On Fri, 10 Jul 2015 07:53:16 +0200 (CEST), Gabriele Bulfon wrote:
As Yuri suggested, I discovered all my previous post is actually around
zvol wce, not the sync property of zvol, nor iscsi.
That wce flag on older zvol was still showing as disabled, using Robert
Milkowski wce_zvol.
But using "smtfadm list-lu -v", I finally got the answer: all of older
kernel (2012) zvol are running with Write Cache Enabled,
they were created like this by default by create-lu.
So, the questions:
1. how much is the risk of having wce enabled on a zvol shared to vmware?
2. am I really running in total async mode with wce enabled, or sync
requests are honored and normal ones cached?
3. is there any real difference in this scenario if zvol is iscsi-shared
(iqn) or fc-shared (wwn)?
4. considering zvols are all sync=standard, what is the difference (in
this case) of turning wce=off or sync=always?
I have the idea that sync=always will have a severe impact on perfs,
while wce=off should impact only sync requests.
5. should I turn off wce through Milkowski command (through ioctl) or
"smtfadm modify-lu" setting wcd=on?
You don't need the 3d party tool, using stmfadm is fine.
6. Is there anything else I can do to put these zvols in good safety
without affecting perfs too much?
7. last but not least, the wce commit Yuri refers to (changing wce
default to off) is of June 2013.
My wce affected zvols are of Apr/May 2012.
A lot of posts about wce/sync/zvol between 2010 and 2012 always says
that zfs default is wce=off.
Does this mean the wce=on issue was introduced somewhere in between?
Defaults for zvol's wce was and is off, but when you created LU using
`stmfadm create-lu` (and not specifying the wcd setting explicitly), the
bug in STMF code would treat it as wcd=false (hence, unconditionally
enabling the write cache on zvol), that commit simply fixed this.
Thanks for any advice.
Gabriele
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