From mboxrd@z Thu Jan 1 00:00:00 1970 From: Adi Kriegisch Subject: Re: blk[front|back] does not hand over disk parameters Date: Mon, 28 Feb 2011 12:54:58 +0100 Message-ID: <20110228115458.GC10906@vrvis.at> References: <20110225164344.GW10906@vrvis.at> <1298887564.5034.584.camel@zakaz.uk.xensource.com> <4D6B8D20020000780003409E@vpn.id2.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Return-path: Content-Disposition: inline In-Reply-To: <4D6B8D20020000780003409E@vpn.id2.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Jan Beulich Cc: "xen-devel@lists.xensource.com" , Ian Campbell List-Id: xen-devel@lists.xenproject.org On Mon, Feb 28, 2011 at 10:55:12AM +0000, Jan Beulich wrote: > >>> On 28.02.11 at 11:06, Ian Campbell wrote: [SNIP] > > It should be trivial to add this in a compatible manner since the > > frontend can just do what it does today if the nodes are missing and the > > backend wouldn't rely on the frontend doing anything useful with the > > information anyway. > > Am I right in understanding that these numbers aren't used by > the block layer itself at all, but just get provided to userspace for > whatever optimization it can do? In that case, I can't really see > how passing through these values can really help general > performance (i.e. for apps not paying attention to these values). AFAIK these values are used by mkfs.* in userspace and by the I/O Schedulers in kernel space to optimize performance. There has been some discussions about that on the kernel mailing lists[1] and there is an interesting document about that available from Mike Snitzer[2]. Those values are important for 4K block size drives, for SSDs and -- as in my case -- for RAID levels with checksums. A quick test with a samba server installed in Dom0 revealed that those values do not need to be honoured by Samba to get full write speed. I/O scheduler seems to be the one that needs those values. -- Adi [1] http://marc.info/?l=linux-ide&m=124058535512850&w=4 [2] http://people.redhat.com/msnitzer/docs/io-limits.txt