[pve-devel] new cache benchmark results

[Alexandre DERUMIER]

seem to occur only with O_DIRECT (so cache=none and cache=directsync)

this bugzilla was about cdrom (with large sector), but I don't know if it's apply on 4K hdd

https://bugzilla.redhat.com/show_bug.cgi?id=608548
" Technical note added. If any revisions are required, please edit the "Technical Notes" field
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    New Contents:
Cause: qemu did not align memory properly for O_DIRECT support

Fix: qemu was changed to use properly aligned memory for I/O requests

Consequence: I/O to device with large sector sizes like CDROMs dit not work in cache=none mode
Result: I/O to devices with large sector sizes like CDROMs work in cache=none mode"

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

De: "Alexandre DERUMIER" <aderumier at odiso.com> 
À: "Dietmar Maurer" <dietmar at proxmox.com> 
Cc: pve-devel at pve.proxmox.com 
Envoyé: Jeudi 8 Novembre 2012 18:06:47 
Objet: Re: [pve-devel] new cache benchmark results 

>>Another problem with cache=none is that it does not work with 4K sector drives. 
>> 
>>User reported problems with 4K iscsi and 4K local disks. 
>> 
Yes, I remember the post in the forum. It doesn't work with cache=none or cache=writeback 

>>Any idea how to solve that? 
This is strange, because it seem to be fixed a long time ago(I have seen some redhat bugzilla from 2011 about it) 
But I don't have hardware to test it :( 


I'll look in my archives tomorrow ;) 


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

De: "Dietmar Maurer" <dietmar at proxmox.com> 
À: "Alexandre DERUMIER" <aderumier at odiso.com>, pve-devel at pve.proxmox.com 
Envoyé: Jeudi 8 Novembre 2012 17:50:58 
Objet: RE: [pve-devel] new cache benchmark results 

> Note : with shared storage, with writeback, I generally see big spike (faster 
> than cache=none), but after big slowdown (near zero) during 5-10s. 
> So I think this is when the host need to flush the datas, it add more 
> overhead. (maybe network latency have an impact...) 

Another problem with cache=none is that it does not work with 4K sector drives. 

User reported problems with 4K iscsi and 4K local disks. 

Any idea how to solve that? 
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