linux-kernel - Re: Failover root devices

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Message-ID: <55FAF8C0.30205@cmpwn.com>
Date:	Thu, 17 Sep 2015 13:30:40 -0400
From:	Drew DeVault <sir@...wn.com>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Failover root devices

> That said, using the term failover for this is probably not the best
> idea, many people associate it almost exclusively with online failover
> and high-availability setups, and trying to do something like that with
> the root file system is just asking for trouble (I'll be happy to go
> into specifics as to why if someone asks).

Do you have a suggestion for another name for this feature? Maybe we can 
just call it "multiple root devices". The issue comes with the 
associated command line options, like "rootfailoverdelay". Perhaps it 
could be called "rootcycledelay". "rootdelay" is the obvious one, but 
it's taken for another feature.

>> 1. Wait rootdelay seconds
>> 2. Check 1st device, not present
>> 3. Recheck 1st device until rootfailoverdelay seconds has passed
>> 4. Move on to 2nd device, present -> boot
>>
>> Or:
>>
>> 1. Wait rootdelay seconds
>> 2. Check 1st device, not present
>> 3. Recheck 1st device until rootfailoverdelay seconds has passed
>> 4. Move on to 2nd device, not present
>> 5. Recheck 2st device until rootfailoverdelay seconds has passed
>> 6. GOTO 2
>>
>> And so on.
> As for this, I'd say default to the first method, and then provide an
> option to switch to the second (both have practical uses).

Sorry to cause confusion - these are actually the same method, but 
handling different scenarios. The first is dealing with the first device 
being nonexistent, and the second device existing. The second is dealing 
with both being nonexistent, and cycling between them until one of them 
shows up. After further thought, though, I think the best solution is a 
bit different: a new command line option called "rootmultiwait" or 
similar, which is a maximum amount of time to wait for the user's first 
choice of root device to become available, then testing all devices 
until that time runs out or the first choice becomes available.

--
Drew DeVault
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