From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ram Ramesh Subject: Re: How to use --freeze-reshape and is it safe? Date: Thu, 14 Aug 2014 01:25:28 -0500 Message-ID: <53EC5658.3050708@gmail.com> References: <53EC4B63.90703@gmail.com> <20140814155633.567baece@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20140814155633.567baece@notabene.brown> Sender: linux-raid-owner@vger.kernel.org To: NeilBrown Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On 08/14/2014 12:56 AM, NeilBrown wrote: > On Thu, 14 Aug 2014 00:38:43 -0500 Ram Ramesh wrote: > >> I was browsing through mdadm man pages to check out --layout options >> when converting 3disk-raid5 to 4disk-raid6 and encountered >> --freeze-reshape switch/arg. I did a quick google and could not get much >> info. Can a user issue this to suspend reshape for a short while? > As --freeze-reshape is only meaningful in combination with --assemble, > this question doesn't really make sense. > > If you are using a sufficiently new kernel and mdadm so that "data_offset" is > adjusted during reshapes so that no 'backup' is needed, then you can > suspend a reshape for a period of time by: > > echo frozen > /sys/block/mdXXX/md/sync_action > > This is perfectly safe. When you want to unfreeze, write 'idle' > to 'sync_action'. md will notice that a reshape is pending and will restart > where it was up to. > > >> Specifically >> >> 1. Is the use (or frequent use) of this switch safe? recommended? >> 2. Can the array be mounted when this switch is used? >> 3. What is correct syntax for the usage? >> 4. Can I use this to manage the reshape load on an array? May be to let >> the disk cool off after a busy hours of seeking to reshape? >> 5. Can I use it as a safe method for shutting down the machine? >> 6. Is there a tutorial/faq/manual that explains in detail the use of >> other mdadm esoteric switches? (like --layout I was searching) > Is it really that esoteric? > If you want to reshape an array, you run "mdadm --grow" and list all the > changes you want to make. Set a new level, a new number of devices, a new > layout, a new chunk size, whatever. mdadm will do it if it can and give an > error if it cannot. > If you want to test it out first then that is extremely sensible. Make some > loop devices and experiment. > > NeilBrown Thanks. The name --freeze-reshape mislead me in to thinking that this is a request to stop reshape just like -fail is to make a drive failed. I used esoteric to mean not routinely used or cannot be interpreted by plain English meaning of the the switch/arg name. While I am at this, let me ask the --layout question also. Does conversion from raid5 to raid6 do --layout=left-symmeric-6 first and then distribute Q through second pass with --layout=left-symmetric? If not, will the reshape be faster if I did it in two phases? Ramesh