[pve-devel] scsi-hd vs scsi-generic with iSCSI

[Daniel Berteaud]

Hi 

After some investigations, I found a way to mitigate two issues I have when working with ZFS over iSCSI (in my case ZoL / LIO) : 



    * First, and more important issue is that drive-mirror (with a running guest) will make the guest FS panic if the source storage is ZFS over iSCSI. See [ https://bugzilla.proxmox.com/show_bug.cgi?id=2384 | https://bugzilla.proxmox.com/show_bug.cgi?id=2384 ] 
    * Second, and minor issue : when resizing a guest live, the guest is not automatically notified about the new size, we have to rescan the device for the new size to be seen. See [ https://bugzilla.proxmox.com/show_bug.cgi?id=2335 | https://bugzilla.proxmox.com/show_bug.cgi?id=2335 ] 

After some digging, I found that when using scsi-hd (instead of the default scsi-generic), everything is working correctly. I understand scsi-generic is supposed to perform better as it bypass the SCSI emulation layer in qemu. But, I prefer a fully working and stable setup over a +5% more performant but without live move storage. (BTW, perf penalty with scsi-hd isn't huge, there are even some workflow in which it performs better, but I'm still measuring it right now) 

In /usr/share/perl5/PVE/QemuServer.pm scsi-generic is curretly hardcoded for any iscsi backed device (using libiscsi from qemu). 

What would be the best way to handle this ? 

For the first issue, I'm not the only impacted, and it seems to exists no matter what iSCSI target is used (ZoL + LIO, ZoL + SCST, FreeNAS with ctld, OmniOS with Comstar). Should we set scsi-hd for any ZFS over iSCSI storage ? Should we add a setting on the storage ? (some uses might prefer the small perf boost of scsi-generic and are not interested in disk move) 

What do you think ? 

Cheers, 
Daniel 

-- 


[ https://www.firewall-services.com/ ] 	
Daniel Berteaud 
FIREWALL-SERVICES SAS, La sécurité des réseaux 
Société de Services en Logiciels Libres 
Tél : +33.5 56 64 15 32 
Matrix: @dani:fws.fr 
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