On 07/20/2014 10:00 AM, Tomasz Torcz wrote: > On Sun, Jul 20, 2014 at 01:53:34PM +0000, Duncan wrote: >> TM posted on Sun, 20 Jul 2014 08:45:51 +0000 as excerpted: >> >>> One week for a raid10 rebuild 4x3TB drives is a very long time. >>> Any thoughts? >>> Can you share any statistics from your RAID10 rebuilds? >> >> >> At a week, that's nearly 5 MiB per second, which isn't great, but isn't >> entirely out of the realm of reason either, given all the processing it's >> doing. A day would be 33.11+, reasonable thruput for a straight copy, >> and a raid rebuild is rather more complex than a straight copy, so... > > Uhm, sorry, but 5MBps is _entirely_ unreasonable. It is order-of-magnitude > unreasonable. And "all the processing" shouldn't even show as a blip > on modern CPUs. > This "speed" is undefendable. > I wholly agree that it's undefendable, but I can tell you why it is so slow, it's not 'all the processing' (which is maybe a few hundred instructions on x86 for each block), it's because BTRFS still serializes writes to devices, instead of queuing all of them in parallel (that is, when there are four devices that need written to, it writes to each one in sequence, waiting for the previous write to finish before dispatching the next write). Personally, I would love to see this behavior improved, but I really don't have any time to work on it myself.
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