iSCSI; CRC/Checksum; Data Header Digest

iSCSI related applications
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iSCSI; CRC/Checksum; Data Header Digest

Post by millhaus » Fri Apr 27, 2012 10:46 pm

Hello Community,

during creation of targets I find the optional setting of "CRC/Checksum" with two checkboxes. "Data Digest" and "Header Digest".
Due to the fact the we sometimes have issues on reconnecting to an iSCSI target on a TS809-U after a reboot I thought about activating this feature.
On the system connection logs of the QNAP is only documented like "iSCSI - Discovery Session - Login Fail".

But I actually do not know what exactly these features do mean?
SCSI CRC Checksum: Data/Header Digest?

a) Does this only play a role on the time for the authentication?
b) or will this checksum be used for the whole data traffic (reading and writing)?

On my short research I found it has to do something with internal SCSI checksum commands to correct (if necessary) corrupted SCSI blocks to the SAN target. ... ps=446&hl=

Due to the fact that in iSCSI the SCSI commands also do rely on the error correction of TCP/IP already by Layer2/3 it shouldn't be necessary of using the iSCSI Data/Header Digest, too?

So I guess: (please correct me if I am wrong)
Usage of iSCSI CRC Data/Header Digest:
+ Less chance of sending corrupt SCSI blocks to the iSCSI target SAN
- bigger headover --> less performance in writing/reading to/from an iSCSI target SAN

Can someone please give a bit more precise input about this feature with hopefully some test/benchmarks or simple experiences?

Many thanks

PS: 1x NAS TS809U (8x HDD @ R5): Several targets with several LUNs with enabled CHAP authentication, CRC/checksum inactive so far.
Clients connecting to NAS: Win Server2k8R2 x64 SP1 with MS Software iSCSI Initiator through Gigabit Ethernet switched LAN Cat6

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Re: iSCSI; CRC/Checksum; Data Header Digest

Post by seamanjeff » Fri Jan 11, 2013 6:31 am

unanswered.. wow fail

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Re: iSCSI; CRC/Checksum; Data Header Digest

Post by schumaku » Fri Jan 11, 2013 7:12 am

seamanjeff wrote:unanswered.. wow fail

Wrong questions, expectations set too high - not a failure ....

I am guessing that most are using iSCSI on a switched LAN, and not on a comparably unreliable Internet connection.

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Re: iSCSI; CRC/Checksum; Data Header Digest

Post by vonSchnoid » Sun Mar 03, 2013 5:20 pm

To clarify the question:

The CRC/Data Header Digest is used only in unreliable environments where one can expect a lot of errors.
Since iSCSI resides on top of the normal TCP protocol there can sometimes be necessary with extra error checking.
So in general if you are on a local LAN with good cabling and switches there is no need to activate this feature.
And yes, there will be an overhead from the extra calculating, i don't know how big impact it will have though.

Here you can read more, if somewhat brief: ... 44(v=ws.10).aspx

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Re: iSCSI; CRC/Checksum; Data Header Digest

Post by vinix » Fri Dec 08, 2017 11:10 am

Just to add some information to this aged question.

I made a test on my Gbps environment, the speed dropped to 30~40 MB/s if I enabled all 2 digest checkboxs. I can get about 100 MB/s If I unchecked them.

Test environment:
NAS: TS-831x
Switch: D-Link DGS-1210-16
MB: Gigabyte GA-H61MA-D3V (rev. 2.0)
NIC: Realtek 8111E/8111F on board
HD: Seagate ST2000DM001 via SATA 3Gb/s slot

So, in my opinion, if you are in a LAN, disable digest check is better, unless you think the network quality is poor.

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Re: iSCSI; CRC/Checksum; Data Header Digest

Post by vmqn » Mon May 28, 2018 6:41 am

vinix, that was a super helpful contribution. thanks.

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