[one-users] Setting the "SHAREABLE" attribute

[Gareth Bult]

Sorry, my fault, attention is elsewhere ..

I actually use "writethrough" rather than "writeback" .. so with the exception of the quote from the
mailing list, where I said "writethrough", please read "writeback" ... apologies.

I "used" to use writeback, but then read a few horror stories and tried writethrough .. and when used
with vdc-store, writethrough .vs. writeback makes almost no difference anyway as vdc-store buffers the
output "and" is migration safe ...

I've done (literally) thousands of migrations using VM's under IO load with writethrough cache (damn! 
I just typed  writeback again and had to correct! losing my mind!) while testing with ON, never had a 
problem or corruption - so for me writethrough is certainly safe enough to make me want to use the
sharable flag .. :)

-- 
	
Gareth Bult 
“The odds of hitting your target go up dramatically when you aim at it.” 
See the status of my current project at http://vdc-store.com 


----- Original Message -----
From: "Stefan Kooman" <stefan at bit.nl>
To: "Gareth Bult" <gareth at linux.co.uk>
Cc: "Javier Fontan" <jfontan at opennebula.org>, users at lists.opennebula.org
Sent: Thursday, 30 January, 2014 2:59:33 PM
Subject: Re: [one-users] Setting the "SHAREABLE" attribute

Quoting Gareth Bult (gareth at linux.co.uk):
> Mmm, there has been some discussion around this ..
> 
> Technically using cache=writeback "should" be safe for migration.
> (which is the method I use)
> 
> So .. I need "cache=writeback" and "sharable" for migration to happen.
> (the performance hit with cache=off is an unacceptable performance hit, at least for me ..)
> 
> From the QEMU lists, 2012;
> "In short, if you're using a recent kernel with ext3 or ext4, cache=writeback is absolutely safe. If you're using an older version of ext3, cache=writeback is still safe but ext3 itself isn't. cache=writeback can make the situation worse."

Hmm, the guys at big blue think otherwise [1]:

With caching set to writeback mode, both the host page cache and the
disk write cache are enabled for the guest. Because of this, the I/O
performance for applications running in the guest is good, but the data
is not protected in a power failure. As a result, this caching mode is
recommended only for temporary data where potential data loss is not a
concern.

Gr. Stefan

[1]:
http://pic.dhe.ibm.com/infocenter/lnxinfo/v3r0m0/index.jsp?topic=%2Fliaat%2Fliaatbpkvmasynchio.htm

-- 
| BIT BV  http://www.bit.nl/        Kamer van Koophandel 09090351
| GPG: 0xD14839C6                   +31 318 648 688 / info at bit.nl