[gpfsug-discuss] HAWC (Highly available write cache)

[Tejas Rao]

In my case GPFS storage is used to store VM images (KVM) and hence the 
small IO.

I always see lots of small 4K writes and the GPFS filesystem block size 
is 8MB. I thought the reason for the small writes is that the linux 
kernel requests GPFS to initiate a periodic sync which by default is 
every 5 seconds and can be controlled by "vm.dirty_writeback_centisecs".

I thought HAWC would help in such cases and would harden (coalesce) the 
small writes in the "system" pool and would flush to the "data" pool in 
larger block size.

Note - I am not doing direct i/o explicitly.



On 8/1/2016 14:49, Sven Oehme wrote:
> when you say 'synchronous write' what do you mean by that  ?
> if you are talking about using direct i/o (O_DIRECT flag), they don't 
> leverage HAWC data path, its by design.
>
> sven
>
> On Mon, Aug 1, 2016 at 11:36 AM, Tejas Rao <raot at bnl.gov 
> <mailto:raot at bnl.gov>> wrote:
>
>     I have enabled write cache (HAWC) by running the below commands.
>     The recovery logs are supposedly placed in the replicated system
>     metadata pool (SSDs). I do not have a "system.log" pool as it is
>     only needed if recovery logs are stored on the client nodes.
>
>     mmchfs gpfs01 --write-cache-threshold 64K
>     mmchfs gpfs01 -L 1024M
>     mmchconfig logPingPongSector=no
>
>     I have recycled the daemon on all nodes in the cluster (including
>     the NSD nodes).
>
>     I still see small synchronous writes (4K) from the clients going
>     to the data drives (data pool). I am checking this by looking at
>     "mmdiag --iohist" output. Should they not be going to the system pool?
>
>     Do I need to do something else? How can I confirm that HAWC is
>     working as advertised?
>
>     Thanks.
>
>
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