Re: 1 week to rebuid 4x 3TB raid10 is a long time!

[ashford@whisperpc.com]

This is the cause for the slow reconstruct.

> I believe the problem here might be that a Btrfs rebuild *is* a strenuous
> random read (+ random-ish write) just by itself.

If you assume a 12ms average seek time (normal for 7200RPM SATA drives),
an 8.3ms rotational latency (half a rotation), an average 64kb write and a
100MB/S streaming write speed, each write comes in at ~21ms, which gives
us ~47 IOPS.  With the 64KB write size, this comes out to ~3MB/S, DISK
LIMITED.

The on-disk cache helps a bit during the startup, but once the cache is
full, it's back to writes at disk speed, with some small gains if the
on-disk controller can schedule the writes efficiently.

Based on the single-threaded I/O that BTRFS uses during a reconstruct, I
expect that the average write size is somewhere around 200KB. 
Multi-threading the reconstruct disk I/O (possibly adding look-ahead),
would double the reconstruct speed for this array, but that's not a
trivial task.

The 5MB/S that TM is seeing is fine, considering the small files he says
he has.

Peter Ashford