Re: raid resync speed

[Stan Hoeppner <stan@hardwarefreak.com>]

On 3/20/2014 10:35 AM, Bernd Schubert wrote:
> On 3/20/2014 9:35 AM, Stan Hoeppner wrote:
>> Yes.  The article gives 16384 and 32768 as examples for
>> stripe_cache_size.  Such high values tend to reduce throughput instead
>> of increasing it.  Never use a value above 2048 with rust, and 1024 is
>> usually optimal for 7.2K drives.  Only go 4096 or higher with SSDs.  In
>> addition, high values eat huge amounts of memory.  The formula is:

> Why should the stripe-cache size differ between SSDs and rotating disks?

I won't discuss "should" as that makes this a subjective discussion.
I'll discuss this objectively, discuss what md does, not what it
"should" do or could do.

I'll answer your question with a question:  Why does the total stripe
cache memory differ, doubling between 4 drives and 8 drives, or 8 drives
and 16 drives, to maintain the same per drive throughput?

The answer to both this question and your question is the same answer.
As the total write bandwidth of the array increases, so must the total
stripe cache buffer space.  stripe_cache_size of 1024 is usually optimal
for SATA drives with measured 100MB/s throughput, and 4096 is usually
optimal for SSDs with 400MB/s measured write throughput.  The bandwidth
numbers include parity block writes.

array(s)		bandwidth MB/s	stripe_cache_size	cache MB

12x 100MB/s Rust	1200		1024			 48
16x 100MB/s Rust	1600		1024			 64
32x 100MB/s Rust	3200		1024			128

3x  400MB/s SSD		1200		4096			 48
4x  400MB/s SSD		1600		4096			 64
8x  400MB/s SSD		3200		4096			128

As is clearly demonstrated, there is a direct relationship between cache
size and total write bandwidth.  The number of drives and drive type is
irrelevant.  It's the aggregate write bandwidth that matters.

Whether this "should" be this way is something for developers to debate.
 I'm simply demonstrating how it "is" currently.

> Did you ever try to figure out yourself why it got slower with higher
> values? I profiled that in the past and it was a CPU/memory limitation -
> the md thread went to 100%, searching for stripe-heads.

This may be true at the limits, but going from 512 to 1024 to 2048 to
4096 with a 3 disk rust array isn't going to peak the CPU.  And
somewhere with this setup, usually between 1024 and 2048, throughput
will begin to tail off, even with plenty of CPU and memory B/W remaining.

> So I really wonder how you got the impression that the stripe cache size
> should have different values for differnt kinds of drives.

Because higher aggregate throughputs require higher stripe_cache_size
values, and some drive types (SSDs) have significantly higher throughput
than others (rust), usually [3|4] to 1 for discrete SSDs, much greater
for PCIe SSDs.

Cheers,

Stan