On 14 Jan 2009 at 4:25, HIMANSHU wrote:

> 
> IET target is connected by MS initiator.Data transfer is going on on
> the connected disk on initiator.
> 
> TCP connection failed(because of any reason,service failure,problems
> in kernel modules)  at say 't1' second.Initiator kept trying for
> reconnect.
> 
> Initiator actually recognizes that target is down at say 't2' seconds.
> 
> Then for the time between 't1' and 't2',data is written on the cache
> on initiator side giving illusion of data transfer.After some time,it
> gave delayed write error.

It's not an illusion of data transfer, but a shift in responsibility: Having 
buffered the data to write, the kernel is responsible to finally write the 
data. 
In HP-UX (without iSCSI) messages may look like these:

vmunix: LVM: WARNING: VG 64 0x1f0000: LV 1: Some I/O requests to this LV are 
waiting indefinitely for an unavailable PV. These requests will be queued until 
the PV becomes available (or a timeout is specified for the LV).
[...]
vmunix: LVM: NOTICE: VG 64 0x1f0000: LV 1: All I/O requests to this LV that 
were 
waiting indefinitely for an unavailable PV have now completed.

Naturally if the user panics in between, data is lost. The sysadmin has the 
responsibility to restore the network connection in the iSCSI case.

If you application relies that the data is written upon return of a write(), it 
should use the O_SYNC (in Linux) open() flag.

> 
> So data is not actually stored on the target.
> 
> What can we do here?Using high capacity network can be the solution in
> case of dedicated framework so that initiator will recognize the
> failure as early as possible.

I'd consider this type of failure the same as if you unplug the IDE or SCSI 
cable 
while the system is running. If your system is very stable, it can recover from 
the situation if the cable is reconnected. However it's usually best to avoid 
such 
situations.

Just my opinion....

Regards,
Ulrich


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