On 2011-10-31, at 12:53 PM, Sunil Mushran wrote: > On 10/31/2011 03:27 AM, Tao Ma wrote: >> OK, so let me explain why the big cluster length works. >> >> In the new bigalloc case if chunk size=64k, and with the linux-3.0 >> source, every file will be allocated a chunk, but they aren't contiguous >> if we only write the 1st 4k bytes. In this case, writeback and the block >> layer below can't merge all the requests sent by ext4. And in our test >> case, the total io will be around 20000. While with the cluster size, we >> have to zero the whole cluster. From the upper point of view. we have to >> write more bytes. But from the block layer, the write is contiguous and >> it can merge them to be a big one. In our test, it will only do around >> 2000 ios. So it helps the test case. > > Am I missing something but you cannot zero the entire cluster because > block_write_full_page() drops pages past i_size. > > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=5693486bad2bc2ac585a2c24f7e2f3964b478df9 With ext4_ext_zeroout->blkdev_issue_zeroout() it submits the zeroing request directly to the block layer (with cloned ZERO_PAGE pages) and skips the VM entirely. Cheers, Andreas -- Andreas Dilger Principal Engineer Whamcloud, Inc. -- 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
