Re: Maximum transaction rate

John R Pierce wrote:
> Stefan Kaltenbrunner wrote:
>> So in my understanding LVM is safe on disks that have write cache
>> disabled or "behave" as one (like a controller with a battery backed
>> cache).
>
> what about drive write caches on battery backed raid controllers? do
> the controllers ensure the drive cache gets flushed prior to releasing
> the cached write blocks ?

If LVM/dm is lying about fsync(), all this is moot. There's no point
talking about disk caches.

BTW. This discussion is continuing on the linux-lvm mailing list.
https://www.redhat.com/archives/linux-lvm/2009-March/msg00025.html
I have some PG databases on LVM systems, so I need to know for sure
I have have to move them elsewhere. It seemed to me the right place
for asking about the issue.

Someone there pointed out that fsycn() is not LVM's responsibility.

Correct. For sure, there's an API (or more than one) a filesystem uses
to force a flush on the underlying block device, and for sure it has to
called while inside the fsync() system call.

So "lying to fsync()" maybe is more correct than "lying about fsync()".

.TM.