[Linux-cluster] gfs tuning

Terry td3201 at gmail.com
Thu Jun 19 15:30:57 UTC 2008

On Thu, Jun 19, 2008 at 9:49 AM, Wendy Cheng <s.wendy.cheng at gmail.com> wrote:
> Terry wrote:
>> On Tue, Jun 17, 2008 at 5:22 PM, Terry <td3201 at gmail.com> wrote:
>>> On Tue, Jun 17, 2008 at 3:09 PM, Wendy Cheng <s.wendy.cheng at gmail.com>
>>> wrote:
>>>> Hi, Terry,
>>>>> I am still seeing some high load averages.  Here is an example of a
>>>>> gfs configuration.  I left statfs_fast off as it would not apply to
>>>>> one of my volumes for an unknown reason.  Not sure that would have
>>>>> helped anyways.  I do, however, feel that reducing scand_secs helped a
>>>>> little:
>>>> Sorry I missed scand_secs (was mindless as the brain was mostly occupied
>>>> by
>>>> day time work).
>>>> To simplify the view, glock states include exclusive (write), share
>>>> (read),
>>>> and not-locked (in reality, there are more). Exclusive lock has to be
>>>> demoted (demote_secs) to share, then to not-locked (another demote_secs)
>>>> before it is scanned (every scand_secs) to get added into reclaim list
>>>> where
>>>> it can be purged. Between exclusive and share state transition, the file
>>>> contents need to get flushed to disk (to keep file content cluster
>>>> coherent).  All of above assume the file (protected by this glock) is
>>>> not
>>>> accessed (idle).
>>>> You hit an area that GFS normally doesn't perform well. With GFS1 in
>>>> maintenance mode while GFS2 seems to be so far away, ext3 could be a
>>>> better
>>>> answer. However, before switching, do make sure to test it thoroughly
>>>> (since
>>>> Ext3 could have the very same issue as well - check out:
>>>> http://marc.info/?l=linux-nfs&m=121362947909974&w=2 ).
>>>> Did you look (and test) GFS "nolock" protocol (for single node GFS)? It
>>>> bypasses some locking overhead and can be switched to  DLM in the future
>>>> (just make sure you reserve enough journal space - the rule of thumb is
>>>> one
>>>> journal per node and know how many nodes you plan to have in the
>>>> future).
>>>> -- Wendy
>>> Good points.  I could try the nolock feature I suppose.  Not quite
>>> clear on how to reserve journal space.  I forgot to post the cpu time,
>>> check out this:
>>>  4822 root      10  -5     0    0    0 S    1  0.0   2159:15 dlm_recv
>>>  4820 root      10  -5     0    0    0 S    1  0.0 368:09.34 dlm_astd
>>>  4821 root      10  -5     0    0    0 S    0  0.0 153:06.80 dlm_scand
>>>  3659 root      10  -5     0    0    0 S    0  0.0 134:40.14 scsi_wq_4
>>>  4823 root      11  -5     0    0    0 S    1  0.0 109:33.33 dlm_send
>>>  367 root      10  -5     0    0    0 S    0  0.0 103:33.74 kswapd0
>>> gfs_glockd is further below so not so concerned with that right now.
>>> It appears turning on nolock would do the trick.  The times aren't
>>> extremely accurate because I have failed this cluster between nodes
>>> while testing.
>> Here is some more testing information....
>> I created a new volume on my iscsi san of 1 TB and formatted it for
>> ext3. I then used dd to create a 100G file.  This yielded roughly 900
>> Mb/sec.  I then stopped my application and did the same thing with an
>> existing GFS volume.  This gave me about 850 Kb/sec.  This isn't an
>> iscsi issue.  This appears to be a load issue and the number of I/O
>> occurring on these volumes.  That said, I would expect that performing
>> the changes I did would result in a major performance improvement.
>> Since it didn't, what are my other points I could consider?   If its a
>> GFS issue, ext3 is the way to go.  Maybe even switch to using
>> active-active on my NFS cluster.   If its a backend disk issue, I
>> would expect to see the throughput on my iscsi link (bond1) be fully
>> utilized.  Its not.  Could I be thrashing the disks?  This is an iscsi
>> san with 30 sata disks.  Just bouncing some thoughts around to see if
>> anyone has any more thoughts.
> Really need to focus on my day time job - its worload has been climbing ...
> but can't help to place a quick comment here ..
> The 900 MB/s vs. 850 KB/s difference looks like a caching  issue - that is,
> for 900 MB/s, it looks like the data was still lingering in the system cache
> while in 850 KB/s case, the data might already hit disk. Cluster filesystem
> normally syncs more by its nature. In general, ext3 does perform better in
> single node environment but the difference should not be as big as above.
> There are certainly more tuning knobs available (such as journal size and/or
> network buffer size) to make GFS-iscsi "dd" run better but it is pointless.
> To deploy a cluster filesystem for production usage, the tuning should not
> be driven by such a simple-mind command. You also have to consider the
> support issues when deploying a filesystem. GFS1 is a little bit out of date
> and any new development and/or significant performance improvements would
> likely be in GFS2, not in GFS1. Research GFS2 (googling to see how other
> people said about it) to understand whether its direction fits your need (so
> you can migrate from GFS1 to GFS2 if you bump into any show stopper in the
> future). If not, ext3 (with ext4 actively developed) is a fine choice if I
> read your configuration right from previous posts.
> -- Wendy

I wrote off the difference between ext3 and gfs performance with my
simple dd command as nothing.  I wanted to ensure I wasn't seeing some
other issue.  I am happy with 800-900 regardless of the filesystem.
I'm going to see if I can get some performance metrics off the SAN and
go from there.

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