Mike Christie, on 02/25/2009 08:38 PM wrote:
> Another reason a lot of distros do not support it is because a common 
> problem we always hit is that users will write out some data, then start 
> modifying it again. But the kernel will normally not do do a sync write 
> when you do a write. So once the write() returns, the kernel is still 
> sending it through the caches, block, scsi, and iscsi layers. If you are 
> writing to the data while the it is working its way through the iscsi 
> layers, the iscsi layer could have done the digest calculation, then you 
> could modify it and now when the target checks it the digest check will 
> fail. And so this happens over and over and you get digest errors all 
> over the place and the iscsi layers fire their error handling and retry 
> and retry, and in the end they just say forget it and do not support 
> data digests.

During testing of iSCSI-SCST with data digests enabled with open-iscsi 
initiator I've regularly once in several hours seen data digests errors. 
I was going to investigate it, but had no time. Now I know the reason.

Thanks for the explanation!

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