[DRBD-user] Slow buffered read performance

Andreas Kurz andreas at hastexo.com
Tue Jan 24 15:13:35 CET 2012

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


On 01/24/2012 10:59 AM, samppah at neutraali.net wrote:
> Hello all,
> 
> I'm running DRBD on two following machines:
> Scientific Linux 6.1
> Intel Xeon E5620 Quad Core
> 12GB Ram
> LSI 9280-4i4e + CacheCade 1.0 (CacheCade is currently disabled)
> 12x 1TB Seagate Constellation SAS 7200 RPM drives
> 
> DRBD has been configured in master/slave -fashion and there is 10 GbE
> dedicated link between machines for DRBD traffic.
> Drives have been configured in RAID-10 mode with 64 kB Stripe Size.
> 
> DRBD version is 8.4.0 (api:1/proto:86-100).

Have you tried to reproduce that with drbd 8.4.1 ?

Regards,
Andreas

-- 
Need help with DRBD?
http://www.hastexo.com/now

> 
> Couple of weeks ago I had to do some maintenance work on node1 (normally
> the master node) so I moved all services to run on node2. Everything
> went well and since machines are identical I didn't bother to move
> services back to node1. Few days later I noticed that performance was
> somewhat degraded but difference was so minimal that I didn't focus on
> it at all.
> 
> A little later I was asked to do some simple read performance tests.
> Everything looked ok when doing direct reads on DRBD-device:
> 
> dd if=/dev/drbd0 of=/dev/null bs=1M iflag=direct
> ^C17317+0 records in
> 17316+0 records out
> 18157142016 bytes (18 GB) copied, 21.3465 s, 851 MB/s
> 
> But with buffered reads things get slow:
> 
> dd if=/dev/drbd0 of=/dev/null bs=1M
> ^C11131+0 records in
> 11130+0 records out
> 11670650880 bytes (12 GB) copied, 105.299 s, 111 MB/s
> 
> However, the underlying disk seems to be fine:
> 
> dd if=/dev/sdb of=/dev/null bs=1M
> ^C14087+0 records in
> 14086+0 records out
> 14770241536 bytes (15 GB) copied, 19.8579 s, 744 MB/s
> 
> I moved services back to node1 and the problem was gone (dd
> if=/dev/drbd0 of=/dev/null bs=1M, 37312528384 bytes (37 GB) copied,
> 54.8519 s, 680 MB/s). Now I started to investigate what caused the
> issues and moved services to node2 and performance problems hit again so
> I thought that there has to be something wrong on node2. I compared
> settings between the two machines to make sure they are really identical
> and found nothing strange between them. Raid sets are fine and there are
> no error messages in log files. At this point I decided to reboot node1
> and after that moved services back to it. After the reboot performance
> dropped also on node1 and I haven't been able to find out anything that
> could really help getting performance up again.
> 
> So it seems like DRBD has huge effect when it comes to buffered reads.
> It may very well be that I have forgotten to do some sysctl or such
> tuning after reboot but I can't figure out what it could be. Any ideas
> how to work this out or is this expected behaviour?
> 
> I'm currently using following settings on my disks:
> 
> echo deadline > /sys/block/sdb/queue/scheduler
> echo 0 > /sys/block/sdb/queue/iosched/front_merges
> echo 150 > /sys/block/sdb/queue/iosched/read_expire
> echo 1500 > /sys/block/sdb/queue/iosched/write_expire
> echo 32000000 > /proc/sys/vm/dirty_background_bytes
> echo 384000000 > /proc/sys/vm/dirty_bytes
> echo 1024 > /sys/block/sdb/queue/nr_requests
> 
> and current DRBD resource configuration:
> 
> resource drbd0 {
>         device /dev/drbd0;
>         disk /dev/sdb1;
>         meta-disk internal;
> 
>         options {
>                 cpu-mask 15;
>         }
> 
>         net {
>                 protocol C;
>                 max-buffers 8000;
>                 max-epoch-size 8000;
>                 unplug-watermark 16;
>                 sndbuf-size 0;
>         }
> 
>         disk {
>                 al-extents 3389;
>                 disk-barrier no;
>                 disk-flushes no;
>         }
> 
>         on node1 {
>                 address 10.10.10.1:7789;
>         }
>         on node2 {
>                 address 10.10.10.2:7789;
>         }
> }
> 
> Best regards,
> Samuli Heinonen
> 
> _______________________________________________
> drbd-user mailing list
> drbd-user at lists.linbit.com
> http://lists.linbit.com/mailman/listinfo/drbd-user



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