Re: Optimizing write performance to a few large files in a small cluster

Robin Jonsson <Robin.Jonsson@xxxxxxx> · Tue, 11 Mar 2014 09:37:53 +0000

Alexander:
I have also experienced the stalls you are explaining. This was in a 2 brick setup running replicated volumes used by a 20 node HPC. 

In my case this was solved by: 

* Replace FUSE with NFS
	* This is by far the biggest booster
* RAM disks for the scratch directories (not connected to gluster at all)
	* If you’re not sure where these directories are, run ‘gluster volume top <volume> write list-cnt 10’
* 'tuned-adm profile; tuned-adm profile rhs-high-throughput’ on all storage bricks
* The following volume options
	* cluster.nufa: enable
	* performance.quick-read: on
	* performance.open-behind: on
* Mount option on clients
	* noatime
		* Use only where access time isn’t needed.
		* Major booster for small file writes in my case. Even with the FUSE client.

Hope this helps, 

Regards,
Robin

On 10 Mar 2014, at 19:06 pm, Alexander Valys <avalys@xxxxxxxxxx> wrote:

A quick performance question.

I have a small cluster of 4 machines, 64 cores in total.  I am running a scientific simulation on them, which writes at between 0.1 and 10 MB/s (total) to roughly 64 HDF5 files.  Each HDF5 file is written by only one process.  The writes are not continuous, but consist of writing roughly 1 MB of data to each file every few seconds.    

Writing to HDF5 involves a lot of reading the file metadata and random seeking within the file,  since we are actually writing to about 30 datasets inside each file.  I am hosting the output on a distributed gluster volume (one brick local to each machine) to provide a unified namespace for the (very rare) case when each process needs to read the other's files.  

I am seeing somewhat lower performance than I expected, i.e. a factor of approximately 4 less throughput than each node writing locally to the bare drives.  I expected the write-behind cache to buffer each write, but it seems that the writes are being quickly flushed across the network regardless of what write-behind cache size I use (32 MB currently), and the simulation stalls while waiting for the I/O operation to finish.  Anyone have any suggestions as to what to look at?  I am using gluster 3.4.2 on ubuntu 12.04.  I have flush-behind turned on, and have mounted the volume with direct-io-mode=disable, and have the cache size set to 256M.  

The nodes are connected via a dedicated gigabit ethernet network, carrying only gluster traffic (no simulation traffic).

(sorry if this message comes through twice, I sent it yesterday but was not subscribed)
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