Re: Very long raid5 init/rebuild times

[Stan Hoeppner <stan@hardwarefreak.com>]

On 2/4/2014 12:55 PM, Phillip Susi wrote:

> On 2/4/2014 1:43 PM, Stan Hoeppner wrote:
>> Everything we've been discussing has been about maximizing write 
>> throughput.  The fact that you argue this at this point makes it
>> crystal clear that you don't have no understanding of the
>> differences in the read/write paths and how buffer cache affects
>> each differently.  Further discussion is thus pointless.
> 
> I am intimately familiar with the two code paths, having written
> several applications using them, studied the kernel code extensively,
> and been one of the original strong advocates for the kernel to grow
> direct aio apis in the first place, since it worked swimmingly well on
> WinNT.
> 
> So I say again: switching to direct aio, while saving a decent chunk
> of cpu time, makes very little difference in streaming write
> throughput.  If it did, there would be something terribly broken with
> the buffer cache if it couldn't keep the disk queues full.

If all this is true, then why do you keep making a tangential arguments
that are not relevant?

I never argued that the buffer cache path is slower.  It is in fact much
faster in most cases.

I argued that accurately measuring the actual data throughput at the
disks isn't possible when writing through buffer cache.  At least not in
a straightforward manner as with O_DIRECT.  I've made the point in the
last two or three replies.  Yet instead of directly addressing that,
rebutting that, you keep making these tangential irrelevant arguments...

-- 
Stan