Re: Disk I/O slowness

> I'm getting some weird latency issues with fsync on the RAID disk
> (/home on /dev/md125): (Caveat: I'm doing a file transfer that is
> writing to /home, so I might expect some latency, but nothing
> like this.)
>
> $ for i in {1..5}; do time sync; done 2>&1 | grep real
> real	0m21.647s
> real	0m11.770s
> real	0m5.234s
> real	0m3.023s
> real	0m0.462s

Why do you expect "nothing like this"?

I have no idea if your numbers are normal or not, but 21s on a 3-disk
RAID writing at max speed (i.e. 6Gb/s) is 6Gbx21 =~ 16GB.

So if the file transfer has accumulated 16GB of data in the write queue,
it's perfectly normal for `sync` to take 20s.
And if the file transfer goes at about 400MB/s, you'll have transferred
about 8GB during those 20s, so it stands to reason that the next sync
will still have 8GB to write to disk which will take another 10s,
and then 5s for the next, ...


=== Stefan