[Gluster-users] EC planning

[Serkan Çoban]

Hi Xavier and thanks for your answers.

Servers will have 26*8TB disks.I don't want to loose more than 2 disk for
raid,
so my options are HW RAID6 24+2 or 2 * HW RAID5 12+1,
in both cases I can create 2 bricks per server using LVM and use one brick
per server to create two distributed-disperse volumes. I will test those
configurations when servers arrive.

I can go with 8+1 or 16+2, will make tests when servers arrive. But 8+2 will
be too much, I lost nearly %25 space in this case.

For the client count, this cluster will get backups from hadoop nodes
so there will be 750-1000 clients at least which sends data at the same
time.
Can 16+2 * 3 = 54 gluster nodes handle this or should I increase node count?

I will check the parameters you mentioned.

Serkan

On Tue, Oct 13, 2015 at 1:43 PM, Xavier Hernandez <xhernandez at datalab.es>
wrote:

> +gluster-users
>
>
> On 13/10/15 12:34, Xavier Hernandez wrote:
>
>> Hi Serkan,
>>
>> On 12/10/15 16:52, Serkan Çoban wrote:
>>
>>> Hi,
>>>
>>> I am planning to use GlusterFS for backup purposes. I write big files
>>> (>100MB) with a throughput of 2-3GB/sn. In order to gain from space we
>>> plan to use erasure coding. I have some questions for EC and brick
>>> planning:
>>> - I am planning to use 200TB XFS/ZFS RAID6 volume to hold one brick per
>>> server. Should I increase brick count? is increasing brick count also
>>> increases performance?
>>>
>>
>> Using a distributed-dispersed volume increases performance. You can
>> split each RAID6 volume into multiple bricks to create such a volume.
>> This is because a single brick process cannot achieve the maximum
>> throughput of the disk, so creating multiple bricks improves this.
>> However having too many bricks could be worse because all request will
>> go to the same filesystem and will compete between them in your case.
>>
>> Another thing to consider is the size of the RAID volume. A 200TB RAID
>> will require *a lot* of time to reconstruct in case of failure of any
>> disk. Also, a 200 TB RAID means you need almost 30 8TB disks. A RAID6 of
>> 30 disks is quite fragile. Maybe it would be better to create multiple
>> RAID6 volumes, each with 18 disks at most (16+2 is a good and efficient
>> configuration, specially for XFS on non-hardware raids). Even in this
>> configuration, you can create multiple bricks in each RAID6 volume.
>>
>> - I plan to use 16+2 for EC. Is this a problem? Should I decrease this
>>> to 12+2 or 10+2? Or is it completely safe to use whatever we want?
>>>
>>
>> 16+2 is a very big configuration. It requires much computation power and
>> forces you to grow (if you need to grow the gluster volume at some
>> point) in multiples of 18 bricks.
>>
>> Considering that you are already using a RAID6 in your servers, what you
>> are really protecting with the disperse redundancy is the failure of the
>> servers themselves. Maybe a 8+1 configuration could be enough for your
>> needs and requires less computation. If you really need redundancy 2,
>> 8+2 should be ok.
>>
>> Using values that are not a power of 2 has a theoretical impact on the
>> performance of the disperse volume when applications write blocks whose
>> size is a multiple of a power of 2 (which is the most normal case). This
>> means that it's possible that a 10+2 performs worse than a 8+2. However
>> this depends on many other factors, some even internal to gluster, like
>> caching, meaning that the real impact could be almost negligible in some
>> cases. You should test it with your workload.
>>
>> - I understand that EC calculation is performed on client side, I want
>>> to know if there are any benchmarks how EC affects CPU usage? For
>>> example each 100MB/sn traffic may use 1CPU core?
>>>
>>
>> I don't have a detailed measurement of CPU usage related to bandwidth,
>> however we have made some tests that seem to indicate that the CPU
>> overhead caused by disperse is quite small for a 4+2 configuration. I
>> don't have access to this data right now. When I have it, I'll send it
>> to you.
>>
>> I will also try to do some tests with a 8+2 and 16+2 configuration to
>> see the difference.
>>
>> - Is client number affect cluster performance? Is there any difference
>>> if I connect 100 clients each writing with 20-30MB/s to cluster vs 1000
>>> clients each writing 2-3MB/s?
>>>
>>
>> Increasing the number of clients improves performance however I wont' go
>> over 100 clients as this could have a negative impact on performance
>> caused by the overhead of managing all of them. In our tests, the
>> maximum performance if obtained with ~8 parallel clients (if my memory
>> doesn't fail).
>>
>> You will also probably want to tweak some volume parameters, like
>> server.event-threads, client.event-threads,
>> performance.client-io-threads and server.outstanding-rpc-limit to
>> increase performance.
>>
>> Xavi
>>
>>
>>> Thank you for your time,
>>> Serkan
>>>
>>>
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>>> Gluster-users mailing list
>>> Gluster-users at gluster.org
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>>>
>>>
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