On Sat, Mar 26, 2011 at 10:44 PM, Ted Ts'o <tytso@xxxxxxx> wrote: > On Sat, Mar 26, 2011 at 08:25:23PM -0400, Jidong Xiao wrote: >> >> But my question is, why data=journal could outperform data=ordered, >> for the data=journal mode, you have to write the data and metadata >> blocks into the journal, but for the data=ordered mode, you only have >> to write the metadata blocks into the journal. If, in some certain >> cases, the former mode can avoid seeks, then the same behavior should >> apply to the latter mode. So it's really odd that the former mode can >> outperform the latter mode. > > When executing an fsync(), in data=ordered mode you have to write the > data data blocks into the journal and wait for the data blocks to be > written. This requires generally will require extra seeks. In > data=journaled mode, the data blocks can be written directly into the > sjoujournal without needing to seek. > > Of course eventually the data and metadata blocks will need to be > written to their permanent locations before the journal space can be > reused. But for short bursty write patterns, the fsync() latency will > be much smaller in data=journal mode. > Thank you Ted, it is really helpful! So the difference is: data=ordered mode: fsync() will return only if the meta data blocks have been written into the journal and the data blocks have been written into the disk. data=journal mode: fsync() returns if the meta data and data have been written into the journal. The journal is contiguous, so data=journal mode means no seeking needed, therefore, fsync() would return more quicker. If, we perform read from and write to the disk simultaneously, like following example: First, write data to the filesystem as quickly as possible: Rapid writing while true do dd if=/dev/zero of=largefile bs=16384 count=131072 done While data was being written to the test filesystem, read 16Mb of data from the same filesystem on the same disk, timing the results: Reading a 16Mb file time cat 16-meg-file > /dev/null In this case, if we conduct the experiment in data=journal mode and data=ordered mode respectively, since write latency is much smaller in data=journal mode, the disk will focus more on the read operation, hence, the read operation will also finish earlier than it do in the data=ordered mode. Am I understanding correctly? Regards Jidong _______________________________________________ Ext3-users mailing list Ext3-users@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/ext3-users
