Write ordering in Ext4

Eric Sandeen sandeen at redhat.com
Tue Jun 4 17:33:25 UTC 2013

On 6/4/13 12:17 PM, Arul Selvan wrote:
> thanks that answered my question. One more question, is it possible to stop the delayed block allocation in ext4 ?

If you mean turn off delayed allocation, look no further than the mount
options documented in the kernel tree, Documentation/filesystems/ext4.txt:

nodelalloc              Disable delayed allocation.  Blocks are allocated
                        when the data is copied from userspace to the
                        page cache, either via the write(2) system call
                        or when an mmap'ed page which was previously
                        unallocated is written for the first time.

Out of curiosity, why do you want to turn off delalloc?


>>>> Andreas Dilger <adilger at dilger.ca> 6/3/2013 8:17 PM >>>
> On 2013-06-02, at 23:33, "Arul Selvan" <Rarul at novell.com> wrote:
>> Greetings. I am Arul Selvan works for Novell. I am exploring the Ext4 architecture, more specifically i would like to understand the write ordering, basically the same blocks is modified more than once, how the write is ordered. Could you point me the doc or the specific source file to look.
> Writes in memory to the same file are serialized by i_mutex, but may
> modify the same page in memory repeatedly.
> When that page us being written to disk, it will be marked with the
> page writeback flag, in order to stabilize the content, and allow consistent
> checksums (e.g. for MD RAID or disks with T10-DIF). This may block
> any further writes from modifying the same page as it is being
> submitted to disk, depending on the kernel version and the
> requirements of the underlying storage. Once the disk write has been
> finished, the writeback bit is cleared and the page can be modified again.
> In all cases, the writes to a single page are ordered, but there is no
> _guarantee_ about writes to different data blocks being ordered.
> The ext4 journal will in fact impose some order on data writes,
> by ensuring that the data from all writes associated with a transaction
> are flushed before the data for the next transaction.
> Since fsync() of any file commits the current transaction, this has
> the side-effect that any fsync causes all older writes to be committed.  This is NOT required by POSIX, and applications that depend on this behavior are not portable to/safe on other filesystems.
> Cheers, Andreas
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