[Gluster-devel] Sharding - what next?

[Lindsay Mathieson]

Hi Guys, sorry for the late reply, my attention tends to be somewhat 
sporadic due to work and the large number of rescue dogs/cats I care for :)

On 3/12/2015 8:34 PM, Krutika Dhananjay wrote:
> We would love to hear from you on what you think of the feature and 
> where it could be improved.
> Specifically, the following are the questions we are seeking feedback on:
> a) your experience testing sharding with VM store use-case - any bugs 
> you ran into, any performance issues, etc

Testing was initially somewhat stressful as I regularly encountered file 
corruption. However I don't think that was due to bugs, rather incorrect 
settings for the VM usecase. Once I got that sorted out it has been very 
stable - I have really stressed failure modes we run into at work - 
nodes going down while heavy writes were happening. Live migrations 
during heals. gluster software being killed while VM were running on the 
host. So far its held up without a hitch.

To that end, one thing I think should be made more obvious is the 
settings required for VM Hosting:

    quick-read=off
    read-ahead=off
    io-cache=off
    stat-prefetch=off
    eager-lock=enable
    remote-dio=enable
    quorum-type=auto
    server-quorum-type=server

They are  quite crucial and very easy to miss in the online docs. And 
they are only recommended with noo mention that you will corrupt KVM 
VM's if you live migrate them between gluster nodes without them set. 
Also the virt group is missing from the debian packages.

Setting them does seem to have slowed sequential writes by about 10% but 
I need to test that more.


Something related - sharding is useful because it makes heals much more 
granular and hence faster. To that end it would be really useful if 
there was a heal info variant that gave a overview of the process - 
rather than list the shards that are being healed, just a aggregate 
total, e.g.

$ gluster volume heal datastore1 status
volume datastore1
- split brain: 0
- Wounded:65
- healing:4

It gives one a easy feeling of progress - heals aren't happening faster, 
but it would feel that way :)


Also, it would be great if the heal info command could return faster, 
sometimes it takes over a minute.

Thanks for the great work,

Lindsay
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