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

[Tejas Rao]

I am not 100% sure what the workload of the VMs is. We have 100's of VMs 
all used differently, so the workload is rather mixed.

I do see 4K writes going to "system" pool, they are tagged as "logData" 
in 'mmdiag --iohist'. But I also see 4K writes going to the data drives, 
so it looks like everything is not getting coalesced and these are 
random writes.

Could these 4k writes labelled as "logData" be the writes going to HAWC 
log files?


On 8/1/2016 15:50, Dean Hildebrand wrote:
>
> Hi Tejas,
>
> Do you know the workload in the VM?
>
> The workload which enters into HAWC may or may not be the same as the 
> workload that eventually goes into the data pool....it all depends on 
> whether the 4KB writes entering HAWC can be coalesced or not. For 
> example, sequential 4KB writes can all be coalesced into a single 
> large chunk. So 4KB writes into HAWC will convert into 8MB writes to 
> data pool (in your system). But random 4KB writes into HAWC may end up 
> being 4KB writes into the data pool if there are no adjoining 4KB 
> writes (i.e., if 4KB blocks are all dispersed, they can't be 
> coalesced). The goal of HAWC though, whether the 4KB blocks are 
> coalesced or not, is to reduce app latency by ensuring that writing 
> the blocks back to the data pool is done in the background. So while 
> 4KB blocks may still be hitting the data pool, hopefully the 
> application is seeing the latency of your presumably lower latency 
> system pool.
>
> Dean
>
>
> Inactive hide details for Tejas Rao ---08/01/2016 12:06:15 PM---In my 
> case GPFS storage is used to store VM images (KVM) and heTejas Rao 
> ---08/01/2016 12:06:15 PM---In my case GPFS storage is used to store 
> VM images (KVM) and hence the small IO.
>
> From: Tejas Rao <raot at bnl.gov>
> To: gpfsug main discussion list <gpfsug-discuss at spectrumscale.org>
> Date: 08/01/2016 12:06 PM
> Subject: Re: [gpfsug-discuss] HAWC (Highly available write cache)
> Sent by: gpfsug-discuss-bounces at spectrumscale.org
>
> ------------------------------------------------------------------------
>
>
>
> 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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