[DRBD-user] Initial Sync - Fast then really slow

Lars Ellenberg Lars.Ellenberg at linbit.com
Fri Apr 9 06:49:54 CEST 2004

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

/ 2004-04-08 18:27:32 -0500
\ Travis Kriza:
> >
> >please reproduce it without heartbeat...  "by hand"
> >Thanks.
> Lars,
> 	I actually experienced that fast then slow condition actually 
> 	without heartbeat.


>  (I was wondering what had been happening originally and 
> was manually starting the drbd processes on each box).  Anyways, after 
> setting up NTP and resyncing the clocks (and a reboot), I was able to 
> achieve pretty good throughput speeds.

Oh, I just wondered: Did you really measure the "slow", or do you rely
solely on the output of /proc/drbd ?

If the latter, and you happen to have your syslog from that time, and
you happen to know (or are able to figure) the wall clock time passed
between "Sync Started" and "Sync Done", please recalculate and verify
that one.

> However, now I'm noticing I may have too much of a bottleneck between 
> running a software RAID 1 on each box PLUS the drbd raid of those raid 
> 1 volumes.  I'm running some iozone tests first to see if I get 
> noticeable improvements in performance.  (Basically, I had apache 
> logging via NFS to this pair of boxes, and the box ended up scaling out 
> of control with load --- that is, the apache box went up to like a load 
> of 20 whereas the nfs box went up to maybe a 2).  Anyways, I was 
> thinking to improve performance, it may make sense to switch the shared 
> (drbd shared) drives to be based on a raid0 volume versus the raid 1.  
> (I was also thinking it may be wiser to either pipe the apache logs to 
> write to both spots, or simply do a regular rsync interval for the 
> logs).  Any thoughts on this?

For me, the main question(s) in (Computer/Network/Data) Security is
not "how can I prevent things from happening" (which is of course still
an important question), but:

  What does it mean (to me) WHEN things are happening,
  what do I lose worst case, how can I cope,
  how fast can I recover?

So, if you don't mind losing some minutes of apache log (worst case),
then rsync should be just fine.

Your load problem is probably mostly with apache log on NFS,
not that much with DRBD...
Your log grows that fast, that it turns out to be a performance problem?
Hard to believe that this is the first bottleneck you hit.
BTW, if you happen to have your apache session files on (NFS on top of)
DRBD, think about having them on DRBD on top of RAM disks ...
If both go down, they have lost their meaning anyways...
As long as one stays up, you still have them.

> And, can anyone recommend an easy way to convert a raid 1 into a raid 
> 0?  doing partition management during install is easy with disk druid, 
> but i'm lousy at disk management when it comes command line.  Never had 
> to deal with it that much.

Won't comment on that...
you may come back and blame me if things go wrong :)

	Lars Ellenberg

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