Buffer I/O error and cache writeback

[kev974]

Hello,

I'm using ISCSI with multipath-tools for storage with LVM on IT.Everything is working fine exept one thing.
I've noticed a problème with my VMs when cache=writeback is activated and virtio for disk images.When i restart open-iscsi when VMs are UP i've got the following errors in /var/log/syslog on the node:

Kernel : [..] Buffer I/O error on dev dm-4, logical block 776513, lost async page write.

So for these VMs we have lots of errors .When we force stopping the VMs the Buffer I/O errors stop and the VMs get UP as normally.

I've switched back VM disk image in cache=none (default) and try restarting open-iscsi services and the there are no mode Buffer I/O errors.

Is those errors normal for cache writeback?am i doing something wrong?

Thanks for your advices.

Sincerely yours,

 

[dietmar]

I guess that is normal, because you loose data from the cache (I would not restart iscsi while VM are running).

 

[kev974]

Thanks for your answer Dietmar.
But if iscsi connexion is lost between the SAN and the node it would happen the same thing no?
We switch cache=writeback because it seems to improve vm disk performance.Is it even safe to keep it or should we use another cache mode?

Sincerely yours,

 

[dietmar]

Is it even safe to keep it or should we use another cache mode?

That depends on the OS and application you run inside the guest. A reasonable database can recover from such errors. But yes, some people think cache=none is safer.