Re: bluestore object overhead

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Does the bluestore min alloc size apply for 4k block-size files [1]?

[1] https://github.com/ceph/ceph/blob/master/src/common/config_opts.h#L1063

On Wed, Apr 19, 2017 at 4:51 PM, Gregory Farnum <gfarnum@xxxxxxxxxx> wrote:
> On Wed, Apr 19, 2017 at 1:49 PM, Pavel Shub <pavel@xxxxxxxxxxxx> wrote:
>> On Wed, Apr 19, 2017 at 4:33 PM, Gregory Farnum <gfarnum@xxxxxxxxxx> wrote:
>>> On Wed, Apr 19, 2017 at 1:26 PM, Pavel Shub <pavel@xxxxxxxxxxxx> wrote:
>>>> Hey All,
>>>>
>>>> I'm running a test of bluestore in a small VM and seeing 2x overhead
>>>> for each object in cephfs. Here's the output of df detail
>>>> https://gist.github.com/pavel-citymaps/868a7c4b1c43cea9ab86cdf2e79198ee
>>>>
>>>> This is on a VM with all daemons & 20gb disk, all pools are of size 1.
>>>> Is this the expected amount of overhead per object? Is there anyway to
>>>> tweak bluestore settings?
>>>
>>> You're going to need to be clearer about what you mean by 2x overhead.
>>> Bluestore itself has a minimum size beneath which it will journal
>>> objects and then copy them into place, which might be considered 2x
>>> overhead. If you're talking about total number of cluster-wide disk
>>> ops, there's also a CephFS log which journals metadata updates that
>>> get flushed out to backing objects later, which might be considered 2x
>>> overhead. But I don't know what you mean just based on a ceph df. :)
>>> -Greg
>>
>> Sorry, I meant the disk space taken up by the files. I have a dataset
>> with lots of small files, my sample set 2.5gb in total size and 5gb on
>> a filesystem with a 4kb block size. When put the files inside ceph
>> bluestore they take up 6gb. Does bluestore have an internal block
>> size? Is there a way to adjust it? For comparison I created a
>> filestore OSD with 2kb block size and the data took up only 4.5gb.
>
> I can't speak with authority on bluestore, but at those total sizes I
> think you're just seeing the effects of the internal journaling.
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-- 
Jason
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