[ceph-users] WAL/DB size

Alfredo Deza adeza at redhat.com
Fri Sep 7 13:08:27 PDT 2018


On Fri, Sep 7, 2018 at 3:31 PM, Maged Mokhtar <mmokhtar at petasan.org> wrote:
> On 2018-09-07 14:36, Alfredo Deza wrote:
>
> On Fri, Sep 7, 2018 at 8:27 AM, Muhammad Junaid <junaid.fsd.pk at gmail.com>
> wrote:
>
> Hi there
>
> Asking the questions as a newbie. May be asked a number of times before by
> many but sorry, it is not clear yet to me.
>
> 1. The WAL device is just like journaling device used before bluestore. And
> CEPH confirms Write to client after writing to it (Before actual write to
> primary device)?
>
> 2. If we have lets say 5 OSD's (4 TB SAS) and 1 200GB SSD. Should we
> partition SSD in 10 partitions? Shoud/Can we set WAL Partition Size against
> each OSD as 10GB? Or what min/max we should set for WAL Partition? And can
> we set remaining 150GB as (30GB * 5) for 5 db partitions for all OSD's?
>
>
> A WAL partition would only help if you have a device faster than the
> SSD where the block.db would go.
>
> We recently updated our sizing recommendations for block.db at least
> 4% of the size of block (also referenced as the data device):
>
> http://docs.ceph.com/docs/master/rados/configuration/bluestore-config-ref/#sizing
>
> In your case, what you want is to create 5 logical volumes from your
> 200GB at 40GB each, without a need for a WAL device.
>
>
>
> Thanks in advance. Regards.
>
> Muhammad Junaid
>
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>
> should not the db size depend on the number of objects stored rather than
> their storage size ? or is the new recommendation assuming some average
> object size ?

The latter. You are correct that it should depend on the number of
objects, but the objects vary in size depending on the type of
workload. RGW objects are different than RBD. So we are
taking a baseline/average for object sizes and recommending based on
that, which is roughly 4% the size of the data device


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