On 10/2/13 9:47 AM, Jan Kara wrote: > On Tue 01-10-13 12:08:17, Benjamin LaHaise wrote: >> While investigating a performance regression during migration of the >> Solace product from an older kernel running ext3 to a 3.x kernel running >> ext4, the change in allocation policies between ext3 and ext4 were found >> to have caused a 10-50% decrease (depending on the test) in I/O >> throughput. In order to extract more parallelism from the filesystem, >> this particular use-case has 100 subdirectories off of the root >> directory of an ext4 filesystem in which files are created in a >> round-robin fashion. The subdirectories are used in order to increase >> the number of metadata operations that can occur in parallel. With the >> older setup on ext3, files were created sequentially, while using ext4 >> resulted in the files being spread out across block groups. >> >> To avoid this change in allocation policies, introduce the noorlov mount >> parameter to ext4. This parameter changes allocation policy such that new >> subdirectories in the filesystem are allocated in the same block group >> as the parent subdirectory. With the subdirectories in the same block >> group, the allocation policy once again results in files being laid out >> sequentially on disk, restoring performance. > Frankly, I'm not very fond of a mount option for tweaking inode allocation > policy. OTOH the regression is large enough that we should address it > somehow. > > So I suppose if your application doesn't use the root directory as a base > but some other directory on ext4 filesystem, everything is OK, isn't it? > Because the root directory is special in the Orlov allocator and that is > where the randomness happens. > > If I'm right about the source of the problem, we could use TOPDIR inode > flag to handle this. Currently Orlov allocator treats directories with > TOPDIR flag set the same way as the root directory. Sadly the root > directory itself is hardcoded in the allocator but we could remove that > just keep the check for TOPDIR flag. To handle backward compatibility, > we would set TOPDIR for root inode during mount first time we mount the fs > with the new kernel (needs some flag in the superblock). > > Hum, so when I wrote this I'm not sure this is that much better than a > mount option. But it's a possibility :). What do others think? I'm right with you on thinking a mount option should be a last resort. One thing I'm curious about - what changed from ext3 to ext4? I thought both defaulted to orlov and the same type of allocation behavior, more or less. I guess one change is that the "oldalloc" mount option went away. (if it does come back, it should probably mirror what we had before, which was "oldalloc" not "noorlov" right?) -Eric > Honza > -- 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
