[Lustre-discuss] 1.8.1 test setup achieved, what about maximum mdt size

[Brian J. Murrell]

On Tue, 2009-10-20 at 10:15 -0600, Andreas Dilger wrote:
> 
> It is true that the MDT device can become full at some point, but this
> happens fairly rarely given that most Lustre HPC users have very large
> files, and the size of the MDT is MUCH smaller than the space needed for
> the file data.

Indeed.  For some (very) anecdotal experience, witness my own very small
Lustre filesystem usage:

$ lfs df
UUID                 1K-blocks      Used Available  Use% Mounted on
mds1_UUID             18348668   1327240  15972852    7% /mnt/lustre[MDT:0]
client-OST0000_UUID   20642428  14883944   4709844   72% /mnt/lustre[OST:0]
client-OST0001_UUID   20642428  14908260   4685528   72% /mnt/lustre[OST:1]
client-OST0002_UUID   20642428  15055492   4538296   72% /mnt/lustre[OST:2]
client-OST0003_UUID   20642428  14905716   4688072   72% /mnt/lustre[OST:3]
client-OST0004_UUID   20642428  14871520   4722268   72% /mnt/lustre[OST:4]

filesystem summary:  103212140  74624932  23344008   72% /mnt/lustre

$ lfs df -i
UUID                    Inodes     IUsed     IFree IUse% Mounted on
mds1_UUID              5242880   2109580   3133300   40% /mnt/lustre[MDT:0]
client-OST0000_UUID    1310720    208666   1102054   15% /mnt/lustre[OST:0]
client-OST0001_UUID    1310720    538201    772519   41% /mnt/lustre[OST:1]
client-OST0002_UUID    1310720    388754    921966   29% /mnt/lustre[OST:2]
client-OST0003_UUID    1310720    292766   1017954   22% /mnt/lustre[OST:3]
client-OST0004_UUID    1310720    469037    841683   35% /mnt/lustre[OST:4]

filesystem summary:    5242880   2109580   3133300   40% /mnt/lustre

As you can see, my MDT, at just less than 20% of the size of my total
OST storage is quite oversized (by 10x perhaps?) for the data I am
storing, and I store lots of small files -- Lustre, kernel and other
misc source trees.  I don't do any striping however, which helps keep
MDT usage lower.

> If you are using LVM you can increase the size of the MDT device and
> resize the filesystem to add more inodes to the filesystem.

Ahhh.  I don't think I knew that resizing actually increased inode
counts.

Just for the experience of it, (and when I can find a moment to do it) I
will probably transplant my MDT into a newly created (on LVM of course,
given I do everything on LVM) device, much smaller than my current one.
Indeed, I could try just shrinking the existing one, but I want to
create a new one from scratch, complete with a mountconf- style UUID and
move the MDT data into it.

b.

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