[Gluster-users] Arbiter brick size estimation

[Ravishankar N]

On 03/05/2016 03:45 PM, Oleksandr Natalenko wrote:
> In order to estimate GlusterFS arbiter brick size, I've deployed test setup
> with replica 3 arbiter 1 volume within one node. Each brick is located on
> separate HDD (XFS with inode size == 512). Using GlusterFS v3.7.6 + memleak
> patches. Volume options are kept default.
>
> Here is the script that creates files and folders in mounted volume: [1]
>
> The script creates 1M of files of random size (between 1 and 32768 bytes) and
> some amount of folders. After running it I've got 1036637 folders. So, in
> total it is 2036637 files and folders.
>
> The initial used space on each brick is 42M . After running script I've got:
>
> replica brick 1 and 2: 19867168 kbytes == 19G
> arbiter brick: 1872308 kbytes == 1.8G
>
> The amount of inodes on each brick is 3139091. So here goes estimation.
>
> Dividing arbiter used space by files+folders we get:
>
> (1872308 - 42000)/2036637 == 899 bytes per file or folder
>
> Dividing arbiter used space by inodes we get:
>
> (1872308 - 42000)/3139091 == 583 bytes per inode
>
> Not sure about what calculation is correct.

I think the first one is right because you still haven't used up all the 
inodes.(2036637 used vs. the max. permissible 3139091). But again this 
is an approximation because not all files would be 899 bytes. For 
example if there are a thousand files present in a directory, then du 
<dirname> would be more than du <file> because the directory will take 
some disk space to store the dentries.

>   I guess we should consider the one
> that accounts inodes because of .glusterfs/ folder data.
>
> Nevertheless, in contrast, documentation [2] says it should be 4096 bytes per
> file. Am I wrong with my calculations?

The 4KB is a conservative estimate considering the fact that though the 
arbiter brick does not store data, it still keeps a copy of both user 
and gluster xattrs. For example, if the application sets a lot of 
xattrs, it can consume a data block if they cannot be accommodated on 
the inode itself.  Also there is the .glusterfs folder like you said 
which would take up some space. Here is what I tried on an XFS brick:
[root at ravi4 brick]# touch file

[root at ravi4 brick]# ls -l file
-rw-r--r-- 1 root root 0 Mar  8 12:54 file

[root at ravi4 brick]# du file
*0       file**
*
[root at ravi4 brick]# for i in {1..100}
 > do
 > setfattr -n user.value$i -v value$i file
 > done

[root at ravi4 brick]# ll -l file
-rw-r--r-- 1 root root 0 Mar  8 12:54 file

[root at ravi4 brick]# du -h file
*4.0K    file**
*
Hope this helps,
Ravi


> Pranith?
>
> [1] http://termbin.com/ka9x
> [2] http://gluster.readthedocs.org/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/
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