memory as long as it is also possible to just drop pages that are not dirty). You can probably achieve this by: - having a large commit interval etc., like you have now - setting laptop_mode to a very large value, e.g. a couple of hours. That will trigger a sync if and only if there has been *no* disk activity at all for hours on end -- i.e., pretty much never. And the other write-reducing VM features of laptop_mode will still be enabled. It would perhaps be a good thing to split these mechanisms into separate knobs. Write batching (the sync-after-disk-activity stuff and also the dirty_ratio / dirty_background_ratio changes) are a completely separate mechanism from write avoidance (the other mechanism I mentioned). Cheers, Bart -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
