Re: using CentOS as an iSCSI server?

[Les Mikesell <lesmikesell@xxxxxxxxx>]

Ross Walker wrote:
> On Oct 21, 2009, at 9:47 AM, Rainer Duffner <rainer@xxxxxxxxxxxxxxx>  
> wrote:
> 
>> Chan Chung Hang Christopher schrieb:
>>> I suspect so. After all, it is just seen as a disk as far as md is
>>> concerned and it will do the same normal thing if you unplugged a  
>>> single
>>> disk from the array.
>>>
>>
>> But the latency over the net is much higher.
>> Who knows if the kernel can handle this in all situations?
> 
> I'm sure the kernel can handle the slowness, it's the cache  
> consistency one has to be careful with in these setups. With so many  
> caching devices in the chain, one must make sure the write and read  
> cache is consistent throughout.

Journaled file systems should take care of the consistency issues. 
However you are adding some new failure modes and making this work 
depends on the right software layers seeing errors at the right time.  A 
target disk error will probably propagate back quickly so the md can 
kick the device out, but what if the error is in the network connection 
or the OS disk on the target?  Will you sit doing 20 minutes of TCP 
retries before the upper layers see an error - and what kind of error 
will they get?

-- 
   Les Mikesell
    lesmikesell@xxxxxxxxx
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