[OpenIndiana-discuss] Fileserver performance with log and cache devices?

[Mark]

On 3/12/2011 9:34 a.m., Per Sjoholm wrote:
> On 12/02/2011 07:44 PM, Gary Mills wrote:
>> On Thu, Dec 01, 2011 at 08:59:48AM -0600, Gary Mills wrote:
>>> I'm in the process of setting up a fileserver running oi_151a. How
>>> can I determine the performance improvement from adding log and cache
>>> devices? I'm using filebench with the fileserver personality. Is
>>> that my best choice? So far, I've tried filebench directly on the
>>> server and also on an NFS client. What statistics should I be
>>> watching?
>> So, can nobody answer my questions?
>>
> http://constantin.glez.de/blog/2011/02/frequently-asked-questions-about-flash-memory-ssds-and-zfs
>
> # How Do I Know if an SSD Will Really Help My Write Performance?
> <http://constantin.glez.de/blog/2011/02/frequently-asked-questions-about-flash-memory-ssds-and-zfs#write>
>
> # How Do I Know if an SSD Will Really Help My Read Performance?
> <http://constantin.glez.de/blog/2011/02/frequently-asked-questions-about-flash-memory-ssds-and-zfs#read>
>
>
>>> To begin, I'm using a single 1 TB SAS disk on the server, without log
>>> or cache devices. I intend to add an Intel 311 SSD for the log and
>>> Intel 320 SSDs for the cache. I'd like to ensure that these improve
>>> NFS performance. I'd like CIFS and zfs send performance to be
>>> reasonable as well.
>
The simple answer is, they can help, but I doubt you will see much 
improvement with a single disk, since that is the major bottleneck.

The standard disk i/o performance guidelines like spindle count and rpm 
still apply.

Read cache is only useful if the same file or data is being read 
multiple times, since the cache needs to be populated.

For writes, if the server has plenty of memory and a zpool with an 
optimal spindle count is as fast as an ssd, then it won't give as much 
benefit, although it does have the benefit of being non-volatile.
I seem to recall mention that the dedicated zil is only used if it's 
needed.

I have great performance from a FC target using 3 x SAT2-MV8 and 20 x 
2Tb sata disks without an ssd zil.

Mark.