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 <[email protected]>
To: gpfsug main discussion list <[email protected]>
Date: 08/01/2016 12:06 PM
Subject: Re: [gpfsug-discuss] HAWC (Highly available write cache)
Sent by: [email protected]
------------------------------------------------------------------------
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 <[email protected]_
<mailto:[email protected]>> 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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