Re: RAID 5,6 sequential writing seems slower in newer kernels

[Phil Turmel <philip@turmel.org>]

On 12/02/2015 09:51 PM, Dallas Clement wrote:
> On Wed, Dec 2, 2015 at 8:38 PM, Phil Turmel <philip@turmel.org> wrote:
>> On 12/02/2015 09:33 PM, Dallas Clement wrote:
>>
>>> I'm not sure that the sync=1 has any effect in this case where I've
>>> got direct=1 set (for non buffered I/O).  I think the sync=1 flag only
>>> matters for buffered I/O.  I really shouldn't be setting that flag at
>>> all.
>>
>> It's substantially different from direct=1.  O_DIRECT just bypasses the
>> kernel's caches.  O_SYNC flushes the file data and filesystem metadata,
>> and kills the device caches and queues.
> 
> Isn't O_SYNC only applicable for buffered I/O or going through the
> kernel caches?  If I'm using O_DIRECT, seems like it should just
> ignore this flag.

O_SYNC is orthogonal to whether the kernel caches are involved.  It is
about ensuring that data *and* metadata are safely written all the way
to permanent media.

Phil