Re: [Qemu-discuss] Understanding QEMU Cache Modes and Emulated Disk Write Cache

[Andrew Martin]

----- Original Message -----
> From: "Narcis Garcia" <address@hidden>
> To: address@hidden
> Sent: Thursday, August 18, 2016 1:10:12 AM
> Subject: Re: [Qemu-discuss] Understanding QEMU Cache Modes and Emulated Disk 
> Write Cache
> 
> Sure.
> 
> 
> El 17/08/16 a les 23:09, Andrew Martin ha escrit:
> > ----- Original Message -----
> >> From: "Narcis Garcia" <address@hidden>
> >> To: address@hidden
> >> Sent: Wednesday, August 17, 2016 1:16:19 PM
> >> Subject: Re: [Qemu-discuss] Understanding QEMU Cache Modes and Emulated
> >> Disk Write Cache
> >>
> >> In the host:
> >> $ sysctl -n vm.dirty_expire_centisecs
> >> $ sysctl -n vm.dirty_writeback_centisecs
> >>
> >> https://www.kernel.org/doc/Documentation/sysctl/vm.txt
> > 
> > Thanks, this is what I was looking for! Don't you need to add these values
> > together for both
> > the kernel running in the guest and on the host to find out the maximum
> > time before a flush?
> > 

Actually I think I was wrong - only the values for the guest kernel should
matter because once the dirty_writeback_centisecs period is hit, it issues a
flush which will bypass the host page cache and flush the dirty data to
permanent storage. Thus the maximum time from write to permanent storage for a
guest is just (vm.dirty_expire_centisecs + vm.dirty_writeback_centisecs)/100
seconds.

Andrew