Initiator; Disk Management with Write Cache for iSCSI

[millhaus]

Hi Community,

looking on the diskmgmt.msc of our 2008R2 we found some irritating disk settings. On machine A the disk write cache was enabled for all iSCSI devices. On machine B the disk write cache was disabled for all iSCSI devices. Both servers do handle backups with mostly read-ops instead of write-ops (4/1 [read/write].

!!!Please NOTE I am speaking of the Write Cache for the filesystem of the MS initiator in the disk management and NOT the write cache on the QNAP storage!!!
After a short research I found out following:
Active write cache: More performance (but also some do say you do not really benefit at all)
No write cache: Less performance, but more stability, less chance of filesystem damage or data loss.

Question: 1) Therefore I am asking you for advice: What should we use and why? Personally, stability and redundancy have the highest priority. Performance comes only after.
Question: 2) I thought this write cache should only be available to removable storage media like USB, SD devices, but not “local” iSCSI devices. Why does Microsoft offer this solution to it, anyway?

Thanks

PS: TS-859U+ @ 3.8.4 Build 20130816
6x HDDs @ RAID6 (1x hot spare, 1x cold spare)
iSCSI and SNMP only

[storageman]

I doubt Qnap is "FUA-capable storage systems", therefore leave it disabled.
I wouldn't worry about this setting, the 859 is more likely to be the bootleneck over any windows setting.
Read performance is improved by host processor mostly with ISCSI.
http://blogs.technet.com/b/storageserve ... erver.aspx

[millhaus]

I wanted to disable the caching, but I can't?!?
When I try to it tells me I can not disable it.
Also, on the event viewer it tells me something like this (translated by me) "Warnung; Disk - the driver noticed that the write cache is active. the data could be corrupted".

After a little research, it is probably due to the (i)SCSI-Controller and the connected disk(s).
But, it is a VM (2008R2 x64 SP1) virtualized by Citrix XenServer 5.6SP2 and is using its internal MS iSCSI initiator.

On the other machine B (basically the same setup), the disk caching is not active. But machine B has a TS809U connected, the machine A has two TS859U-RP+ connected.

All 3 QNAPs have setting: hardware > NO ext4 write cache.

Do you know how to disable the write cache? Or is it - by default - somehow active on the 2x TS859U?

[storageman]

Sorry looks like you can't alter it anyway. I think this warning maybe a misnomer.

http://social.technet.microsoft.com/For ... rverhyperv

Try updating to 4.0.5.

[millhaus]

Thanks for the link;
AFAIK, I'll quote the most important post:

...however if on your san everything is disabled, of course it will be disabled on the hyper v host too, and you cant enable it, since on the array is disabled.

same for virtual machines, exception for virtualized DC were the disabling write cache rule is in action.

to sum it up.. you must control your cache on your raid array.. you enable it will be on the os too, you disable it and will be disabled....

Thus, QNAP fw seems to be buggy? How can I check if write cache is enabled via CLI? On WebUI it seems to be disabled. Also on the kmesg logs of QNAP, there is no hint that it may be enabled.

V4xxx seems to be a bit overloaded with SOHO-features we dislike pretty much. Unfortunately, QNAP decided to go back to multimedia, kiddie stuff KMUs do not need. We want stability and reliability. We had bad experience on one of our TS859 devices after upgrade to v4xxx. After the upgrade, SMB services with MS AD auth did no more work.
Is v4.0.5 now more tested and performant?

Thanks