Ty! Boyack escribió:
> 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?
>
> 2) Is this type of bursty traffic normal?
>
> 3) This may be related to the queue_depth filling up - is there an easy 
> way to see the depth of the queue in real time? (Not the setting, but 
> the actual state of the queue)
>
> I know the setup is a bit convoluted, so here another view of it.  The 
> physical data blocks are on the left, becoming more layered to the right 
> until we hit our "production" servers. (This is all pre-production now)
>
> SATA_disks -> Raid6_controller -> IET(blockio) -> (next box) -> 
> open-iscsi -> md_raid5 -> lvm -> Logical_volume -> IET(blockio) -> (next 
> box) -> open-iscsi -> filesystem_on_production_server
>
> It is when writing to that filesystem the problem becomes pronounced.
>
> Any insight that anyone has on this would be very welcome.
>   
I haven't tried make a RAID on iSCSI targets but my bet is that adding 
another RAID 5 layer makes it more difficult. RAID 5 performs in writes 
worse than RAID 1 since RAID 5 requires 2 writes and 2 reads when 
performing a writing to the RAID. Try RAID 1 over the iSCSI targets to 
see if the performance improves.

Miguel




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