[pve-devel] qemu 1.2 :default cache mode writeback

Alexandre DERUMIER aderumier at odiso.com
Thu Sep 6 14:28:55 CEST 2012


Damn, I have redone test with a 3.6 guest kernel, and now it's work fine.

So writeback in guest is the default if nocache= is defined.
Don't know if on the host side, cache is enable (none or writeback)

But if I understand the default cache on qemu 1.1 was writethrough in guest. (so directsync or writethrough on host).
And not none, as it's in pve-manager currently.
I'll do same test with qemu 1.1 to be sure.



qemu 1.2

ide
---
no cache defined :  hdparm -W /dev/sda : on
cache=none  : hdparm -W /dev/sda : on
cache=writeback : hdparm -W /dev/sda : on
cache=writethrough : hdparm -W /dev/sda : off
cache=directsync : hdparm -W /dev/sda : off


virtio-scsi
---------------------
no cache defined : sdparm --get WCE /dev/sda : on
cache=none :  sdparm --get WCE /dev/sda : on
cache=writeback :  sdparm --get WCE /dev/sda : on
cache=writethrough :  sdparm --get WCE /dev/sda : off
cache=directsync :  sdparm --get WCE /dev/sda : off

lsi-scsi
---------------------
no cache defined : sdparm --get WCE /dev/sda : on
cache=none :  sdparm --get WCE /dev/sda : on
cache=writeback :  sdparm --get WCE /dev/sda : on
cache=writethrough :  sdparm --get WCE /dev/sda : off
cache=directsync :  sdparm --get WCE /dev/sda : off

virtio (guest kernel 3.6-rc4)
------------------------------
no cache defined : cat /sys/block/vda/cache_type : write_back
cache=none :  cat /sys/block/vda/cache_type : : write_back
cache=writeback :   cat /sys/block/vda/cache_type : write_back
cache=writethrough :   cat /sys/block/vda/cache_type : write_through
cache=directsync :   cat /sys/block/vda/cache_type : write_through


----- Mail original ----- 

De: "Alexandre DERUMIER" <aderumier at odiso.com> 
À: pve-devel at pve.proxmox.com 
Envoyé: Jeudi 6 Septembre 2012 13:38:58 
Objet: Re: [pve-devel] qemu 1.2 :default cache mode writeback 

firsts test (rc2) 

qcow2 + ide 
----------- 
no cache defined : hdparm -W /dev/sda : off 
cache=none : hdparm -W /dev/sda : on 
cache=directsync : hdparm -W /dev/sda : off 
cache=writethrough : hdparm -W /dev/sda : off 
cache=writeback : hdparm -W /dev/sda : on 



so the default mode seem to be writethrough or directsync ..??. 

I'll do more tests with scsi disk, and also with kvm 1.1 


----- Mail original ----- 

De: "Alexandre DERUMIER" <aderumier at odiso.com> 
À: pve-devel at pve.proxmox.com 
Envoyé: Jeudi 6 Septembre 2012 12:18:09 
Objet: Re: [pve-devel] qemu 1.2 :default cache mode writeback 

I have found the commit, 
I'm not sure it's related to the host cache, but maybe more the cache inside the guest ? 




"Now all major device models (IDE, SCSI, virtio) can choose between 
writethrough and writeback at run-time, and virtio will even revert 
to writethrough if the guest is not capable of sending flushes. So 
we can change the default to writeback at last. 

Tested, for lack of a better idea, with a breakpoint on bdrv_open 
and all cache choices one by one. 

Signed-off-by: Paolo Bonzini <pbonzini at redhat.com> 
--- 
blockdev.c | 1 + 
1 file modificato, 1 inserzione(+) 

diff --git a/blockdev.c b/blockdev.c 
index 8669142..7c83baa 100644 
--- a/blockdev.c 
+++ b/blockdev.c 
@@ -377,6 +377,7 @@ DriveInfo *drive_init(QemuOpts *opts, int default_to_scsi) 
} 
} 

+ bdrv_flags |= BDRV_O_CACHE_WB; 
if ((buf = qemu_opt_get(opts, "cache")) != NULL) { 
if (bdrv_parse_cache_flags(buf, &bdrv_flags) != 0) { 
error_report("invalid cache option"); 
-- 
1.7.11.2 

" 


----- Mail original ----- 

De: "Alexandre DERUMIER" <aderumier at odiso.com> 
À: pve-devel at pve.proxmox.com 
Envoyé: Jeudi 6 Septembre 2012 12:13:48 
Objet: [pve-devel] qemu 1.2 :default cache mode writeback 

I just see that in the release note: 

block: default cache mode is now writeback 


Maybe do we need to warn proxmox users and update pve-manager ? 

(default was cache=none if not defined) 




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