[Beowulf] Big storage

[Loic Tortay]

According to Bruce Allen:
>
> Thanks.  In what I wrote I was using 1 TB = 10^12 bytes and 20 TB = 2 x
> 10^13 bytes.
>
> When you say that the max capacity with two system disks is 19.6 TB, do
> you mean 19 600 000 000 000 bytes or do you mean 19.6 x 1024^4 = 21 550
> 427 904 410 bytes?
>
My figures are with 1 TB = 2^40 bytes.


One important aspect of ZFS is that the amount of space dedicated to
metadata is not fixed in advance.

So 19.6 TB is the usable space when the filesystem is really empty.


Creating many (hundreds of thousands or millions) files with a size
less than 512 bytes would reduce the available space significantly.

A file less than 512 bytes large uses one block (512 bytes) for the
"znode" (meta-data) and one block for the data.

You can actually overwhelm the filesystem with meta-data (we first
noticed this the "hard way").


Loïc.
-- 
| Loïc Tortay <tortay at cc.in2p3.fr> -     IN2P3 Computing Centre     |