Ty! Boyack wrote:
> Folks,
> 
> I'm working on a setup where I connect to several iSCSI targets, then 
> setup raid-5 on those iSCSI target volumes (The iSCSI targets are 
> themselves raid devices, we are adding a raid-5 layer here to cover the 
> case of a controller/target failure).  We use this very large 
> raid5-of-iscsi-targets as an LVM volume group so that we can carve out 
> logical volumes that safely span across multiple iscsi targets.  We then 
> present these LVM volumes as iSCSI targets using IET.
> 
> In general, this seems to work OK, but when shoving large writes to the 
> constructed iscsi target we see very bursty writes.  It will write 
> quickly for about 30 seconds, then falter to almost nothing for 20 
> seconds, then resume quick operations, and repeat.  This is visible both 
> from the fact that during the 20 seconds of slow the filesystem is 
> unresponsive, and also that the network use goes drastically lower 
> during that time.
> 
> I'm wondering a couple of things:
> 
> 1) Is there any known problem with passing through two layers of iscsi?
> 

Unless you are using a really old version of open-iscsi there should not 
be any problems. With 2 layers we should be ok even with old versions. 
Are you using XFS though? And what kernel?

> 2) Is this type of bursty traffic normal?
> 

No. What version of open-iscsi are you using? I noticed something 
similar for another bugreport with iqstor targets which was fixed with 
this. Could you try it out?
http://www.open-iscsi.org/bits/open-iscsi-2.0-868-test1.tar.gz

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