On Wed, Apr 18, 2012 at 08:09:02AM -0700, Andreas Dilger wrote: > Looking at these performance numbers again, it would seem better if ext4 _was_ zero filling the whole file and converting the whole thing to initialized extents instead of leaving so many uninitialized extents behind. > > The file size is 256MB, and the disk would have to be doing only 3.5MB/s for linear streaming writes to match the performance that you report, so a modern disk doing 50MB/s should be able to zero the whole file in 5s. > > It seems the threshold for zeroing uninitialized extents is incorrect. EXT4_EXT_ZERO_LEN is only 7 blocks (28kB normally), but typical disks can write 64kB as easily as 4kB, so it would be interesting to change EXT4_EXT_ZERO_LEN to 16 and re-run your test. > > If that solves this particular test case, it wont necessarily the general case, but is still a useful fix. If you submit a patch for this, please change this code to compare against 64kB instead of a block count, and also to take s_raid_stride into account if set, like: > > ext_zero_len = max(EXT4_EXT_ZERO_LEN * 1024 >> inode->i_blkbits, > EXT4_SB(inode->i_sb)->s_es->s_raid_stride); > > This would write up to 64kB, or a full RAID stripe (since it already needs to seek that spindle), whichever is larger. It isn't perfect, since it should really align the zero-out to the RAID stripe to avoid seeking two spindles, but it is a starting point. Hi Andreas, I set EXT4_EXT_ZERO_LEN to 16 and run the same benchmark again. the result is the same as before. I notice this commit (3977c965) and it set EXT4_EXT_ZERO_LEN to 7. But in commit log, it doesn't describe why this value is set to 7. As you said, I believe that the disk writes 64K as easily as as 4k in modern disk. So maybe we can consider to set it to 16 or RAID stripe. :) Regards, Zheng -- 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
