Ulrich Windl wrote:
> Hello,
> when browsing the open-iscsi feature list, I found:
> - PDU header Digest;

Is that from suse's docs or open-iscsi.orgs?

> Does this mean that data digests are not supported? A bugzilla at readhat 
> near mid 
> of 2007 seems to confirm this.

Data digests were working but when upstream did the scatterlist changes 
to the kernel it broke data digests. We have not found the cause yet.

For Red Hat, they do not support them for different reasons depending on 
the version and arch. For example in RHEL5, the big endien crypto digest 
code is busted. It needs a fix from upstream, and I think in general 
there is still some other bugs in the digest code.

> I see the performance impact, but is there another reason against 
> implementing it? 
> Can I safely activate it on the target, or will it cause problems?

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.

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