From mboxrd@z Thu Jan 1 00:00:00 1970 From: aristizb@ualberta.ca Subject: Re: Linux md and iscsi problems Date: Mon, 04 Feb 2008 09:53:35 -0700 Message-ID: <20080204095335.n26tlbaav4s8sk08@webmail.ualberta.ca> References: <20080201115919.5qx4ih3zeogswc4w@webmail.ualberta.ca> <18340.58431.163856.338970@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; DelSp=Yes format=flowed Content-Transfer-Encoding: 7BIT Return-path: In-Reply-To: <18340.58431.163856.338970@notabene.brown> Content-Disposition: inline Sender: linux-raid-owner@vger.kernel.org To: Neil Brown Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Good morning. Quoting Neil Brown : > On Friday February 1, aristizb@ualberta.ca wrote: >> >> >> Summarizing, I have two questions about the behavior of Linux md with >> slow devices: >> >> 1. Is it possible to modify some kind of time-out parameter on the >> mdadm tool so the slow device wouldn't be marked as faulty because of >> its slow performance. > > No. md doesn't do timeouts at all. The underlying device does. > So if you are getting time out errors from the iscsi initiator, then > you need to change the timeout value used by the iscsi initiator. md > has no part to play in this. It just sends a request and eventually > gets either 'success' or 'fail'. > 1. What seems strange to me is that under the same conditions running the test only with iscsi, the initiator never fails. I had some problems with iscsi on the past and they were solved changing the time-out parameters on the initiator side, but now that I added the md layer I am getting errors with the slow device. >> >> 2. Is it possible to control the "buffer" size of the RAID?, in other >> words, can I control the amount of data I can write to the local disc >> before I receive an acknowledgment from the slow device when I am >> using the write-behind option. > > No. md/raid1 simply calls 'kmalloc' to get space to buffer each write > as the write arrives. If the allocation succeeds, it is used to > perform the write lazily. If the allocation fails, the write is > performs synchronously. > > What did you hope to achieve by such tuning? It can probably be > added if it is generally useful. > > NeilBrown > > 2. The idea here is to implement remote replication, by having a RAID-1 I can create a mirror of my local disk and use it for backup or for any other purposes on a centralized location. Changing the buffer will allow me to improve performance on the writing, so a user will always experience local writing speed, while I am still sending data across the network to the mirror device. Thanks a lot for your time. Juan Aristizabal.