On Mon, Feb 7, 2011 at 2:42 PM, Nico Williams <n...@cryptonector.com> wrote:
> On Mon, Feb 7, 2011 at 1:17 PM, Yi Zhang <yizhan...@gmail.com> wrote:
>> On Mon, Feb 7, 2011 at 1:51 PM, Brandon High <bh...@freaks.com> wrote:
>> Maybe I didn't make my intention clear. UFS with directio is
>> reasonably close to a raw disk from my application's perspective: when
>> the app writes to a file location, no buffering happens. My goal is to
>> find a way to duplicate this on ZFS.
>
> You're still mixing directio and O_DSYNC.
>
> O_DSYNC is like calling fsync(2) after every write(2). fsync(2) is
> useful to obtain
> some limited transactional semantics, as well as for durability
> semantics. In ZFS
> you don't need to call fsync(2) to get those transactional semantics, but you
> do
> need to call fsync(2) get those durability semantics.
>
> Now, in ZFS fsync(2) implies a synchronous I/O operation involving
> significantly
> more than just the data blocks you wrote to. Which means that O_DSYNC on ZFS
> is significantly slower than on UFS. You can address this in one of two ways:
> a) you might realize that you don't need every write(2) to be durable, then
> stop
> using O_DSYNC, b) you might get a fast ZIL device.
>
> I'm betting that if you look carefully at your application's requirements
> you'll
> probably conclude that you don't need O_DSYNC at all. Perhaps you can tell us
> more about your application.
>
>> Setting primarycache didn't eliminate the buffering, using O_DSYNC
>> (whose side effects include elimination of buffering) made it
>> ridiculously slow: none of the things I tried eliminated buffering,
>> and just buffering, on ZFS.
>>
>> From the discussion so far my feeling is that ZFS is too different
>> from UFS that there's simply no way to achieve this goal...
>
> You've not really stated your application's requirements. You may be
> convinced
> that you need O_DSYNC, but chances are that you don't. And yes, it's possible
> that you'd need O_DSYNC on UFS but not on ZFS.
>
> Nico
> --
>
Please see my previous email for a high-level discussion of my
application. I know that I don't really need O_DSYNC. The reason why I
tried that is to get the side effect of no buffering, which is my
ultimate goal.
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