Ext4 sync or async

pitm0d · 10-31-2018, 02:56 PM

Hi,
is ext4 filesystem sync or async by default. Shall I explicitly set async mount option to make it perform I/O writes in asynchronous way?
BR,

JeremyBoden · 10-31-2018, 04:58 PM

I think all disk filesystems (including ext4) are basically async, so no specifier is necessary.

async / sync are really only relevant for remote I/O.

berndbausch · 10-31-2018, 05:01 PM

According to the generic mount man page, you can’t say that for sure:

Code:

       Some of these options could be enabled or disabled by default in the
       system kernel.  To check the current setting see the options in
       /proc/mounts.  Note that filesystems also have per-filesystem
       specific default mount options (see for example tune2fs -l output for
       extN filesystems).

Also note the commit mount option:

Code:

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

pitm0d · 11-04-2018, 11:12 AM

Dumpe2fs/tune2fs -l doesn't show if it's sync or async.

syg00 · 11-04-2018, 07:35 PM

Quote:

Originally Posted by pitm0d

Shall I explicitly set async mount option to make it perform I/O writes in asynchronous way?

All write are done in an apparently synchronous way.

For truly asynchronous I/O, applications have to be sufficiently sophisticated to manage multi-threaded sleep and respond to I/O completion events. Not many are that sophisticated - almost all just leave it to the operating system.
What are you really asking ?.

Perhaps read this before responding.

berndbausch · 11-04-2018, 08:59 PM

To come back to your original question:

Quote:

Originally Posted by pitm0d

Shall I explicitly set async mount option to make it perform I/O writes in asynchronous way?

The manual page for ext4 doesn't mention any sync or async mount options. This explains why no such option appears in the output of tune2fs.

Regarding the behaviour of ext4, I suggest you look at the commit and data options in the ext4 man page. By default, data is sync'ed every 5 seconds. For more asynchronicity (is that a word?) set data=writeback, or change the commit value to a larger time interval.

You may also want to read this LWN article about ext4 and data loss.

Thanks for the question, by the way. It forced me to research the subject and learn something.

JeremyBoden · 11-05-2018, 10:42 AM

The hardware won't necessarily physically obey the async/sync "preference".
For example a SATA disk will normally accept writes in the order it receives them, but they will be executed in the order it considers to be most efficient.
You will get this happening with any hardware that has a cache.

It's possible for the OS to instruct the device that certain I/O is to be carried out synchronously though...

The sync command might be relevant, if you need internal caches to be rolled out to disk.