Re: Confining scrub to a subvolume

[Duncan <1i5t5.duncan@cox.net>]

Christoph Anton Mitterer posted on Wed, 30 Dec 2015 20:28:00 +0100 as
excerpted:

> On Wed, 2015-12-30 at 18:26 +0000, Duncan wrote:
>> That should work.  Cat the files to /dev/null and check dmesg.  For
>> single mode it should check the only copy.  For raid1/10 or dup,
>> running two checks, ensuring one is even-PID while the other is
>> odd-PID, should work to check both copies, since the read-scheduler
>> assigns copy based on even/odd PID.  Errors will show up in dmesg, as
>> well as cat's STDERR.

> That doesn't seem very reliable to me, to be honest... plus it wouldn't
> work in any RAID56 or dupN (with n!=2) case, when that gets sooner or
> later implemented.

Well, yes, but right now except on raid56... and there's a good chance 
it'll work for a year at least, as I've seen no first-patches yet to 
implement n-way (which I'm sure looking forward to), after which perhaps 
he'll have implemented the multi-btrfs on partitions or lvm thing that I 
actually prefer, myself.

Meanwhile, it's a pretty clever solution, I think. =:^)

> Also, I'd kinda guess (or better said: hope) that the kernel's cache
> would destroy these efforts, at least when the two reads happen mostly
> in parallel.

I was thinking run them in parallel, but you're right, you'd have to run 
them serially and dumpcache between runs.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman