In the host: $ sysctl -n vm.dirty_expire_centisecs $ sysctl -n vm.dirty_writeback_centisecs
https://www.kernel.org/doc/Documentation/sysctl/vm.txt El 17/08/16 a les 20:03, Andrew Martin ha escrit: > ----- Original Message ----- >> From: "Jakob Bohm" <[email protected]> >> To: "Andrew Martin" <[email protected]> >> Cc: [email protected] >> Sent: Wednesday, August 17, 2016 10:32:39 AM >> Subject: Re: [Qemu-discuss] Understanding QEMU Cache Modes and Emulated Disk >> Write Cache >> >> On 17/08/2016 16:56, Andrew Martin wrote: >>> >>> >>> ----- Original Message ----- >>>> From: "Jakob Bohm" <[email protected]> >>>> To: [email protected] >>>> Sent: Wednesday, August 17, 2016 9:34:32 AM >>>> Subject: Re: [Qemu-discuss] Understanding QEMU Cache Modes and Emulated >>>> Disk Write Cache >>>> >>>> On 17/08/2016 15:21, Andrew Martin wrote: >>>>> According to this page, cache=writeback enables the disk write cache: >>>>> http://www.ibm.com/support/knowledgecenter/linuxonibm/liaat/liaatbpkvmguestcache.htm >>>>> >>>>> Is this the write cache on the physical disk itself, or the write cache >>>>> on >>>>> the >>>>> qcow2 image (does the qcow2 even have a disk write cache)? Does this >>>>> cache >>>>> get >>>>> bypassed when fdatasync(2) calls come in from the guest? >>>>> >>>>> When is the guest's page cache ever used? >>>> Whenever the Guest OS feels like it, because the Guest OS doesn't (in >>>> principle) know the virtual disk is virtual. >>> Would this primarily be for reads then, since write(2) is written to the >>> host >>> page cache? >> >> The write(2) calls we are talking about are the virtual hardware disk >> writes done by the guest OS, qemu does not know or care if the Guest OS >> caches data before doing those writes. >> >>> >>> Does QEMU create an emulated disk cache for the qcow2 image, or is that >>> just >>> the host page cache? >> >> I don't know. I presume that qemu at least keeps some of the qcow2 >> metadata in memory to avoid excessive host disk I/O. >> > Thanks for the clarification. One final question - how can I calculate the > delay > between when the data is written and when it is flushed to permanent storage? > Is > this just the period between subsequent fdatasync(2) calls when I strace the > QEMU > process? > > Thanks, > > Andrew >
