ZFS: How to enable cache and logs.

[Alexander Leidinger]

Quoting ©imun Mikecin <numisemis at gmail.com> (from Thu, 12 May 2011  
10:02:42 +0200):

>
> On 12. svi. 2011., at 08:44, Daniel Kalchev wrote:
>
>> On 12.05.11 05:26, Danny Carroll wrote:
>>>
>>>  - Don't use SSD for the Log device.  Write speed tends to be a problem.
>> It all depends on your usage. You need to experiment, unfortunately.
>
> What is the alternative for log devices if you are not using SSD?
> Rotating hard drives?
>
> AFAIK, two factors define the speed of log device: write transfer  
> rate and write latency.

There is also bus contention (either on the SCSI bus, or in the SATA  
channel/controller, or on the PCI-whatever (e/X/y) bus).

> You will not find a rotating hard drive that has a write latency  
> anything near the write latency of even a slowest SSD you can find  
> on the market.
> On the other hand, only a very few rotating hard drives have a write  
> transfer rate that can be compared to SSD's.

And if your PCI-something bus is not saturated but your SCSI/SATA  
controller struggles with the work which is thrown at it, a separate  
log device (normal HD) on another controller could free up the  
pool-controller(s) up to a situation where it can handle all requests  
at full speed and the log-controller can provide the additional  
throughput at full speed which the pool-controller was not able to  
satisfy.

What you do in this case is that you add more spindles (disks)  
dedicated to sync-write operations. The normal RAID-common-knowledge  
of "adding more spindles for more performance" applies here, just that  
it is specially for sync-write operations. The generic hint to have  
them faster than the pool-disks is an answer for the worst case. As  
always, the worst case for one person may not be the worst case for  
another persons workload.

Bye,
Alexander.

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