From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Ty! Boyack" Subject: Re: Is it possilble to be "delay tolerant" or have "slow dropout" of unavailable components? Date: Tue, 08 Apr 2008 12:24:02 -0600 Message-ID: <47FBB842.2030902@nrel.colostate.edu> References: <47FBA3BB.2030007@nrel.colostate.edu> <200804081948.41959.Markus@hochholdinger.net> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <200804081948.41959.Markus@hochholdinger.net> Sender: linux-raid-owner@vger.kernel.org To: Markus Hochholdinger Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Markus Hochholdinger wrote: > hi, > > Am Dienstag, 8. April 2008 18:56 schrieb Ty! Boyack: > >> I'm curious if there is a way to have a raid set (raid5 in my case, but >> this could apply to any raid level) that could tolerate a component >> device being unavailable for a period of time. >> > > for RAID1 there is "--write-mostly" and "--write-behind=". Don't know if this > is already available for RAID5. > > There's also the option "--bitmap=" which can speedup a resync when > temporarily disconnecting one device. > > > Thanks - I was looking at those options, but it seems that the 'write-behind' option would need to be applied to ALL devices. It seems to indicate a difference in the devices - one is fast, one is slow, and the slow one is indicated with write-behind. In my case, I think all are fast except in the case of a failure, in which case I'd like to have some delay before it gets declared bad to see if it comes back. As for the 'bitmap' option - I think this has a lot of potential, and might work IF there was an automatically re-add a failed device. With the bitmap I see the following sequence taking place: 1) Device in a raid5 goes away for some reason (iscsi reboot, network glitch, etc.) but the component is really still good. 2) raid5 marks device as bad, starts tracking changes in bitmap 3) device comes back online 4) device is re-added to raid5 5) Resync occurs fast because of the bitmap. So... Perhaps I'm asking for the wrong thing. Is there a way to detect a recovery after a failure, and have it automatically repair the raid set? Right now, without the automation, it is possible, and likely, that an operator cannot respond in time to avoid having the bitmap fill up, and then we are into a long resync. More critically, we would be running with a degraded array from the point of failure until an operator can fix it and the resync finishes, which is frightening. -Ty! -- -===========================- Ty! Boyack NREL Unix Network Manager ty@nrel.colostate.edu (970) 491-1186 -===========================-