09 February 2019

# Ext4

## Journaling Levels

Ext4 Filesystem has the following text:

This mode is the slowest except when data needs to be read from and written to disk at the same time where it outperforms all others modes.

The reason:

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.

Peter says:

Write latency is actually much longer: because it requires two writes instead of one. It is fsync latency as mentioned above that is smaller, because it depends only on the first write to what is in effect a small log based filesystem. This distinction matters a great deal, because it is the reason why “short bursty write patterns” is the qualification above. For long write patterns things are very different as the journal eventually fills up. For any given size it will also fill up a lot faster for ‘data=journal’.

Jidong question Write latency is actually much longer:

In addition, I am actually not very clear why you said that write() latency is longer while fsync() latency is shorter, I am trying to repeat what you said, please point out if I am incorrect:

1. Normally we call write() syscall first and then call fsync() to flush the data.
2. The write() returns as long as the data is written into page caches while the fsync() returns only if the data have been written into a stable store.
3. Although write() latency for data=journal mode is much longer because it requires two writes instead of one, however, since the write() means writing to page cache, so the actually cost is not so high, compared to the fsync() syscall where we have to write into disk and may require disk seeks. So we can mainly focus on the fsync() system call.

I agree with Jidong. And I think that by saying Write latency is actually much longer: because it requires *two* writes instead of one, Peter means that write involves a write of data blocks to the journal and a write of data blocks to its original location in filesystem.

commit=nrsec (*) Ext4 can be told to sync all its data and metadata every ‘nrsec’ seconds. The default value is 5 seconds.

I guess that a write() will only write the data blocks and metadata in the cache without a following fsync(). The reason is that fsync() syncs both the data and metatdata to the disk. If write() ensures a sync of metadata, then the sync of metadata by fsync() does not make sence.