I got my iSCSI R3 Ent setup running, and have been successful in creating drives, mapping them to my RHEL box, etc. I have 3 targets setup on my iSCSI box for testing. Everything seems ok so far.
However, I noted that when I added the third target, they got mapped to different devices in my initiator's system. For example, initially they were:
disk 1 -> /dev/sdb
disk 2 -> /dev/sdc
But when I added the third drive, it became /dev/sdc and disk 2 moved to /dev/sdd.
I am using the iscsi-initiator-utils rpm that is included with RHEL 4.5. I imagine its just a matter of somehow statically mapping this, but can anyone shed some light on how I can make sure that the same targets get mapped to the same devices? I cannot find any articles online that discuss doing this.
One more question, if I can indulge someone... Write back caching. I have options to do this in my RAID controller (Areca card w/o battery backup), in the iSCSI software, and it appears that Linux also does it at least to some degree. While I don't expect power issues in my data center, and I have redundant power supplies in the iSCSI box, I am concerned about other issues that could cause a lot of problems if I had too much/multiple layers of write caching going on (i.e. if the iSCSI box were to crash, and/or the amount of time needed for it to be safe to shutdown the iSCSI boxes if needed). FWIW, I do have a second open-e iSCSI box I am replicating the primary one too, for safety. Not sure how that figures into the picture.
It seems high risk to enable write back at each option - any thoughts on where it would be most advantageous? Let Linux do it? Let the RAID card do it? With all three doing it, its REALLY fast to write, but it seems quite dangerous too.
Thanks for any help on my iSCSI config, and/or your opinions on write back caching. I think this setup will really rock once I get the tweaks in place.
Concerning the WB each has there responsibilities to perform so when all are using WB "a means for Write-back caching" - it will perfrom better for each component transfer. So I really do not see a danger with setting these options. IMHO
Awesome. I feel stupid now as I actually did read that, but missed the part that says:
It is extremely important to use disk labels when working with iSCSI storage. Delays in network traffic may cause the LUNs to be discovered in a different order the next time the system is booted. The only way to guarantee that the correct partition will be mounted at the proper mount point is to use disk labels to identify them.
Oh well, guess it was too easy - forest for the trees and all that.
Also, re: WB caching, I will crank it all up then, I guess. Now I just have to start tweaking to improve the read speeds.
Help configuring iSCSI in Linux, and write back ?
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