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Re: [Linux-cluster] gfs tuning
- From: Terry <td3201 gmail com>
- To: "linux clustering" <linux-cluster redhat com>
- Subject: Re: [Linux-cluster] gfs tuning
- Date: Thu, 19 Jun 2008 10:30:57 -0500
On Thu, Jun 19, 2008 at 9:49 AM, Wendy Cheng <s wendy cheng gmail com> wrote:
> Terry wrote:
>> On Tue, Jun 17, 2008 at 5:22 PM, Terry <td3201 gmail com> wrote:
>>> On Tue, Jun 17, 2008 at 3:09 PM, Wendy Cheng <s wendy cheng gmail com>
>>>> 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
>>>> Sorry I missed scand_secs (was mindless as the brain was mostly occupied
>>>> day time work).
>>>> To simplify the view, glock states include exclusive (write), share
>>>> 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
>>>> 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
>>>> 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
>>>> answer. However, before switching, do make sure to test it thoroughly
>>>> 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
>>>> journal per node and know how many nodes you plan to have in the
>>>> -- 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:
>>> PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
>>> 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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