Hi,
few days i'm reading Memcached documentation and blogs... and i don't
understand how objects are stored.
*My test*
3 slabs :
- 96.0 Bytes
- 120.0 Bytes
- 240.0 Bytes
Everywhere, it's told :
- if data is < 96 Bytes, it will be stored in Slabs1 (96B)
- if data > 96B and < 120B, it will be stored in Slabs2 (120B)
- if data > 120B, it will be stored in Slabs3 (240B)
- etc.
BUT, for example, when i'm creating an *30B* object, it's stored in Slab2 (
*120B*), and NOT in Slab1 (*96B*).
External sources:
> For example, the default size for the smallest block is 88 bytes (40 bytes
> of value, and the default 48 bytes for the key and flag data). If the size
> of the first item you store into the cache is less than 40 bytes, then a
> slab with a block size of 88 bytes is created and the value stored.
> =>
> https://dev.mysql.com/doc/mysql-ha-scalability/en/ha-memcached-using-memory.html
*WRONG*
A slab class is a collection of pages divided into same sized chunks. Each
> slab class is referenced to by its chunk size. So we’ll have Slab class
> 80kb, Slab class 100kb and so on. When an object needs to be stored, its
> size determines where it gets stored. So if the object is larger than 80kb
> but less than 100kb, it gets stored into Slab class 100kb.
> =>
> http://returnfoo.com/2012/02/memcached-memory-allocation-and-optimization-2/
*WRONG*
*How i create an object:*
data=$(pwgen 30 -c 1)
> echo -e 'set 30 0 3600 30\r\n'$data'\r'| nc ${memcached_server} 11211
So, when 30B object is creating :
- key name : 30 = 2 bytes
- value: 30 characters = 30 bytes
- tags : 0 = 1 bytes
=> All = 33 bytes
If i add the default 48b *as explained on Mysql website* : 33 + 48 = 81B
... so < Slab1 (91B)... but always stored in Slab2 (120B)
So, *the size used to store object* in the good Slab *is not*:
- object value size
- sum of KEY, VALUE and TAGS in bytes
KEY size : 1 character = 1 B
VALUE size : 1 character = 1 B
TAGS size : 1 character = 1 B
... as read everywhere
So, It seems that: (SUM of KEY+VALUE+TAGS )
- For slab1 96.0 Bytes, data stored if <= 31 B (SUM of 2+28+1 )
- For slab2 120.0 Bytes, data stored if <= 55 B (SUM of 2+52+1 )
- For slab3 152.0 Bytes, data stored if <= 87 B (SUM of 2+84+1 )
- For slab4 192.0 Bytes, data stored if <= 126 B (SUM of 3+122+1 )
- For slab5 240.0 Bytes, data stored if <= 174 B (SUM of 3+170+1 )
- etc.
My configuration :
- Chunk Size : 48
- Chunk Growth Factore: 1,25
- Max Bytes: 64.0 MBytes
So, someone could explain me how the data is stored in the right Slabs???
How calculate it???
Thank you
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