Re: raid resync speed

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 03/20/2014 05:19 PM, Eivind Sarto wrote:
>
> On Mar 20, 2014, at 8:36 AM, Bernd Schubert <bernd.schubert@fastmail.fm> wrote:
>
>> On 03/20/2014 04:35 PM, Bernd Schubert 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?
>>> 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.
>>
>> Sorry, I forgot to write 'cpu usage', so it went to 100% cpu usage.
>>
>>>
>>> So I really wonder how you got the impression that the stripe cache size
>>> should have different values for differnt kinds of drives.
>>>

> The hash chains for the stripe cache become long if you increase the stripe cache.  There are only 256
> hash buckets.  With 32K stripe cache entries, the average length of a hash chain will be 128 and that will
> increase contention for the lock protection the chain.
>

Yes, this is a implementation detail. But that make a difference between 
SSDs and rotating disks... (which was my point here).