[pve-devel] diskstats : wchar vs write_bytes

[Alexandre DERUMIER]

I read the doc, for reads I understand

rchar : read from the real underlying storage
read_bytes : read from kvm host buffer cache

but for writes I don't understand...
I don't use writeback, so data should be write directly to storage.

vm use cache=none.

So data must be write to the cache of the storage, then flushed each time the vm do a fsync, or barrier occur.
I'll try to do some tests with writethrough to see how are stats.

Thanks 

Alexandre


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

De: "Dietmar Maurer" <dietmar at proxmox.com> 
À: "Alexandre DERUMIER" <aderumier at odiso.com>, pve-devel at pve.proxmox.com 
Envoyé: Samedi 28 Avril 2012 09:05:11 
Objet: RE: [pve-devel] diskstats : wchar vs write_bytes 

> I was checking rrd io stats for a vm I was installing, and something strange. 
> 
> I have a lot of writes, but 
> wchar < rchar 
> but 
> write_bytes > read_bytes 
> 
> 
> write_bytes,read_bytes seem to be the correct values, reported in qemu 
> monitor also 
> I also check on my storage,write_bytes is the correct value. 
> 
> what's the difference between wchar and write_bytes ? 

Cache? Details are available here: 

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=Documentation/filesystems/proc.txt;h=b7413cb46dcb1f1a331068861e766a6c03a5da40;hb=HEAD#l1387 

- Dietmar 




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