Re: 2.6.17 odd hd slow down

[Jens Axboe]

On Wed, Sep 06 2006, Damon LaCrosse wrote:
> 
> Hi all,
> experimenting a little the 2.6 block device layer I detected under
> some circumstances a net slowness in the disk throughput. Strangely
> enough, in fact, my IDE disk reported a significant performance drop
> off in correspondence of certain access patterns.
> 
> Following further investigations I was able to simulate this ill
> behavior in the following piece of code, clearly showing a non
> negligible hard-disk slow down when the step value is set greater than
> 8. These result in fact far below the hard-disk real speed
> (30~70MB/sec), as correctly measured instead in correspondence of low
> STEP values (<8). In particular, with step of 512 or above, the
> overall performance scored by the disk results below 2MB/sec.

You are effectively approaching seeky writes, I bet it's just the drive
firmware biting you. Repeat the test on a different drive, and see if
you see an identical pattern.

> At first I thought to a side-effect of the queue plug/unplug
> mechanism: the scattered accesses involve the unplug timeout to each
> bio. So, I added the BIO_RW_SYNC flag that - AFAIK - should force the
> queue unplugging. Unfortunately nothing changes.
> 
> Now, as it is quite possible that I'm missing something, the question
> is: is there an effective way of doing scattered disk accesses using
> bios? In other words, how can I fix the following program in order to
> get disk full speed for steps > 8?

I don't think the io path has anything to do with this. Why are you
expecting non-sequential writes to continue to be fast? They wont be.

-- 
Jens Axboe

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