Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
Mark,
On 01/23/2009 05:01 PM, Mark Watts wrote:
> The following test was used to obtain a write average:
> # for loop in `seq 1 10`; do echo "Writing to /empty, loop $loop"; dd
> if=/dev/zero of=/empty bs=$(( 1024 * 1024 * 1024 )) count=4; done
You're not writing to your disk, but to your page/buffer cache. Add
"oflag=direct".
> When writing to the / volume the average is 277 MB/s
This is probably going to dip significantly if you _really_ start
measuring your disk writes (i.e. by adding oflag=direct). In your setup
I would expect this to be in the 150-180 MB/s range.
> When writing to the /shared volume the average is 65.1 MB/s
>
> When writing to the /shared volume with no-disk-flushes and no-md-flushes
> enabled the average is 67.4 MB/s
This is still a bit slow, though the difference when being compared with
a real disk write measurement is likely to be significantly less.
> Can anyone suggest why DRBD is causing such a drastic slow-down when ultimatly
> writing data to the same set of spindles; the only difference being the
> addition of the DRBD layer.
Firstly, I repeat, because you hadn't been writing to your disk, but to
your page cache (i.e. memory). But:
> syncer {
> al-extents 257;
> }
... is probably way too low for your setup. Try upping that one to 1801,
maybe as far as 3389.
Still, this:
> syncer {
> rate 100M;
> }
... is likely to kill your foreground I/O performance during any resync,
even when you've upped your al-extents to the values suggested above.
And of course, you'll never reach your anywhere near your 150 MB/s if
your replication link is Gigabit Ethernet, as that tops out at 110 MB/s.
These resources may provide additional insights:
http://www.linbit.com/en/education/on-demand-webinars/drbd-performance-tuning/
http://www.drbd.org/users-guide/s-configure-syncer-rate.html
Hope this helps.
Cheers,
Florian
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