[Linux-cluster] GFS (1 & partially 2) performance problems

[Steven Whitehouse]

Hi,

On Mon, 2010-06-14 at 14:00 +0200, Michael Lackner wrote:
> Hello!
> 
> I am currently building a Cluster sitting on CentOS 5 for GFS usage.
> 
> At the moment, the storage subsystem consists of an HP MSA2312
> Fibrechannel SAN linked to an FC 8gbit switch. Three client machines
> are connected to that switch over 8gbit FC. The disks themselves are
> 12 * 15.000rpm SAS configured in RAID-5 with two hotspares.
> 
> Now, the whole storage shall be shared (single filesystem), here GFS
> comes in.
> 
> The Cluster is only 3 nodes large at the moment, more nodes will be
> added later on. I am currently testing GFS1 and GFS2 for performance.
> Lock Management is done over single 1Gbit Ethernet Links (1 per
> machine).
> 
> Thing is, with GFS1 I get far better performance than with the newer
> GFS2 across the board, with a few tunable parameters set, for writes
> GFS1 is roughly twice as fast.
> 
What tests are you running? GFS2 is generally faster than GFS1 except
for streaming writes, which is an area that we are putting some effort
into solving currently. Small writes (one fs block (4k default) or less)
on GFS2 are much faster than on GFS1.

> But, concurrent reads are totally abysmal. The total write performance
> (all nodes combined) sits around 280-330Mbyte/sec, whereas the
> READ performance is as low as 30-40Mbyte/sec when doing concurrent
> reads. Surprisingly, single-node read is somewhat ok at 180Mbyte/sec,
> but as soon as several nodes are reading from GFS (version 1 at the
> moment) at the same time,  things turn ugly.
> 
Reads on GFS2 should be much faster than GFS1, so it sounds as if
something isn't working correctly for some reason. For cached data,
reads on GFS2 should be as fast as ext2/3 since the code path is
identical (to the page cache) and only changes if pages are not cached.
GFS1 does its locking at a higher level, so there will be more overhead
for cached reads in general.

Do make sure that if you are preparing the test files for reading all
from one node (or even just a different node to that on which you sre
running the read tests) that you need to sync them to disk on that node
before starting the tests to avoid issues with caching.

> This is strange, because for writes, global performance across the
> cluster increases slightly when adding more nodes. But for reads, the
> oppsite seems to be true.
> 
> For read and write tests, separate testfiles were created and read for
> each node, with each testfile sitting in its own subdirectory, so no
> node would access another nodes file.
> 
That sounds like a good test set up to me.

> GFS1 created with the following mkfs.gfs parameters:
> "-b 4096 -J 128 -j 16 -r 2048 -p lock_dlm"
> (4kB blocksite, 16 * 128MB journals, 2GB resource groups,
> Distributed LockManager)
> 
> Mount Options set: "noatime,nodiratime,noquota"
> 
> Tunables set: "glock_purge 50, statfs_slots 128, statfs_fast 1, 
> demote_secs 20"
You shouldn't normally need to set the glock_purge and demote_secs to
anything other than the default. These settings no longer exist in GFS2
since it makes use of the shrinker subsystem provided by the VM and is
auto-tuning. If your workload is metadata heavy, you could try boosting
the journal size and/or the incore_log_blocks tunable.

> 
> Also, in /etc/cluster/cluster.conf, I added this:
> <dlm plock_ownership="1" plock_rate_limit="0"/>
> <gfs_controld plock_rate_limit="0"/>
> 
> Any ideas on how to figure out what's going wrong, and how to
> tune GFS1 for better concurrent read performance, or tune GFS2
> in general to be competitive/better than GFS1?
> 
> I'm dreaming about 300MB/sec read, 300MB/sec write sequentially
> and somewhat good reaction times while under heavy sequential
> and/or random load. But for now, I just wanna get the seq reading
> to work acceptably fast.
> 
> Thanks a lot for your help!
> 
Can you try doing some I/O direct to the block device so that we can get
an idea of what the raw device can manage? Using dd both read and write,
across the nodes (different disk locations on each node to simulate
different files).

I'm wondering if the problem might be due to the seek pattern generated
by the multiple read locations,

Steve.